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Selective Attention Modulates Peripheral Auditory Function

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Title:
Selective Attention Modulates Peripheral Auditory Function
Creator:
Srinivasan, Sridhar
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[Gainesville, Fla.]
Florida
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University of Florida
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english
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Degree:
Doctorate ( Ph.D.)
Degree Grantor:
University of Florida
Degree Disciplines:
Psychology
Committee Chair:
SMITH,DAVID WILLIAM
Committee Co-Chair:
KEIL,ANDREAS
Committee Members:
EBNER,NATALIE CHRISTINA
LE PRELL,COLLEEN GARBE
Graduation Date:
8/9/2014

Subjects

Subjects / Keywords:
Acoustic noise ( jstor )
Audio frequencies ( jstor )
Auditory cortex ( jstor )
Auditory stimulation ( jstor )
Cochlea ( jstor )
Ears ( jstor )
Fine structure ( jstor )
Selective attention ( jstor )
Signals ( jstor )
Visual stimuli ( jstor )
Psychology -- Dissertations, Academic -- UF
attention -- dpoae -- finestructure -- otoacoustic -- selective
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bibliography ( marcgt )
theses ( marcgt )
government publication (state, provincial, terriorial, dependent) ( marcgt )
born-digital ( sobekcm )
Electronic Thesis or Dissertation
Psychology thesis, Ph.D.

Notes

Abstract:
There is increasing evidence that attending to an auditory stimulus selectively in the presence of multimodal stimuli results in changes in responses at the most peripheral levels of the auditory system. This is in apparent agreement with measures at more central levels that show response enhancement to attended stimuli, compared to responses to unattended stimuli. However, the direction of the response changes at peripheral levels has not been established conclusively, with some studies showing enhancement of the attended signal response, and some others showing no effects or the opposite effect (attenuation). It is generally accepted that the descending auditory (corticofugal) pathways that synapse at different stages of the ascending auditory pathways and down to the level of the cochlea are responsible (among other proposed functions) for mediating these effects of endogenous attention. While the effects of attention at the auditory periphery have been characterized using a variety of measures, including brainstem auditory evoked potentials, cochlear potentials, and otoacoustic emissions, it is not clear how they may be reconciled with the changes observed at more central levels in the brain. To explore the question of how the corticofugal system mediates selective attention and to attempt to reconcile the differing effects at the central and peripheral auditory levels, a series of experiments are conducted, looking at a) intra-modal attention in the presence of binaural stimulation, b) responses to attended and unattended stimuli at different frequencies in the cochlea. ( en )
General Note:
In the series University of Florida Digital Collections.
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Includes vita.
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Includes bibliographical references.
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Description based on online resource; title from PDF title page.
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This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Thesis:
Thesis (Ph.D.)--University of Florida, 2014.
Local:
Adviser: SMITH,DAVID WILLIAM.
Local:
Co-adviser: KEIL,ANDREAS.
Statement of Responsibility:
by Sridhar Srinivasan.

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UFRGP
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Copyright Srinivasan, Sridhar. Permission granted to the University of Florida to digitize, archive and distribute this item for non-profit research and educational purposes. Any reuse of this item in excess of fair use or other copyright exemptions requires permission of the copyright holder.
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SELECTIVE ATTENTION MODULATES PERIPHERAL AUDITORY FUNCTION By S RIDHAR SRINIVASAN A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA 2014

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© 2014 Sridhar Srinivasan

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To Amma, Appa & SJR

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4 ACKNOWLEDGMENTS I thank my parents for their unconditional support and encouragement . I am greatly indebted to my advisor and mentor Prof. David W. Smith for his gui dance throughout my stay in the program. I thank Prof. Andreas Keil for his counsel and f inancial support . I would like to thank Prof . Colleen G. Le Prell for her advice and for allowing me the use of lab space and equipment. I thank Prof. Natalie C. Ebner for her continued encouragement and interest in my work. I would also like to thank P rof . Christopher Spankovich for allowing me the use of h is equipment and his guidance. I would like to take this opportunity to thank Prof. Neil E. Rowland and Prof. Darragh P. Devine for their continued support. This journey would not have been successful, if not for my friends and colleagues that have supported and encouraged me in various capacities over the years. I am deeply indebted to Debdeep, Selvi, Ryan, Christian, and Aditi for their confidence in me , an d their unwavering support. Special thanks go to my lab colleagues Kyle, Seth and Wendy.

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5 TABLE OF CONTENTS page ACKNOWLEDGMENTS ................................ ................................ ................................ .. 4 LIST OF TABLES ................................ ................................ ................................ ............ 7 LIST OF FIGURES ................................ ................................ ................................ .......... 8 LIST OF ABBREV IATIONS ................................ ................................ ............................. 9 ABSTRACT ................................ ................................ ................................ ................... 11 CHAPTER 1 INTRODUCTION ................................ ................................ ................................ .... 13 2 EXPERIMENT 1 INTERAURAL ATTENTION MODULATES OUTER HAIR CELL FUNCTION ................................ ................................ ................................ ... 26 Procedure ................................ ................................ ................................ ............... 30 Participants ................................ ................................ ................................ ....... 30 Instrumentation And Stimulus Parameters ................................ ....................... 30 Auditory stimuli ................................ ................................ ........................... 30 Visual stimuli ................................ ................................ .............................. 31 Behavioral Task ................................ ................................ ................................ 32 DPOAE Analysis ................................ ................................ .............................. 33 Statistical Analysis ................................ ................................ ............................ 33 Experimental Procedure ................................ ................................ ................... 34 Results ................................ ................................ ................................ .................... 34 Behavioral Results ................................ ................................ ........................... 34 DPOAE Results ................................ ................................ ................................ 35 Discussion ................................ ................................ ................................ .............. 37 3 EXPERIMENT 2 SELECTIVE ATTENTION AND FINE STRUCTURE DPOAE ... 45 Procedure ................................ ................................ ................................ ............... 52 Participants ................................ ................................ ................................ ....... 52 Instrumentation And Stimulus Parameters ................................ ....................... 52 Auditory stimuli ................................ ................................ ........................... 53 Visual stimu li ................................ ................................ .............................. 53 Behavioral Task ................................ ................................ ................................ 54 DPOAE Analysis ................................ ................................ .............................. 54 Statistical Analysis ................................ ................................ ............................ 56 Experimental Procedure ................................ ................................ ................... 57 Result s ................................ ................................ ................................ .................... 58 Behavioral Results ................................ ................................ ........................... 58

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6 DPOAE Results ................................ ................................ ................................ 58 Discussion ................................ ................................ ................................ .............. 60 4 EXPERIMENT 3 SELECTIVE ATTENTION AND FINE STRUCTURE DPOAE IN THE PRESENCE OF CONTRALATERAL NOISE ................................ ............. 70 Procedure ................................ ................................ ................................ ............... 77 Participants ................................ ................................ ................................ ....... 77 Instrumentation And Stimulus Parameters ................................ ....................... 78 Auditory stimuli ................................ ................................ ........................... 78 Visual stimuli ................................ ................................ .............................. 79 Behavioral Tas k ................................ ................................ ................................ 80 DPOAE Analysis ................................ ................................ .............................. 81 Statistical Analysis ................................ ................................ ............................ 83 Experimental Procedure ................................ ................................ ................... 83 Results ................................ ................................ ................................ .................... 84 Behavioral Results ................................ ................................ ........................... 84 DPOAE Results ................................ ................................ ................................ 85 Discussion ................................ ................................ ................................ .............. 86 5 GENERAL DISCUSSION AND FUTURE DIRECTIONS ................................ ...... 101 LIST OF REFERENCES ................................ ................................ ............................. 106 BIOGRAPHICAL SKE TCH ................................ ................................ .......................... 119

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7 LIST OF TABLES Table page 3 1 Experimental Design for visual and auditory attention ................................ ........ 69 4 1 Experimental Design for visual and auditory attention with noise ....................... 99 4 2 Behavioral Results ................................ ................................ ............................ 100

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8 LIST OF FIGURES Figure page 2 1 Schematic of three different attending conditions.. ................................ ............. 41 2 2 Individual DPOAE contours for ten participants recorded during three attending conditions. ................................ ................................ ........................... 42 2 3 Difference in absolute overall DPOAE onset level. ................................ ............. 43 2 4 Average DPOAE contours for ten participants recorded during three attending conditions . ................................ ................................ ........................... 44 3 1 Swept frequency paradigm for DPOAE measurement.. ................................ ..... 64 3 2 Graph showing a typical DPOAE and its components ................................ ........ 65 3 3 Graph showing a typical generator component DPOAE during different attention conditions. ................................ ................................ ............................ 66 3 4 gDPOAE in visual attention and auditory attention conditions ............................ 67 3 5 gDPOAE V A difference between visual attention and auditory attention conditions. ................................ ................................ ................................ .......... 68 4 1 Swept frequency paradigm for DPOAE measurement. ................................ ...... 94 4 2 Graph showing a typical DPOAE and its components ................................ ........ 95 4 3 gDPOAE V A difference between attend visual with noise and attend auditory with noise conditions . ................................ ................................ ......................... 96 4 4 Example of whole fs DPOAE frequenc y maxima analysis ................................ .. 97 4 5 ANOVA of frequency shift dataset in groups ................................ ...................... 98

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9 LIST OF ABBREVIATIONS A/A CN Ratio of auditory with no noise DPOAE and auditory with CN frequency DPOAE ABR Auditory brainstem response ANF Auditory nerve fiber ANOVA Analysis of variance ASSR Auditory steady state response BBN Broadband noise CAP Compound action potential CEOAE Click evoked otoacoustic emission CM Cochlear microphonic CN Contralateral noise DPOAE Distortion product otoacoustic emission EOAE Evoked otoacoustic emission FFT Fast Fourier transform fs DPOAE Fine structure of DPOAE gDPOAE Generator component of DPOAE gDPOAE V A Difference in gDPOAE between visual and auditory attention conditions IC Inferior colliculus IFFT Inverse fast Fourier transform IHC Inner hair cell LSF Least squares fit MEM Middle ear muscle MOC Medial olivocochlear efferent OAE Otoacoustic emission

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10 OHC Outer hair cell rDPOAE Reflection component of DPOAE SFOAE Stimulus frequency otoacoustic emission SOAE Spontaneous otoacoustic emission SOC Superior olivary complex TEOAE Transient evoked otoacoustic emission VNTB Ventral nucleus of trapezoid body

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11 Abstract of Dissertation Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy SELECTIVE ATTENTION MODULA TES PERIPHERAL AUDITORY FUNCTION By Sridhar Srinivasan August 2014 Chair: David W. Smith Major: Psychology There is increasing evidence that attending to an auditory stimulus selectively in the presence of multimodal stimuli results in changes in responses at the most peripheral levels of the auditory system. This is in apparent agreement with measures at more central levels that show response enhancement to attended stimuli, compared to responses to unattended stimuli. However, the direction of the response changes at peripheral levels has not been established conclusively, with some studies showing enhancement of the attended signal response, and some others showing no effects or the opposite effect (attenuation). It is generally accepted that the descending auditory (corticofugal) pathways that synapse at different stages of the ascending auditory pathways and down to the level of the cochlea are responsible (among other proposed functions) for mediating these effects of endogenous attention. While the effects of attention at the auditory periphery have been characterized using a variety of measures, including brainstem auditory evoked potentials, cochlear potentials, and otoacoustic emissions, it is not clear how they may be reconciled with the changes observed at more central levels in the brain. To explore the question of how the corticofugal system mediates selective attention and to attempt to reconcile the differing effects at the

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12 central and peripheral auditory levels, a series of experiments are conducted , looking at a) intra modal attention in the presence of binaural stimulation, b) responses to attende d and unattended stimuli at different frequencies in the cochlea.

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13 CHAPTER 1 INTRODUCTION Selective attention is a key feature of all sensory modalities. Because our sensory environment comprises a constant influx of complex, overlapping stimuli, selective attention is necessary to minimize interference from extraneous, competing stimuli. Selective attention is the process by which we attend to one stimulus to the exclusion of other presented stimuli either in the sa me sensory modality or in multiple sensory modalities. The quintessential example of this mechanism is the cocktail party effect (Cherry, 1953). Despite the presence of a crowded, cacophonous background, a listener can actively focus attention on a singl e individual. This voluntary attentional shift enhances the stimulus signal, increasing the signal to noise ratio and resulting in greater perceptual salience. Differential stimulus processing in the cerebral cortex has been proposed as one potential exp lanation for this phenomenon ( Hillyard et al. , 19 87 ). For instance, studies in human observers have shown enhanced sensory signal representation of attended stimuli in the auditory cortex. Specifically, attending to an auditory stimulus has been shown t o enhance the late event related potential P50 and N1 components at latencies of 50 ms or greater, corresponding to responses from areas above the thalamus ( Woldorff & Hillyard , 1991 ). Conversely, evoked responses with latencies of 20 ms or less, consisti ng of cochlear or brainstem responses, have not been shown to be affected by changes in the focus of attention (Collet & Duclaux, 1986 ; Connolly et al. , 1989 ; Hirschhorn & Michie, 1990). In addition to the ascending auditory pathways in humans, there exist anatomical connections from the primary auditory cortex descending down to regions of the thalamus, the inferior colliculi, and to the superior olivary complex (SOC) ( Winer, 2005;

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14 Schofield, 2011 ). Evidence for local feedback loops exists at every level of this corticofugal pathway (f or review see Schofield, 2011). Ipsilateral and contralateral efferent connections then extend from the medial (MOC) and lateral (LOC) aspects of the superior olivary complex down to the outer (OHC) and inner (IHC) hair cell s in the cochlea (for review see Guinan, 2006; Robertson, 2009). Supplementing the anatomical evidence for descending connections from the auditory cortex to the cochlea, there exists evidence of a functional connection between the auditory cortex an d coc hlea. The function of the descending corticofugal pathway has been explored in a number of species including humans ( Khalfa et al ., 2001; Perrot et al ., 2006), bats (Xiao & Suga, 2002), mice (Liu et al ., 2010) and chinchillas ( León et al. , 2012). In human patients undergoing evaluation for surgical relief of epileptic symptoms, Perrot and colleagues (2006) showed that electrical stimulation of the contralateral primary auditory cortex resulted in a significant decrease in transient evoked o toacoustic emissions in response to click stimuli , a measure of peripheral cochlear OHC responses. Surgical ablation of the auditory cortex in human patients to minimize epilepsy attacks has been shown to decrease medial olivocochlear function (Khalfa et al. , 2001). MOC function, while resection of the anterior temporal lobe increased MOC activity. Likewise, León and colleagues (2012) showed that blocking ongoing basal auditory cortex activity in c hinchillas modulated the cochlear response, decreasing the cochlear microphonic component. Xiao & Suga (2002) showed that cortical electrical stimulation, through the corticofugal pathway, reduces the amplitude of, and shifts the frequency of the cochlear electrophysi ological responses in bats. Electrical stimulation of the auditory cortex in

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15 mice has also been shown to modulate response magnitudes, latencies and receptive fields of ipsilateral cochlear neurons (Liu et al. , 2010). The final segment of this corticofugal pathway, starting from the superior olive down to the cochlea, comprises the lateral (L) and medial (M) olivocochlear (OC) neurons (Warr & Guinan, 1979 ; Warr, 1992 ). The lateral olivocochlear neurons (LOC) originate mostly from the later al superior olive (LSO) and synapse mainly on dendrites of auditory nerve fibers beneath the inner hair cells (IHC). The medial olivocochlear neurons (MOC) receive afferent signalling from the ipsilateral and contralateral cochlear nuclei and descend to s ynapse on the base of outer hair c ells (OHC) within the cochlea. The MOC neurons are thick and myelinated, while the LOC neurons are thin and unmyelinated. The outer hair cell system is responsible for amplifying auditory stimuli though its activ e mechan ical response to sound. The amplified stimuli are then trans duced by the inner hair cells. Outer hair cells have the highest resting m embrane potential in the body. The transduction voltage through the outer hair cells drives a fast electromechanical motor action (Holley & Ashmore, 1988 ; Dallos et al. , 1991) that is responsible for amplifying the vibrational response to incident sounds (Neely & Kim , 1986). Medial olivocochlear efferent fibers descend from the medial superior olivary nucleus and synapse at the base of the outer hair cells and, when activated, decrease the resistance of the basolateral wall of outer hair cells and decrease the resting membrane potential. The resulting decrease in the resting potential results in a decr eased mechanical amplification, thereby effectively attenuating the response of the inner hair cells to sound. This amplification is modulated selectively at different frequencies via tonotopically descending connections from MOC efferent neurons,

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16 thereby influencing cochlear frequency selectivity to au ditory stimuli (Robertson, 1984; Liberman & Brown, 1986; Brown, 1989). Activity of the medial olivocochlear efferent pathway is thought to play a role in a variety of auditory functions including protection from acoustic trauma, frequency selectivity, auditory, sound localization, mediation of selective attention and in improvement in detection of signals in noise (for review see Robertson, 2009 ; Guinan, 2010). When the auditory system is exposed to intense sounds, temporary threshold shifts are observed in cochlear activity, which are reduced when the olivocochlear sys tem is activated (Rajan, 1988). Similarly, when the MOC tracts into an ear are sectioned, that ear shows an increase in the temporary thresho ld shifts after noise exposure (Z heng et al. , 1997). The proposal that the MOC tracts function to protect the ear from acoustic trau ma is not without controversy. Kirk & Smith (2003), for example, argue that protection from noise is not a primary functio n of the medial efferent system, as the intense acoustic environments necessary to result in the evolution of a protective mec hanism do not exist in nature. Frequency selectivity has also been proposed to be regulat ed by medial efferent function. Section ing the medial efferent bundle results in an enlargement of the tip segment of the cochlear action potential tuning curve, and decreased frequency sel ectivity (Carlier & Pujol, 1982; Brown et al. , 1983; Bonfils et al. , 1986). Another function of the effer ent system is to produce rapid adaptation to sustained stimuli so that transient and speech like stimuli ( in humans) are detected better. This rapid adaptation has a time constant on the order of 50 500ms (da Costa et al. , 1997), and is also observed in t he response of outer hair cells, and distortion product otoacoustic emissions (DPOAE) in human listeners (Liberman et al. , 1996; Kim

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17 et al. , 2001; Bassim et al. , 2003). More recently studies have shown that the medial efferents may be involved in localizing sound sources in noise by counteracting the effects of background noise ( Andéol et al. , 2011). The effects of selective attention at the auditory periphery have been studied using a variety of measures including otoacoustic emissions (CEOAE, DPOAE) , cochlear poten tials (CAP) and auditory brain stem responses (ABR) . Lukas (1980, 1981) reported that ABRs were suppressed when participants attended to visual stimuli, an d ignored auditory stimuli. Lukas measured ABR when subjects either l istened to binaurally presented tone pips (auditory attention) or counted letters that were flashed rapidly on a screen (visual attention). The ABR potentials showed an increase in latency and a decrease in amplitude during the visual attention condition, physiological evidence consistent with M OC suppression of OHC function while attending to visual task . Earlier studies by Oatman and colleagues (1971, 1976) have also measured changes in ABRs during different attention conditions in behaviorally traine d cats. In these studies, they presented click stimuli to cats when they were engaged in a visual discrimination task and recorded click evoked potentials from the round window, the cochlear nu cleus and the auditory cortex. Oatman and colleagues reported that click evoked responses measured at the round window and the cochlear nucleus were reduced during visual attention conditions, compared to control conditions where the tention was directed elsewhere. In a more recent, conceptua lly similar study, Delano and colleagues (2007) reported reductions in CAP and increases in cochlear microphonics, when chinchillas were trained to attend to visual stimuli, w hile ignoring auditory stimuli. While the CAP is a neural response measured from the auditor y nerve

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18 reflecting the IHC electrical activity, it is, in part, modulated by OHC amplification of the mechanical vibration transduced by the IHC. The CM is believed to be a measure of receptor potentials arising in hair cells, but predominantly reflecting OHC potentials (Dallos & Cheatham , 19 76 ) . Both CM and CAP changes are therefore indirect measures of MOC inhibition of OHC activity . In humans, MOC function can be studied using a non invasive method of measuring outer hair cell activity by means of recording otoacoustic emissions (OAE) in resp onse to tonal auditory stimuli. OAE s are non linear cochlear responses to sound that are generated by the OHCs in the inner ear, travel back through the middle ear, and into the external ear canal where they ca n be recorded u sing a microphone (Kemp, 1978). Various types of OAEs such as SFOAE , distortion product OAEs (DPOAE) and evoked OAEs have been used to study MOC control over cochlear function (Shera & Guinan, 1999; Shera, 2004). In some selective attentio n tasks, decreased OAE amplitudes were observed when participants ignored the auditory stimuli vs. when they attended to the auditory stimuli (Giard et al. , 1994; Giard et al. , 2000). Giard and colleagues (1994) recorded sound EOAE to 1 kHz in one ear and 2 kHz in the opposite ear during a dich otic selective listening task. They reported that the amplitude of EOAEs for attended t ones were larger than for unattended tones. While these studies, and others show an increase in the amplitude of attended audit ory responses at the cortical level (Woldorff et al. , 1987, Hillyard, 1993), there is controversy regarding the effects of selective atten tion at the auditory periphery. Michie and colleagues (1996) and Avan & Bonfils (1992), reported that attending to vi sual tasks produced no measurable change in evoked OAE amplitudes and, in some cases, an increase in evoked OAE amplitudes w hen ignoring auditory

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19 stimuli. Avan & Bonfils (1992) , compared DPOAEs and SFOAEs while listeners attended to, and ignored visual sti mu li. They reported no significant change in the recorded signals acros s the two attending conditions. Michie and colleagues conducted a series of six experiments addressing the questions of whether the MOC system innerva ting the OHC plays a role in selective attention . In the first five experiments, 1 and 2 kHz tone pips were presented to the same ear with the attentional load manipulated by the changing the difficulty of the task and by i ntroducing contralateral noise. In the sixth experiment, aud itory stimuli we re presented to opposite ears. The results suggested no effects due to selective attention on the EOAE amplitude and power; however, in one experiment EOAE power was reduced during the auditory atten ding condition compared to the auditory ignoring condition. In another experiment, Maison and colleagues (2001) recor ded EOAEs in the presence of contralateral tone pips embedded in noise. embedded tone pips, while in another condition, the experimenters asked them to atte nd to and count the tone pips. When participants attended to the embedded tone pips in noise in the contralateral ear, enhanced suppression of EOAE at the same frequency was o bserved in the ipsilateral ear. Similarly Smit h and colleagues (2012) reported increases in DPOAE amplitudes when participants attended to visual stimuli, while ignoring auditory stimul i. In these studies, Smith and colleagues instructed participants to either read a book and count the occurrence of or count either the short or long duration DPOAE eliciting tones. These results were replicated in another study (Srinivasan et al. , 2012), comparing DPOAEs under different, balanced condi tions of intermodal attention. In thi s study participants were

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20 instructed to either attend to a visual Gabor patch and count early/late phase shifts or count either the short or long d uration DPOAE eliciting tones. DPOAE amplitudes were decreased in the auditory attending condition, compare d to the visual attending condition. These results are in contrast with the cortical effects that show an increase in the amplitude of the attended auditory responses. This discrepancy between the cortical and peripheral effects of selective attention raises important questions concerning precisely how the cortical activity (enhancement and attenuation) to attended vs. ignored signals modulates the responses of the OHC a nd IHC in the cochlea. It also raises the issue of understanding the role of the corticofugal pathways, particularly at the level of the MOC system. MOC efferent neurons exit the brain stem and synapse at the base of the OHC and act to sup press OHC acti vity. The MOC neurons release acetylcholine, which opens calcium ion channels at th e base of the OHCs . Calcium ions enter the OHC s leading to the opening of calcium activated potassium channels i n the OHC membrane. This lowers the basolateral wall resis tance of the OHC s, which reduces the drive to the OHC mechanical motor, resulting in decreased cochlear amplification. The combination of this suppressive effect and the tonotopicity of the efferent connections with the out er hair (Cody & Johnstone, 1982; Liberman & Brown, 1986; Robertson, 2009) cells influences peripheral auditory activity to mediate selective attention effects, frequency selectivity and improved signal to noise ratio (Robertson, 2009; Guinan, 2010). MOC efferent neurons have been shown to influence the response of auditory nerve fibers (ANF) to signals in quiet and in background noise; stimulation of the crossed olivocochlear bundle modulates the dynamic range of the auditory nerve, by

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21 suppressing the ANF response to the background noise target signal in the presence of broadband noise (Winslow & Sachs, 1987 ; Kawase & Liberman 1993). MOC efferent activation also results in similar unmasking effects in n eurons of the cochlear nucleus. These effects are manif est as decompression of the output range of the I/O functions as well as a steepened I/O slope, these result in an improvement in intensity discrimination of pure tones in noise (Mulders et al. , 2008). Psychophysical data from multiple studies show that th e effects of attention are different at frequencies away from the expected or attended freq uency band (Scharf et al. , 1987; Dai et al. , 1991), compared with when the signal is presented in the expected frequency band. Scharf and colleagues (1987) presente d participants tones at an expected frequency vs. at unexpected frequencies using a probe signal method. The participants detected tones at an expected frequency at 90%, while unexpected frequency to nes were detected near chance. In one experiment, Dai and colleagues (1991) presented tones within and far away from an attended target frequency band. Within the attended target frequency band, the attenuation of the attention band was similar to that of the auditory filter centered on the target. In the case of away frequencies, an increased signal level was required to elicit similar levels of accuracy as for tones presented i n the attended frequency band. The authors suggested that attenuation produced in the away bands might be a result of a decreased weight assigned to unattended frequency bands. The MOC neurons synapse tonotopically onto the OHC s in a mapping that is generally similar to the cochlear frequency mapping for auditory nerve fibers (Liberman & Brown, 1986; Brown, 1989). Differential eff ects of attention at expected and

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22 unexpected frequencies as measured in Scharf et al. ( 1987 ) , Dai et al. ( 1991 ) can be explained by a possible corticofugal mechanism that influences MOC neuronal activity at attend ed and unattended frequencies. Given that the MOC neurons are known to inhibit the activity of the OHC s (Robertson, 2009), differential MOC initiated inhibition of OHC activity might take place in attended frequency place vs. unattended frequency place in the cochlea. This differential activation serves as a possible explanation for the results observed in peripheral studies assessing OAEs (Michie et al. , 1996; Maison et al. , 2001 ; Smith et al. , 2012; Srinivasan et al. , 2012) where relatively lower otoacoustic emissions were observed during audito ry attention compared to visual attent ion. It is however, not clear how this differential inhibition of OHC activity is mediated by the apparently opposite effect observed in the auditory cortex where attention to auditory stimuli results in a higher even t related potentials compared to attention to visual stimuli (Hillyard et al. , 1987; Woldorff et al. , 1993; Bidet Caulet et al. , 2007; Kauramäki et al. , 2007; Saupe et al. , 2009). In an attempt to resolve the question of how selective attention at the cort ical level might differentially influence the OHC activity at the cochlear level, via the corticofugal pathway, three experiments are proposed. Selective attention is manipulated in a cross modal attention task with intra modal components to compare DPOAE in the atten ded vs. unattended conditions. In this experiment, decoupled binaural stimuli would be presented simultaneously with a visual stimulus and selective attention is manipulated in three different conditions while DPOAEs are recorded from an ear of interest. In one condition, the participa nt will be instructed to attend to auditory stimuli in the ear of interest, while ignoring the stimuli in

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23 the o ther ear and the visual domain. In the second condition, the participant attends to the stimuli in the ear that is not the ear of interest, and in the third condition, the participant attends to the visual stimulus and ignores the auditory stimuli. Based on previous studies, we would expect that in the auditory attending condition to the ear of interest, the recorded distortion product otoacousti c emissions would be lowest and during visual attended condition, they would be the highest. Distortion product OAEs (DPOAEs) are produced by the nonlinear mechanical behavior of the outer hair cells when two tones with a frequency ratio of 1.21 (f2/f1) an d specific level relationshi p are presented to the cochlea. The cubic DPOAE at frequency 2f1 f2, the largest of the DPs as measured in humans, consists of a combination of two components, a generator and a reflection component ( Mauermann et al. , 1999; Tal madge et al. , 1999; Kalluri & Shera, 2001). When two primary tones at frequencies f1 and f2 are presented simultaneously, they interact maximally on the basilar membrane near the cochlear tonotopic place for the tone with frequency f2, generating an ampli tude modulated signal at a frequency that is related to f1 and f2 as 2f1 f2 (inter modulation distortion). The intermodulation distortion component from this cochlear place travels basally, towards the middle ear and is measured as the generator component in the ear canal, while the apical travel of this component results in its amplification as it reaches the cochlear characteris tic place for frequency 2f1 f2. The basal reflection of this emission from the cochlear place for frequency 2f1 f2, measured in the ear canal is consid ered the reflection component. The respective phase delays of the reflection and generator components at the middle ear enable the DPOAEs to be computationally separated into these two components (Long et al. , 2008).

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24 Given that med ial efferent control of outer hair cell activity is known to be tonotopic, the second and third experiments are designed to look at frequency selectivity and how selective attention is mediated via efferent con trol at the auditory periphery. A swept DPOAE measure (Long et al. , 2008) may be used to observe changes in OHC sensitivity due to selective attention at different frequencies when two primary DPOAE eliciting tones are presented simultaneously with visual stimuli to the participant. The tones are sw ept from ~1kHz to ~10kHz and from ~10kHz to ~1kHz at the rate of 7 seconds per swe ep. Frequency anomalies, where the sequential presentation of DP eliciting primaries is reversed for a narrow frequency band (~50 100 Hz), are introduced in some trials at e ither 2kHz or 4kHz. The visual stimulus is a Gabor patch on a gray screen, shifti ng phase at random times. During the auditory attention condition participants will attend to the swept stimuli and count the frequency anomalies at 2kHz (or 4kHz) and ignor e the visual stimuli, while during the visual attention condition, they will attend to the visual stimuli and ignore the auditory stimuli. It is hypothesized that the DPOAE recorded during visual attention will be higher than during auditory attention. Fi nally, the variation in OHC sensitivity due to selective attention, as mediated by the MOC, across different frequencies may be studied with DPOAE eliciting tones presented with background noise. Medial efferent control of outer hair cell activity has bee n proposed to help detection of signals in noise (Robertson, 2009). While MOC activity in a quiet background has been shown to suppress auditory nerve fiber response to the presented signals (Winslow & Sachs, 1987), in a noisy background, the dynamic range of the ANF firing is increased particularly due to the decreased response

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25 of ANF to the background noise. To study MOC effects on outer hair cell activity to stimuli in the presence of background noise, the third experiment will include contralateral bro adband noise stimulation in addition to swept frequency stimuli. The auditory and visual attention conditions are similar to the second experiment. Contralateral broadband noise has been shown to decrease otoacoustic emissions (Bassim et al. , 2003; Abda la et al. , 2009). Given that the efferent pathway is implicated in selective attention and better signal detection in noise, this experiment will study the effect of selective attention in signals presented with noise. It is hypothesized that DPOAEs reco rded during visual attention will be lower than when recorded during auditory attention.

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26 CHAPTER 2 EXPERIMENT 1 INTERAURAL ATTENTION MODULATES OUTER HAIR CELL FUNCTION Many studies have explored the effects of selective attention on the peripheral auditory system (Meric & Collet 1992 ; 1994; Giard et al. , 1994; Michie et al. , 1996; Maison et al. , 2001; Delano et al. , 2007) and it is generally agreed that manipulating sele ctive attention can produce alterations in the functional characteristics of the cochlea. While relative enhancement of the attended signal and relative attenuation of the unattended signal a re observed cortically (Hillyard , 1993; Kauramäki et al. , 2007), there is no clear consensus on the nature and direction of selective attention eff ects at the auditory periphery. Earlier studies looking at changes in auditory brain stem responses due to selective attention reported increases in amplitude and changes i n wave latency (Lukas , 1980; Brix , 1984) or no change in the ABR (Picton et al. , 1974 ; Connolly et al. , 1989) while participants attended to visual stimuli. More recent studies (Delano et al. , 2007) looked at electrophysiological responses in behaviorally trained chinchillas performing visual and auditory attention tasks; Delano and colleagues (2007) reported changes in the auditory nerve compound action potential (CAP) and the cochlear microphonic (CM) responses to auditory stimuli. They observed relative decreases in CAP and relative increases in the CM during visual attention tasks compared to when the animals performed auditory attention tasks. While the CAP reflects the magnitude of the signal measured in the auditory nerve, the CM is a measure of the outer hair cell activity and an increase in the CM reflects changes in OHC activity . The authors argued that the changes in OHC activity were a result of voluntary selective attention.

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27 Other non invasive measures of outer hair cell function include otoac oustic emissions, which are recorded using a microphone at the outer ear, as a response to a tone or tones at certain frequencies (Kemp , 1978). Otoacoustic emissions are generated by the active mechanical response of the outer hair cells to sound (Yates e t al. , 1992). Some studies have shown that these otoacoustic emission measures reflect alterations in cochlear hair cell function as a result of selective attention (Giard et al. , 1994; Michie et al. , 1996; Maison et al. , 2001; Smith et al. , 2012). Giard and colleagues (1994) measured evoked otoacoustic emissions (EOAE) while participants attended to a tone at a certain frequency. They reported that EOAE to tones in one ear at a certain frequency had larger amplitude when attention was directed to this e ar than when attention was directed to the opposite ear. The experimental paradigm by Giard et al. (1994) recorded different EOAE in response to different frequencies (1kHz and 2kHz) in the two ears. In a later study Michie and colleagues (1996) tried to replicate the results reported by Giard et al. (1994) . After conducting a series of six experiments, however, they reported a null effect due to selective attention. In fact, their data showed a trending effect of selective attention in the opposite dire ction with a decrease in the evoked otoacoustic emission while participants attended to the tone, compared to when they ignored it. More recent studies, including Smith et al. ( 2012 ) , have reported similar changes with decreases in otoacoustic emissions w hen participants attended to the auditory stimulus compared to when they ignored the auditory stimuli and read a book or watched a muted DVD. Smith and colleagues measured distortion product otoacoustic emissions and reported that they were relatively dec reased when

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28 participants attended to an auditory task, compared to when they ignored the auditory stimuli. A significant methodological difference between the earlier studies (Giard et al. , 1994 ; Michie et al. , 1996) and the later ones (Smith et al. , 2012; Srinivasan et al. , 2012), specifically the use of distortion product otoacoustic emissions might explain the apparent discrepancy between the directions of the effects of selective attention. The medial olivocochlear system comprises efferent fibers that originate from the ventral nucleus of the trapezoid body and synapse primarily at the base of the OHC. These MOC efferent fibers form inhibitory connections at the base of the OHC (Robertson , 2009; Guinan , 2010). This seemingly paradoxical finding that attended signal responses are relatively smaller than responses recorded to the same, but ignored stimuli may be explained by the tuning characteristics of MOC tracts within the cochlea (Murugasu & Russell, 1996; Dolan et al. , 1997). Individual MOC fibers synapse onto the base of the OHC in a tonotopic fashion (Brown , 1989). In OAE studies listeners are instructed to attend to primary tones used to produce the non linear OAEs, which are recorded at a tonotopic location remote from th e attended signals (Shera & Guinan, 1999). Given the inhibitory nature of the MOC system on OHC activity, away frequencies may be subject to different levels of amplification by the OHC compared to attended frequencies. This effect can be observed in psy chophysical studies where detection of unexpected tonal frequencies is decreased as tonal frequencies get farther away from the expected frequency (Dai et al. , 1991, Strickland & Viemeister, 1995). Evidence (for review, see Schofield , 20 10, 2011 ) shows th at anatomical connections extend from the auditory cortex, thalamus and various levels of the

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29 brainstem down to the cochlea. The corticofugal pathways extend from the auditory cortex down to the superior olivary complex (SOC). As mentioned above, connect ions extend from the SOC to the base of the OHC. The corticofugal pathways are functionally implicated in studies looking at the effects of electrical stimulation of the auditory cortex on the periphery in many species. Xiao & Suga (2002) showed that sti mulating the auditory cortex in bats resulted in sharpened frequency tuning with an increase in the CM at cortical matched best frequencies and a decrease in CM and a centrifugal frequency shift at non matched best frequencies. In humans, Perrot and colle agues (2006) showed that stimulation of the auditory cortex results in a decrease in OAEs, similar to what was observed in the attentional manipulation studies (Michie et al. , 1996 ; Smith et al. , 2012, Srinivasan et al. , 2012). Besides the corticofugal an d crossed MOC connections it is well documented that the two ears are functionally connected by a rich innervation composed of the uncrossed medial olivocohlear efferent tracts (Warr et al. , 1986). While Le ó n and colleagues (2012) reported the existence o f cortico olivocochlear efferent basal firing activity modulating the CAP and CM responses in chinchillas, there is evidence supporting a functional connection between the two ears. Acoustic or electrical stimulation of one ear suppresses responding in th e contralateral ear (Warren & Guinan, 198 9 ; Smith et al. , 1994; Perry et al. , 1999; Bassim et al. , 2003; Deeter et al. , 2009). This contralateral effect is mediated via the uncros sed medial efferent system . While earlier studies manipulated attention to either ear (Giard et al. , 1994; Michie et al. , 1996) the stimuli were presented at different frequencies. In this experiment, we report

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30 the changes due to attention to either ear while the same tonal stimuli are presented to both ears. Procedure Participants Ten college aged students (18 20 years old, seven females) participated in this experiment. Prior to testing, a brief history was taken from each participant to document ear related complaints, such as current ear congestion or infection, history of ear infections, ear surgery, noise exposure, music player and headphone use and ototoxic and chronic medication use. All experiments were approved by the Institutional Review Board of the University of Florid a. Instrumentation And S timulus P arameters Two digitally generated primary tones (RX6 and RP2.1 DSP processors, Tucker Davis Technologies, Gainesville, FL, USA) were fed individually to two transducers in each ear (Etymotic Research, Elk Grove Village, IL, USA). The primaries and the otoacoustic e missions were measured in the ear canal with a low noise microphone probe (ER 10B+, Etymotic Research), sampled continuously at a rate of 48.83 kHz, digitized (Tucker Davis Technologies), and stored to the hard drive. Auditory stimuli The primary tones f1 and f2 were presented with a frequency ratio f2/f1 = 1.21 and f1 level = 70 dB SPL and f2 level = 65 dB SPL. A DPgram, assessing the sensitivity of the ear across a range of frequencies was constructed by varying the f2 frequency in a 20 step geometric pr ogression from 1.2 to 10.8 kHz, and the frequency producing the strongest DPOAE level in either ear for each subject was selected as the

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31 recorded (ipsilateral) ear during the experiment. The primary tones, though chosen to generate the largest DPOAE in th e recorded ear, were presented binaurally at all times to produce the largest measurable response in the ear selected for study (Liberman et al. , 1996; Bassim et al. , 2003). Responses from the contralateral ear, though collected were not further analyzed. The primary tones presented to each ear were either 1 or 2 s in duration (~20% 1s and ~80% 2 s), presented in a randomized manner with an inter trial interval of 2 s. The onset of both tones was simultaneous, regardless of duration. Using long duration t ones, as opposed to transient or click stimuli to measure DPOAEs offers the advantage of characterizing both the presence of MOC activity, as well as its onset time course (cf., Liberman et al. , 1996; Kim et al. , 2001; Bassim et al. , 2003). The rise/fall times of all stimuli were zero, with the primaries beginning at 0 degrees of phase in order to reduce the effects of frequency splatter. This splatter was further minimized by measuring the DPOAE amplitude at the 2f1 f2 frequency peak, discarding the firs t ~1ms, with a bin width of 11.92 Hz. Visual stimuli All visual stimuli were presented on a flat front of the participant. During each trial, after presentation of a fixation cross, Gabor patches, oriented 45 deg to the right of vertical, were presented in the center of the screen. Each Gabor patch was composed of seven alternating black/white sinusoidal bars whose greatest contrast (99.8% Michelson) was at the center of the stimulus, with a Gaussian decline to the edges. The Gabor patch subtended a visual angle of 2.18 deg rees with 3.21 cycles per degree. After a 1 or 2 s delay (presented randomly in a session, ~20% 1s and ~80% 2s) the patch was phase shifted by 180 deg rees . The

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32 visual stimulus was presented for a total of 3s in each trial with an inter stimulus interval of approximately 2s. During the inter stimulus interval, a fixation cross was present occupying 0.8 deg rees of visual angle. A miniature video camera, connected to a monitor outside of the sound chamber, The onset of visual stimuli and auditory stimuli were presented in each trial were offset, with the visual stimulus onset delayed with respect to the auditory stimulus by a random interval ranging between 500 and 1000 ms from the onset of the auditory stimulus. The random respectively, to identify short duration targets. Post experimental assessment of the strategy adopted by the participants suggested that they focused solely on the to be attended modality to complete the task. Behavioral Task Participants were instructed to press a response key when a target event was detected. Three di fferent attention conditions were tested (Fig ure 2 1 .). In the attend visual condition, participants ignored all (binaural) auditory stimuli and were instructed to respond to the short latency phase change in the (visual) Gabor patch. In the first auditor y attending condition (attend ipsilateral), participants were instructed to attend to the ear in which the DPOAEs were recorded and to report a short duration tone, while ignoring contralateral auditory stimuli and the visual Gabor patch. In the second au ditory attending condition (attend contralateral), participants were instructed to report detection of a short duration tone presented to the contralateral ear, while ignoring the auditory stimulus in the DPOAE recorded ear and the Gabor patch. Each atten tion condition was indicated by text on the screen before each session started. An arrow

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33 presented on the screen prior to each trial served to instruct the participants to attend to the left or the right ear. In attend visual conditions, text on the scre en instructed participants to attend to the visual stimuli. D POAE Analysis The microphone signals were divided and a real time Fast Fourier Transform was performed by a second computer during each trial to monitor the levels of primary signals, as well as to monitor for the presence of the 2f1 f2 DPOAE. During data analysis, a two component exponential was fitted to the DPOAE contours (Sigma Plot; Systat Software, San Jose CA) to facilitate calculation of the magnitude of adaptation, as well as the adaptat ion onset time constant (cf., Kim et al. , 2001). were included in the overall analysis only if the observed rapid adaptation was larger than the measured variance in the adaptation contour; using this criterion gave confidence that MO C activity, necessary for any attentional effects, could be observed Absolute DPOAE levels across attention conditions were estimated by averaging the data points in each trace from 750 ms to 1000 ms. Statistical Anal ysis For DPOAE levels and behavioral accuracy, the results obtained from each participant under each of the three attention conditions were compared statistically using repeated measures analysis of variance (ANOVA) with a within subjects of factor of cond ition (attend visual, attend ipsilateral, attend contralateral). A significant effect was followed by contrast analysis, including linear versus quadratic contrasts among the three means.

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34 Experimental Procedure All experiments were conducted inside of an IAC (Industrial Acoustics Corporation, Bronx, NY, USA) sound attenuating chamber. After initial screening, participants were seated in comfortable reclining chair within the sound booth, detailed instructions were provided, ear tip recording probes wer e recorded and saved to disk, and the responses from the ear with the selected DPOAE (ipsilateral ear), were averaged across subjects. Trials with variations in the primary levels and/or DPOAE responses (greater than the magnitude of rapid adaptation) wer e discarded before calculating averages. At least eight sessions under each condition were conducted with 16 trials in each session. Auditory stimuli of 1 (short) or 2 s (long) duration were pseudo randomly presented binaurally in each session with ~20% of the trials of the short duration. Visual stimuli were presented simultaneously on the screen in front of the participant, starting with a short pseudo randomized delay of 500 1000 ms after auditory stimuli onset. The visual stimuli changed phase eithe r after 1 s (short) or 2 s (long) after onset and stayed on the screen for a total of 3 s before being replaced by a fixation cross. Short visual stimuli were presented in ~20% of the trials in a pseudo randomized fashion in a session (the pseudo random o rder not coupled with that of the auditory stimuli) Results Behavioral R esults The percentage of mean hits (responding to the short duration primary tones) was 93.08 for the DPOAE recording ear attention condition, 86.54 for the contralateral ear attention condition and 84.54 for the visual attention condition. False alarms (respond ing to the long duration primary tones) were 1.17% for the DPOAE recorded

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35 ear attention condition, 1.70% for the contralateral attention condition and 9.63% for the visual attention condition (reporting detection of the short latency phase change in the Ga bor patch). The ANOVA indicated that there was no statistically significant difference between the mean hit percentages across the attend ipsilateral, attend contralateral and attend visual conditions, F(2,16) = 0.665, p = 0.528. False alarm percentages, however, were significantly different across the three conditions resulting in a main effect F(2,16) = 8.699, p = 0.003. A linear contrast F(1,8) = 12.074, p = 0.008 modeled false alarms condition differences with attend ipsilateral < attend contralatera l < attend visual. DPOAE Results Figure 2 2 shows the individual DPOAE contours for 10 listeners under three attending conditions; while attending to the visual Gabor patches and ignoring all auditory tones in both ears (attend visual; solid blue line), w hile attending to the DPOAE eliciting tones in the recorded ear and ignoring the contralateral tones and visual Gabor patch (attend ipsilateral; solid red line), and while attending to the contralateral tones and ignoring the recorded ear tones and visual Gabor patches (attend co ntralateral; dashed red line). While considerable inter subject variability is evident, in general, changes in attending conditions produced parallel shifts in the absolute level of the recor ded DPOAE adaptation contours. The absolute onset level of DPOAEs while participants attended to the visual distractor, and ignored all auditory tones (attend visual), was relatively higher in 8 of 10 participants (Figure 2 3). When listeners ignored the visual distractor and attended to the tones presented in the DPOAE recorded ear (attend ipsilateral), DPOAE onset levels were relatively lower in

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36 overall level in 6 of 10 participants, compared with when they attended to the tones presented in the contralateral ear (attended contralater al). Figure 2 4 shows the average DPOAE adaptation contours measured for the 10 listeners in each of three attending conditions. Consistent with previous results, mean adaptation contours show that DPOAEs recorded during the attend visual condition (sol id blue line) were relatively highest in overall level compared with DPOAEs recorded during both auditory attending conditions (attend ipsilateral; solid red and attend con tralateral; dashed red lines). The results also show that DPOAEs were lowest in ove rall level when listeners responded to target stimuli presented to the ipsilateral, recorded ear (solid red line). Comparing the two auditory attending conditions, on average, the amplitude of the DPOAE recorded while participants attended to the tones pr esented in the contralateral ear was statistically higher in overall level, approximately 0.08 dB, than the amplitude of DPOAEs recorded while attending to the tones presented in the ipsilateral, recorded ear. The data also show that attending to the visu al Gabor patches (attend visual), while ignoring auditory stimuli presented in both ears, resulted in the highest overall DPOAE levels; the mean DPOAE amplitude recorded while listeners ignored all tones presented in both ears, and attended to the visual d istractor was 0.1641 dB higher than the DPOAE amplitude when subjects attended to the ipsilateral, recorded ear and 0.08 dB higher than when listeners attended to the c ontralateral, non recorded ear. The ANOVA indicated that this linear increase of DPOAE means from the lowest levels during attend ipsilateral to the highest levels during attend visual trials was statistically significant, resulting in a main effect of con dition, F(2,18) = 6.4, p<0.01. A linear contrast F(1,9) = 11.3, p<0.01, modeling

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37 condi tion differences as attend ipsilateral < attend contralateral < attend visual, supported the hypothesis that DPOAE means increase as attention is directed away from the recorded ear. Discussion Overall, the present results show attention dependent, paralle l shifts in absolute DPOAE levels recorded during different intra and intermodal attending conditions. Consistent with previous results (Smith et al. , 2012; Srinivasan et al. , 2012), DPOAEs recorded while listeners attended to a visual distractor (attend visual) and ignored the auditory tones, were relatively higher in overall level compared with when participants attended to the same auditory stimuli and ignored the visual Gabor patches. These data, combined with previous studies (Delano et al. , 2007, Sm ith et al. , 2012; Srinivasan et al. , 2012) demonstrate that the effects of attention are clearly evident as modifications in responding at the most peripheral aspects of the auditory system within the cochlea. These findings also support the notion that s elective attention produces a systematic change in outer hair cell activity in the cochlea, via the descending corticofugal and medial efferent system. The current data show that attending to the ipsilateral ear results in DPOAE that are relatively lower i n overall amplitude, compared with DPOAEs recorded in the same ear while participants attended to primary tones presented in the contralateral ear. The effects of the MOC on otoacoustic emissions are well studied with acoustic stimuli presented in the con tralateral ear (for review see Guin a n , 2006). Cubic DPOAEs show a change in amplitude when contralateral broadband noise is presented (Liberman et al. , 1996), similar changes in amplitude are observed in SFOAEs in the presence of contralateral broadband n oise (Guinan et al. , 2003). This effect is mediated via the

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38 uncrossed MOC fibers, which constitute the contralater al olivocochlear reflex (Guinan , 2006). In addition, León and colleagues (2012) showed evidence to suggest basal corticofugal efferent firin g that modulated MOC and LOC firing thereby modulating CAP and CM. The current findings suggest that the interaural pathways also function both directly and indirectly to mediate an attentional mechanism. Cortical attention studies (Woldorff & H i llyard, 1991; Woldorff et al. , 1993) typically used a dichotic listening task to compare responses when participants attended to an auditory stimulus with when they ignored it. Magnetoencephalographic ( MEG ) and electroencephalographic ( EEG ) data from these studies showed increases in middle latency (20 50 ms) and long latency (>100 ms) components when participants attended to the auditory stimulus eliciting those responses, but not at latencies < 20 ms. Earlier studies by Giard et al. (19 94) and Michie et al. (1996) recorded and compared evoked otoacoustic emissions (EOAEs) while manipulating selective auditory attention. Giard and colleagues (1994) presented dichotic stimuli with 1 kHz in one ear and 2 kHz in the opposite ear. The partic ipants were instructed to count occasional targets in the right ear and ignore the tones in the left ear, or ignore the right ear and attend to the left ear. They reported evoked otoacoustic emissions (EOAE) as being higher for the attended tones than whe n they were unattended. Michie and colleagues (1996) employed a similar design in a series of six experiments, using both dichotic stimuli and stimuli presented to one ear alone. In the first five experiments, they presented two different frequency tones to one ear, without stimuli presented to the contralateral ear, and compared EOAEs while manipulating attention to either of those two frequency tones. They reported finding no effect of changes in attention in any of those conditions. In the sixth expe riment, they

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39 employed binaural tones, presented at different frequencies to the two ears, in an attempt to replicate the design by Giard et al. (1994). In that experiment, EOAEs were compared while participants attending to one ear and ignored stimuli pre sented to the other ear, with when the attending conditions were reversed. Under these conditions, they observed a decrease in the amplitude of EOAEs to attended stimuli, the opposite of effects reported by Giard et al. (1994). It is important to point o ut that neither Giard et al. (1994) nor Michie et al. (1996) presented an attentional control condition. Relatively lower DPOAE amplitudes observed in our study may be explained at a first level as resulting from the properties of the MOC on OHC function. The corticofugal pathways descend from the auditory cortex and thalamic regions onto the MOC and exert control on MOC activity (Mulders & Robertson , 2000; Schofield , 2010). The MOC fibers influence the active mechanical function OHCs by altering the cond uctance of the cell membrane and thereby reduce the gain of the cochlear amplifier (Robertson , 2009). This is supported by evidence of increase in CM and decrease of CAP in animals attending to a visual task (Delano et al. , 2007). The difference in DPOA E amplitude across the three attention conditions may be explained by a combination of effects mediated via the corticofugal pathways a nd the uncrossed MOC pathways. DPOAE levels are relatively lower during the contralateral ear attend condition compared to when participants attended to the visual stimuli. This effect may likely be explained by the differential activity in the corticofugal MOC pathways innervating OHCs. The difference in amplitude between the contralateral ear attending condition and t he ipsilateral ear attending condition, however, may involve an effect of uncrossed MOC in addition to the existing corticofugal pathway activity. The

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40 uncrossed MOC system receives input from the contralateral ear and synapses on t he OHC of the ipsilatera l ear. The incremental increase in MOC suppression with activation of both MOC subgroups (crossed and uncrossed) might explain the decreased DPOAE amplitude in the ipsilateral ear attend condition. As with our earlier studies we did not observe a differe nce across attending conditions in the time course of the rapid adaptation component of the DPOAE (Smith et al. , 2012; Srinivasan et al. , 2012). Given that rapid adaptation is unchanged across attending conditions, while a level difference exists, this suggests that at least two different MOC functions are measured rapid adaptation, unaffected by attention and a set point of OHC active mechanism gain, which is under the influence of attention. The present results are in agreement with previous DPOAE st udies (Smith et al. , 2012; Srinivasan et al. , 2012) and EOAE studies (Michie et al. , 1996). This finding is the opposite of previous results from a number of studies reporting different electrophysiological measures of peripheral auditory function (Oatman , 1971, 1976; Lukas , 1980; Delano et al. , 2007). It is also the opposite of the generally accepted effect observed cortically (Woldorff et al. , 1987; Johnson & Zatorre, 2005; Kauram ä ki et al. , 2007), which is a relative increase in the amplitude of attend ed signals. DPOAEs are produced by the non linear mechanical behavior of cochlear OHCs to two primary tones (Wilson, 1980; Probst et al. , 1991; Yates et al. , 1992). In the present experiment, participants are instructed to attend to the DPOAE primary tone s, but the 2f1 f2 DPOAE is recorded at a frequency some cochlear distance below the primaries and depending on the sharpness of MOC tuning, attending to the primaries might result in suppression of the DPOAE response (Dai et al. , 1991; Strickland & Viemeis ter, 1995).

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41 The next two experiments try to answer this question with a sweeping frequency DPOAE measure. Figure 2 1. Schematic of three different attending conditions. In the auditory ignoring condition, participants ignored all binaural auditory stimuli and were instructed respond to the short latency phase change in the Gabor patch (solid blue line). In the first auditory attending condition, participants were instruct ed to attend to the ear in which the DPOAEs were recorded and to report a short duration tone, while ignoring contralateral auditory stimuli and the visual Gabor patch (solid red line). In the second auditory attending condition, participants were instruct ed to report detection of a short duration tone presented to the contralateral ear, while ignored the auditory stimulus in the DPOAE recorded ear and the Gabor patch (dashed red line).

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42 Figure 2 2. Individual DPOAE contours for ten participants record ed during three attending conditions. DPOAE recorded during the visual attention condition, while ignoring all auditory stimuli in both ears (solid blue line); DPOAE recorded while attending to primary tones presented in the recorded ear,

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43 while ignoring to nes presented to the contralateral ear and visual Gabor patches (solid red line), and while attending to the primary tones presented to the contralateral ear, and ignoring the tones presented to the recorded ear and ignoring the visual Gabor patches (dashe d red line). Figure 2 3. Difference in absolute overall DPOAE onset level, from the attend visual condition in auditory attend ipsilateral ear condition and the auditory attend contralateral ear condition for each subject. Nine of 10 subjects showed rel atively lower overall DPOAEs in the attend ipsilateral condition.

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44 Figure 2 4. Average DPOAE contours for ten participants recorded during three attending conditions. DPOAE recorded during the visual attention condition, while ignoring all auditory stimuli in both ears (solid blue line); DPOAE recorded while attending to primary tones presented in the recorded ear, while ignoring tones presented to the contralateral ear and visual Gabor patches (solid red line), and while attending to the primary ton es presented to the contralateral ear, and ignoring the tones presented to the recorded ear and ignoring the visual Gabor patches (dashed red line).

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45 CHAPTER 3 EXPERIMENT 2 SELECTIVE ATTENTION AND FINE STRUCTURE DPOAE The ability to selectively at tend to one stimulus out of many is an important feature of the auditory system in many mammals including humans (Driver , 2001). While selective attention can be effective across sensory modalities and dimensions, directing attention to stimuli of differe nt frequencies in particular is considered to be a function of the descending (corticofugal) auditory pathways (Robles & Delano, 2008). Many functions have been proposed for the auditory medial efferent system, including, improving the detection of stimul i in noise, protection from loud sounds, and the mediation of selective attention (c.f. Guinan, 2006; Robles & Delano , 2008 ; Robertson, 2009 ) . A particular case of intramodal selective attention within the auditory sensory modality requires attention to tonal stimuli at one or more frequencies, while ignoring stimuli at other frequencies. Psychophysical evidence shows that detection of tones at attended frequencies is more accurate than that of tones at unattended frequencies (Greenberg & L arkin , 1968 ; Dai et al. , 1991). While the high rate of stimulus detection at attended frequencies is expected, the decreased accuracy of stimulus detection at unattended frequencies suggests an attenuation of stimuli or signal encoding those stimuli a t th e unattended frequencies. This attenuation might be mediated by decreased amplification of the stimuli by the OHCs. Descending corticofugal pathways extending from the auditory cortex modulate OHC amplific ation via the MOC efferent fibers. The idea of s elective inhibition of OHC activity by the MOC fibers was investigated by Scharf et al. (1997) who reported changes in psychophysical detection tasks in patients undergoing vestibular neurotomy. Scharf et al. observed improved detection of tonal stimuli a t unattended frequencies after patients underwent vestibular

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46 neurotomy for treatment of vertigo, which resulted in section of the olivocochlear bundle. While there were no reported changes in intensity discrimination, frequen cy discrimination , Scharf et a l. obs erved that detection accuracies of tones at unattended frequencies were higher after surgery compared to before while they remained constant at attended frequencies. Selectively attending to a stimulus results in an enhanced representation at the le vel of the auditory cortex, as shown by imaging and electrophysiological studies (Woldorff et al. , 19 87; Kauram ä ki et al. , 2007). Woldorff et al. (1987) reported increased amplitudes of auditory ERPs for attended ear stimuli at central/frontal scalp sites when participants attended to 5 kHz and 3.4 kHz tone pips in a dichotic listening task . They reported that mid late ncy waves at 20 50 ms were affected with a greater positivity to attended ear stimuli. Kauram ä ki et al. (2007) reported that attending to a tonal stimulus (1 kHz) in the presence of notched noise resulted in a greater amplitude of ERP signal 100 ms from stimulus onset, compared to when participants ignored the auditory stimulus. Studies looking at the auditory brainstem and periphery have a lso reported changes in signal representation at various stages of the auditory neuraxis. There is little agreement on the nature of the effects of selective attention at the auditory periphery. Some studies r eported no effects on ABRs (Picton et al. , 19 74; Connolly et al. , 1989), while some others ( Lukas , 1980 ; Brix, 1984 ) report changes similar to that observed in the auditory cortex (Woldorff et al. , 19 87 ) ; an increase in the ABR signal amplitude and a decrease in interpeak latency when participants at tended to the tones compared to when they ignored them .

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47 A variety of both electrophysiological and psychophysical measures have been used to describe the function of the descending auditory pathways, including ABRs and OAEs in humans. Oatman (1971, 1976) measured the clicked evoked CAP from the cochlear nucleus in behaving cats and observed changes due to selective attention with, decreased potentials during a visual discrimination task (i.e., when cats ignored the CAP evoking stimulus). In a more rec ent study, Delano et al. (2007) measured CAP and CM in chinchillas while they performed an auditory or visual discrimination task; they showed that the CAP increased during the auditory discrimination task compared with during the visual discrimi nation task. Physiological studies have explored the use of otoacoustic emissions in determining the effects of selective attention. For example, Puel et al. (1988) recorded evoked otoacoustic emissions (EOAE) in humans and showed that they were altered when the participants engaged in a selective attention task. Likewise, recent studies by Giard et al. (1994) and Michie et al. (1996) showed changes in evoked otoacoustic emissions when participants counted targets at different frequencies. A common eleme nt in many selective auditory attention experiments (Giard et al. , 1994, Michie et al. , 1996) is the use of an auditory frequency discrimination task. In these studies selective attention is manipulated to focus attention to tones at presented at differen t frequencies and the responses were recorded. Dai and colleagues (1991) looked at the ability to filter auditory stimuli and selectively attend to stimuli at one frequency and ignore other frequencies. They presented tones at a central frequency and ins tructed participants to attend to tones at that frequency and perform a two interval forced choice task. They reported that listeners were most sensitive to that

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48 particular frequency and a restricted band of frequencies surrounding that central frequency. They also reported that the shape of the attention band resembled that of the auditory filter, but that the equivalent attenuation of tones at away frequencies suggested the recruitment of multiple auditory filters centered at away freque ncies in the dec ision process. In an earlier study, Scharf and colleagues (1987) presented tones in a two interval forced choice task at expected or varying frequencies . Participants were presented with a cue signal followed by two intervals, one of which contained a to ne pip at the expected cue frequency or at a different frequency. They reported that detection accuracy decreased as tones were presented increasingly away from the expected cue frequency. These studies (Scharf et al. , 1987; Dai et al. , 1991; Scharf et a l. , 1997) suggest that in addition to modulation of amplification at attended frequencies, OHC activity at away or unattended frequencies may be less amplified re sulting in a relatively attenuated signal transduced by the IHC. The increased accuracy of tone detection at unattended frequencies after section of the OCB compared to before (Scharf et al. , 1997) lends support to the idea that psychophysical selective attention can modulate the filtering characteristics of the auditor y system. A growing body of evidence using various direct and indirect physiological measures supports the notion that selective attention can alter peripheral auditory system function. Studies have looked at changes in CAP and CM with manipulations in se lective attention. Oatman (1971, 1976) reported decreases in CAP in cats during periods of visual attention. Delano et al. (2007) measured CAP and CM in trained chinchillas when they performed a visual and auditory discrimination task. They reported dec reases in CAP and, importantly, increases in the CM in the visual attending

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49 condition when compared to the auditory attending condition. While the CAP reflect the activity of the auditory nerve fibers, the CM is believed to be a measure of most ly outer ha ir cell activity (Dallos & Cheatham , 197 6 ). The corticofugal pathways innervate multiple brainstem nuclei and extend to the level of the cochlea via the medial oliv ocochlear efferent (MOC) tracts ( Schofield , 2010 ). The functional significance of these pa thways has been explored by many studies ( Perrot et al. , 2006; Suga, 2008; L é on et al. , 2012 ) in humans and other species . Other studies have looked at more indirect measures of outer hair cell activity, namely otoacoustic emissions . Otoacoustic emissions are acoustic signals detectable via a microphone placed in the ear canal. They are generally considered to reflect OHC activity, the active process that is responsible for sound amplification seen by the IHC (for review see Kemp , 200 2). A variety of evoked otoacoustic emissions have been measured in selective attention paradigms, including evoked otoacoustic emissions (EOAE) in Giard et al. (1994), Michie et al. (1996) and Maison et al. (2001), transient evoked otoacoustic emissions (TEOAE) in Khalfa et al. (2001), distortion product otoacoustic emissions (DPOAE) in Smith et al. (2012) and Srinivasan et al. (2012). Giard et al. (1994) and Meric & Collett (1992) reported decreases in EOAEs during a visual attention task. Giard et al. (1994) presented 1 kHz tones in one ear, a nd 2 kHz tones in the other ear. Participants attended to either ear during different conditions. EOAEs were relatively larger when recorded to attended tones, compared with when subjects ignore d the tones . Meri c & Collet (1992) presented auditory clicks or visual flashes and instructed participants to attend to either stimuli or to a control condition. They reported a decrease of 0.35 dB in the EOAE during visual attention. In an attempt

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50 to replicate Giard et a l. (1994), Michie and colleagues (1996) presented participants with similar dichotic stimuli. They reported a decrease in EOAEs when participants attended to the acoustic stimuli, compared to when they ignored them. More recently Smith et al. (2012) repo rted decreases in DPOAEs when participants performed an auditory discrimination task, compared to when they ignored the same auditory stimuli and attended to visual Gabor patch stimuli or read a book. This apparently opposite effect may be an artifact of the DPOAE being measured. In these studies, including our more recent work, DPOAEs were measured when selective attention was manipulated towards visual and auditory stimuli. The DPOAE is a nonlinear mechanical response to tones presented at two primary frequencies that are in a fixed frequency ratio (~1.2). The recorded frequency, 2f1 f2 Hz, is thought to be produced at a frequency some cochlear distance away from the two primaries. Depending on the sharpness of the MOC tuning, attending to the primari es might result in a suppression of the DPOAE response. Dai et al. (1991) showed that thresholds for distant probes were higher that elicited the same detection performance as for the targets at attended frequencies, suggesting the suppression of frequenc ies aw ay from the attended signal. Attending to a certain frequency might result in a similar suppression of away frequencies, in this case, the DPOAE frequency. This suppression might explain the apparent opposite direction of DPOAE magnitude change during auditory attention in our previous experiments . In an attempt to understand how differential modulation of OHC activity by the MOC system might occur at cochlear positions cor responding to attended and unattended frequencies, a continuous sweeping DPOAE technique (Long et al. , 2008)

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51 was used to measure changes in outer hair cell activity due to selective attention . Primary tone pairs are presented using an up , down frequency sweeping paradigm with micro increments (~ 4 7 Hz) and the DPOAE response is recorded. DPOAEs were initially assumed to arise from the overlap region near the f2 frequency place when f1 and f2 frequency tones were presented ( Harris et al. , 199 2 ; Kummer e t al. , 1995). Recent studies have, however, shown that they are better explained as a vector sum of two components, one (generator/nonlinear distortion product) arising from the overlap region near the f2 place, while the other component (reflection/linea r reflection product) arises from the characteristic place for the 2f1 f2 frequency (Heitmann et al. , 1998; Talmadge et al. , 1999; Shera , 2004). These two components sum constructively or destructively, dependent on their phases (Shaffer et al. , 2003). T he non linear distortion component is generated at the maximal overlap of the two frequencies and travels apically and basally along the cochlea. The basal portion is measured as the generator component. As the energy travels from the overlap region apic ally to the characteristic DP place, it gets amplified and is reflected back to the middle ear and can, in some cases, be larger than the generator component (Talmadge et al. , 1999). The present experiment was aimed to look at changes in the amplitude of the generator component of the DPOAE arising from the attended frequency pair vs. the unattended frequency pair. Frequency quirks or anomalies were introduced at two fixed frequency positions in the up and downsweep and they occurred randomly in a certai n percentage of the sweeps. The participants were instructed to attend to one set of frequency quirks, while ignoring the other set during the auditory attention condition. They ignored the auditory stimuli and attended to a visual Gabor patch in the vis ual

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52 attention condition. The experimental design is shown in Table 3 1. One group attended to quirks at a certain frequency, while the other group of participants attended to quirks at another frequency in the auditory attention conditions . Procedure Participants Fourteen college aged students (18 22 years old, nine females) participated in this experiment. Prior to testing, a brief history was taken from each participant to document ear related complaints, such as current ear congestion or infectio n, history of ear infections, ear surgery, noise exposure, music player and headphone use and ototoxic and chronic medication use. All experiments were approved by the Institutional Review Board at the University of Florida. Instrumentation And Stimulus Pa rameters Custom software for Mac OS computers developed by C.L.Talmadge (Talmadge et al. , 1999) was used to generate the stimuli and record the ear canal signals (RecordAppX). Two ER2 (Etymotic Research, Elk Grove Village, IL) tube phones were connected t o a two port ER10B+ low noise microphone, which was inserted in the ea r canal using a disposable tip. Stimuli were presented via the ER2 tube phones after passing through a buffer (Etymotic Research HF4). The responses were conditioned, pre amplified and filtered (Stanford SR650, 300 to 10000 Hz in battery mode) prior to being digitized by a MOTU UltraLite Mk3 hybrid audio interface (24 bit, 44100 sample s/sec) under computer control. At the start and end of each session, white noise was played through ea ch tube phone in turn, recorded and the levels analyzed to ensure stable probe fit for the duration of the experiment.

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53 Auditory stimuli Tone pairs were presented using an up , down frequency sweeping paradigm (Long et al. , 2008), an f 2 /f 1 ratio of 1.22 wit h the f 2 ranging from 1000 11314 Hz (7 s sweep, 2 s per octave), and stimulus levels L 2 = 65 dB and L 1 = 39 dB SPL + 0.4 * L 2 = 65 dB (Kummer et al. , 1998). These intensities were based on the so paradigm, which was developed to ensure t hat the region of maximum overlap stays fixed when stimulus level is changed to optimize the level of the distortion source (Kummer et al. , 1998) . Sweeps were obtained for this primary level (N ~39) and averaged to increase the signal to noise ratio betwe en the measured DPOAE and background noise. Each run consisted of three sessions per attention condition, with 18 trials per session (~5 tria ls with the frequency anomaly). Each trial consisted of an upsweep and a downsweep pair. The interstimulus interv al was 3 s. The trial presentation was decoupled from the visual target presentation such that the visual task target would not be perceived concurrently with the tone sweep onset. Swept frequency anomalies were introduced in some of the trials (~18%) at two f1 frequencies, 2kHz and 4kHz. This anomaly consisted of a reversal in sweep direction for a short duration; 50Hz traversal duration at 2kHz (~140 ms) and 100Hz traversal duration at 4kHz (~140 ms). Visual stimuli All visual stimuli were presented on a flat ly in front of the participant. During each trial, after presentation of a fixation cross, a Gabor patch, oriented 45 o to the right of vertical, was presented in the center of the scr een. Each Gabor patch was composed of seven alternating black/white sinusoidal bars whose greatest contrast (99.8% Michelson) was at the center of the stimulus, with a

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54 Gaussian decline to the edges. The Gabor patch subtended a visual angle of 2.18 o with 3.21 cycles per degree. After a randomized delay of a few seconds (mean delay 10 s ), the patch was phase shifted by 180 o . Behavioral Task Participants were instructed to count the occurrence of the instructed target event by making a tally response by pencil when an event was detected. Two different attention conditions were tested. In the attend visual condition, participants ignored the auditory stimuli and were instructed to respond to the phase shift in the (visual) Gabor patch. In the attend auditory c ondition, the participants were instructed to attend to the tone sweeps and respond to the sweep frequency anomalies. One group (n = 7 participants) responded to the sweep anomaly at f1 = 2kHz, while the other group (n = 7 participants) responded to the s weep anomaly at f1 = 4kHz. The attend auditory and attend visual sessions were interleaved to remove any confounding effect of fatigue. DPOAE Analysis Spectrograms of the individual sweeps were visually inspected and sweeps contaminated by noise (eg: Body movement, coughing, loud breathing) in the frequency regions of interest were eliminated before averaging. The remaining sweeps with identical stimulus conditions (i.e., sweep direction and stimulus intensity) were averaged to reduce the noise floor and subtract ed to estimate the noise floor. Up and down frequency sweeps were analyzed independently and compared as a cross check. The remaining data analyses were restricted to the up sweep data. The up sweep and down sweep data provide comparable fine s tructure outcomes (Long et al. , 2008). A least squares fit (LSF) procedure first introduced by Long & Talmadge (1997) and modified by Long et al. (2008) was used to extract the amplitude and phase of the

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55 ear canal DPOAE for each averaged sound file using o verlapping H ann windowed segments. The result of the LSF procedure is a band pass filter that changes center frequency a s the DPOAE frequency changes. The bandwidth of the filter is determined by the size (i.e. duration) of the analysis window . Once the whole DPOAE was extracted, analysis software developed by C. L. Talmadge (Long et al. , 2008) based on the program NIPR (Withnell et al. , 2003 ; Long et al. , 2008) was used to separate the nonlinear distortion and linear reflection sources . NIPR (Withnell e t al. , 2003) is a MATLAB TM based (Mathworks, Natick, MA) analysis program that uses an Inverse Fast Fourier Transfer (IFFT) based algorithm to convert the frequency domain complex valued DPOAE amplitude to the time domain, where a time window filter is the n applied to separate the DPOAE sources based on their phase lag . The nonlinear distortion and linear reflection components arrive at the ear canal with different latencies, with the nonlinear distortion component arriving earlier than the linear reflection component (Talmadge et al. , 1999). Figure 3 1 shows a sample D POAE across frequency, with the two primary frequencies f1 and f2 in bold. The cubic DPOAE 2f1 f2 is labeled DP. Figure 3 2 shows a sample of the DPOAE fine structure with the nonlinear distortion and the linear reflection components overlaid. The noise floor is shown in gray. An average of the DPOAE nonlinear distortion component was computed for each band and compared across the visual and auditory attention conditions fo r both groups of participants. The nonlinear distortion sources corresponding to t he attended frequency bands were processed for further analysis. In accordance with the two source model of DPOAE generation (Shaffer et al. , 2003; Shera , 2004), data from certain frequency

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56 bands were extracted and compared. For the f1 = 2 kHz, f2 = 2.43 9 kHz anomaly the nonlinear distortion source band of interest was calculated as (2f1/f2 1)* f2 frequency, 1.28 kHz 1.6 kHz. Similarly, for f1 = 4 kHz, f2 = 4.878 kHz, the nonlinear distortion source band of interest was calculated as 2.56 kHz 3.2 kH z. Statistical Analysis Data from both groups of participants were combined. A comparison between attended frequency band DPOAE amplitude difference between the full distortion product ( DP ) obtained during visual and auditory attention conditions and una ttended frequency band DPOAE difference between the DP obtained during visual and auditory attention conditions was performed. The attended frequency bands for the group 1 would be 2 kHz and 4 kHz for group 2. The unattended frequency bands would be 4 kH z for group 1 and 2 kHz for group 2. The within subject difference in DPOAE generator component amplitude ( g DPOAE V A ) across the two attention conditions (visual and auditory) was computed for a wide frequency range (600 Hz 6000 Hz) for the two groups o f participants. Comparison of the gDPOAE V A difference between the attended and unattended bands was performed. data was combined with the unattended band gDPOAE V A (4 kHz in group 1, 2 kHz in group 2) in one dataset, while the attended band gDPOAE V A (2 kHz in group 1, 4 kHz in group 2) was pooled into another dataset. A paired t test was performed with these two datasets to determine if they differed significantly from each other. curacies were characterized as hits or misses and false alarms. Hits and misses corresponded to the accuracy of the participants in counting the attended band anomalies, while counting an unattended band anomaly was a false alarm. The behavioral results obtained from each participant under each

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57 attention conditions were compared statistically using a t test to determine if the . Experimental Procedure All experimen ts were conducted inside of a double walled IAC sound attenuating chamber. After initial screening, participants were seated in comfortable reclining chair within the sound booth, detailed instructions were provided, eartip recording probes were inserted in the ear and calibrations were performed. At the start and end of each session, white noise was played through each tube phone in turn, recorded and analyzed using a Fast Fourier Transfer (FFT) to evaluate probe fit for consistency and to ensure that le vels near 1000 Hz approximated the required stimulus level for each output. In the case where the levels became inconsistent, the probe position was adjusted and monitored over time to ensure stable positioning. If stable positioning , data collection was stopped and the participant session terminated. In successful experimental sessions, DPOAE responses were recorded and saved to disk, and th e responses were averaged within subjects. The data from unsuccessful experimen t al sessions were not averaged and discarded from the dataset. Auditory up and down sweep stimuli were presented in each session with ~18% of the trials consisting of the sweep anomalies. The visual stimuli changed phase with a random interval (mean interval 10 s). During the auditory attention task, the participants counted sweep anomalies at one frequency using a tally measure with a pencil and ignored the visual stimuli. They counted the visual stimulus changes in the visual attention task and ignored the sweep a nomalies. Auditory and visual attention sessions were interleaved so as to minimize any fatigue effects.

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58 Results Behavioral R esults For group 1, the percentage of mean trials responding to 2 kHz frequency quirks was 93.41 and the percentage of mean trails responding to the Gabor patch shift in the visual attention condition was 92.86. False alarm average in the auditory attend condition was 10.99% corresponding to responding to 4 kHz frequency quirks, while in the visual attend condition it was 0.6 %, corr esponding to responding to no Gabor patch switch. For group 2 , the percentage of mean trials responding to 4 kHz frequency quirks was 94.96 and the percentage of mean trails responding to the Gabor patch shift in the visual attention conditi on wa s 89.29. False alarm average in the auditory attend condition was 13% corresponding to responding to 2 kHz frequency quirks, while in the visual attend condition it was 1.19% , corresponding to responding to no Gabor patch switch . A paired t te st comparing the hit percentages showed no significant difference between the two attention conditions (p = 0.305). The false alarm percentages across the attention conditions showed a significant difference using a paired t test (p = 0.0005) DPOAE Results Figure 3 2 shows on fine structure DPOAE amplitudes and its nonlinear distortion and the linear reflection components with a fre quency range from 1000 6000 Hz. The bold red line represents the whole DPOAE and its quasi periodic fine structure. The nonlinear d istortion component (generator DPOAE source or gDPOAE ) is represented by the dotted line and the linear reflection product (reflection DPOAE source) represented by the thin red line. The noise floor is shown in gray. The nonlinear distortion product and the linear reflection product interact constructively and

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59 destructively to produce the whole DPOAE. signal to noise ratio was >= 6 dB. Figure 3 3 shows an example comparison of the gDPOAE between the auditory a ttention and visual attention conditions. The gDPOAE frequencies were computed for the presumed attention bands 2 2.5 kHz and 4 5 kHz. They are displayed as green bands. The gDPOAE difference in amplitude was computed for these frequency bands (show n in figure 3 4) and averaged. amplitude difference values are shown in figure 3 5. For group 1, the gDPOAE amplitude difference between visual and auditory attention conditions is shown in translucent red stars for individual participants. For group 2, the gDPOAE amplitude difference is shown in translucent blue circles for individual participants. The abscissa displays categorical labels for green bands corresponding to the frequency quirks at f1 = 2 kHz and f1 = 4 kHz. The gDPOAE difference is plotted on the ordinate, with each values joined by translucent lines. The group average gDPOAE amplitude differences are plotted as solid red stars for group 1 and soli d blue circles for group 2. On average, the visual gDPOAE was greater than the auditory gDPOAE amplitude in the attended band for both groups. In contrast, the average visual gDPOAE was lower than the auditory gDPOAE amplitude in the unattended band for bo th groups. In group 1, participants attended to 2 kHz frequency anomalies during the auditory attention condition . On average the attended band gDPOAE in the visual attention condition was greater than the attended band gDPOAE in the auditory attention c ondition by 0.15 dB. The unattended band gDPOAE in the visual attention condition

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60 was relatively lower than the unattend ed band gDPOAE in the auditory attention condition by 0.31 dB. In group 2, participants attended to 4 kHz frequency anomalies during t he auditory attention condition. On average the attended band gDPOAE in the visual attention condition was greater than the attended band gDPOAE in the auditory attention condition by 0. 21 dB. The unattended band gDPOAE in the visual attention condition was relatively lower than the unattend ed band gDPOAE in the auditory attention condition by 0. 51 dB. A paired t test looking at the gDPOAE difference in the attended and unattended bands showed a statistically significant difference (p = 0.006) Table 3 1 unattended band. The nonlinear distortion product DPOAE was compared during the different attention conditions across the attended and the unattended band. Discussion Overall, t he present results show that the fine structure DPOAE amplitude recorded over the attention conditions show significant changes in levels in the nonlinear distortion component when subjects shifted the focus of attention from the auditory stimuli to visual stimuli. One group of participants attended to the frequency anomaly f1 = 2kHz and f2 = 2.439kHz, while another group of participants attended to the frequency anomaly f1 = 4kHz and f2 = 4.878kHz. Both groups ignored the frequency anomalies in the other frequency band. In the visual attention condition, both groups ignored the auditory stimuli and counted the Gabor patch phase shifts. Comparisons of the nonlinear distortion DPOAE component amplitudes were made across the attended frequency anomaly band between auditory and visual attention and across the unattended frequency band between auditory and visual attention conditions. Overall, in the attended band in both groups of participants, the nonlinear distortion

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61 DPOAE component amplitude was lower on average during auditory attention, compared to during visual attention. In the unattended band, in both groups, the nonlinear distortion DPOAE component amplitude was higher on average during auditory attention, compared to during visual attention. In b oth groups, the relative decrease of DPOAE amplitude in attended band during auditory attention condition occurred irrespective of the attended frequency anomaly (i.e., f1 =2 kHz, f2 = 2.439 kHz DPOAE amplitude was relatively lower in group 1 during audito ry attention compared to visual attention, and f1 = 4 kHz, f2 = 4.878 kHz DPOAE amplitude was relatively lower in group 2 during auditory attention compared to visual attention). The unattended band DPOAE amplitude comparison showed the opposite effect, t hat the relative decrease of DPOAE amplitude during visual attention condition occurred irrespective of the unattended frequency anomaly (i.e. in group 1 the unattended band DPOAE amplitude was relatively lower during visual attention condition at f1 = 2 k Hz, f2 = 2.439 kHz, and in group 2 it was relatively lower during visual attention condition at f1 = 4 kHz, f2 = 4.878 kHz). The current findings suggest differential modulation by efferent control of outer hair cell amplification of attended and unattend ed stimuli. In this experiment, DPOAEs were recorded with a sweeping tone pair paradigm with f2 ranging from 1000 Hz to 11314 Hz. The effect of selective attention on the OHC activity at frequencies corresponding to the task was studied (2 2.5 kHz, 4 5 kH z). While the frequency an omalies are presented at f1 = 2 2.05 kHz and f2= 2.439 2.5 kHz (likewise at f1 =4 4.1 kHz and f2 = 4.878 5 kHz), attending to or away from them would result in changes in OHC activity at those frequencies. This implies that all o toacoustic emissions arising from OHC activity in the frequency band 2 2.5 kHz (4 5 kHz) would be

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62 modulated by changes in attention. The nonlinear distortion source is generally considered to be the more dominant of the two sources of DPOAE generation, as seen in Figure 3 2, and is localized to the overlap region near the f2 frequency in a DPOAE eliciting tone pair (Talmadge et al. , 1999). Hence the DPOAE bands whose nonlinear distortion sources were localized to the attention task bands (DPOAE frequencie s whose f2 frequencies lay within the attention task bands) were analyzed to determine if there was any change due to selective attention. The nonlinear distortion sources for the f1 and f2 pair are generated at the maximal overlap region near f2 frequency (Talmadge et al. , 1998; Talmadge et al. , 1999; Shera & Guinan , 1999; Kalluri & Shera , 2001). It is known that the medial efferent fibers synapse tonotopically at the base of the outer hair cells (Liberman & Brown , 1986; Brown , 1989). One function of the descending corticofugal pathways is mediation of selective attention by modulating the activity of the OHC via the MOC fibers (Robertson , 2009). Earlier auditory selective attention studies (Michie et al. , 1996, Maison et al. , 2001) measured changes in O AEs while participants attended to tone pips at different frequencies. Maison and colleagues (2001) presented 1 kHz and 2 kHz tone pips with contralateral acoustic stimulation. They manipulated selective attention by asking participants to count embedded tone pips at a given frequency in contralateral noise. They reported frequency specific changes in the EOAE amplitudes as a result of changes in attention. When participants attended to the embedded tones at a certain frequency, there was a decrease in the EOAE amplitudes at that frequency compared to the no attention condition. More recent studies (Smith et al. , 2012; Srinivasan et al. , 2012) reported decreased DPOAE amplitudes while participants

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63 attended to auditory stimuli compared to when they ignored auditory tonal stimuli and attended to a visual stimulus. In addition electrophysiological studies (Delano et al. , 2007; Le ó n et al. , 2012) have reported changes in CAP and CM as a result of desce nding corticofugal modulation of MOC activity. While Delano et al. (2007) reported a decrease in CAP and increase in CM when chinchillas engaged in a visual attention tas k, León et al. ( 2012 ) reported changes in CAP and CM with non task related modulation via the corticofugal pathways. León et al. (2012 ) applied cryocooling techniques and lidocaine treatment to a small region of the auditory cortex in rats and reported changes in the CAP and CM. While the direction of these changes was inconclusive, the study lends support to the notion that the task related and unrelated modulation of the auditory cortex can mediate changes via the descending corticofugal and MOC efferent pathways on OHC activity. Compared to the previous studies from our lab measuring t he effects of selective attention on DPOAEs, this study reports changes in DPOAE in a wide frequency range (600 6000 Hz) with selective attention manipulations. Using the fine structure technique allowed us to observe changes in DPOAE amplitude in atten ded and unattended frequency bands. The current study utilized frequency targets at 2 kHz and 4 kHz. A better design would include finer grained selective attention manipulation with targets closer in frequency.

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64 Figure 3 1 . Swept frequency paradigm f or DPOAE measurement. Tone pairs at geometrically increasing F1 and F2 frequencies are presented to the ear and the response recorded. Shown here is the cubic DPOAE at 2f1 f2 frequency marked c DP.

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65 Figure 3 2 . Graph showing a typical DPOAE (bold red line) and its components, the nonlinear distortion source (dotted red line) and the linear reflection source (thin red line). The nonlinear distortion source (generator component) is produced at the maximal cochlear overlap region of the tone pairs, w hile the linear reflection source (reflection component) is produced at the 2f1 f2 cochlear place. Both components superimpose to form the DPOAE measured at 2f1 f2 frequency. The noise floor is shown in light gray. The narrower vertical gray bars show freq uency anomalies at frequencies f1 (f2), 2 kHz (2.439 kHz) and 4 kHz (4.878 kHz). The discrete f1, f2 and DP frequencies are shown by vertical lines. The wider gray vertical bars show the nonlinear distortion source frequencies presumed to be manipulated by attention.

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66 Figure 3 3 . Graph showing a typical generator (nonlinear distortion) component DPOAE during different attention conditions . Visual attention condition gDPOAE shown in blue, auditory attention condition gDPOAE shown in red. The two wide green bands show the frequencies under comparison; orange box showing 1280 1600 Hz for the frequency targets at f1 = 2 kHz, f2 = 2.439 kHz; blue box showing 2560 3200 Hz for the frequency targets at f1 = 4 kHz, f2 = 4.878 kHz. gDPOAE were average d across the frequencies and compared across visual and auditory attention. The narrower vertical gray bars show frequency anomalies at frequencies f1 (f2), 2 kHz (2. 439 kHz) and 4 kHz (4.878 kHz). In this example, the participant attended to f1 = 2 kHz (f 2 = 2.439 kHz) targets and ignored the visual Gabor patch during auditory attention condition. In the visual attention condition the participants ignored the auditory stimuli and attended to the Gabor patch visual stimulus.

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67 A B Figure 3 4 . gDPOAE in visual attention (blue) and auditory attention (red) conditions . These inset plots are from Figure 3 3. The gDPOAE value difference between the two attention conditions was computed and averaged over the frequencies in the green bands. A) Orange box showing the gDPOAE band in green corresponding to f1 = 2 kHz, f2 = 2.439 kHz. B) Blue box showing the gDPOAE band in green corresponding to f1 = 4 kHz, f2 = 4.878 kHz. DP DP

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68 Figure 3 5 . gDPOAE V A difference between visual attention and auditory attention conditions . 4 kHz band joined by thin line. Average gDPOAEV A for group 1 with 2 kHz attention shown in red, group 2 with 4 kHz att ention shown in blue.

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69 Table 3 1. Experimental Design for visual and auditory attention 2 kHz frequency anomaly 4 kHz frequency anomaly Gabor patch task Group 1 N = 7 Auditory Attention Attend Ignore Ignore Visual Attention Ignore Ignore Attend Group 2 N = 7 Auditory Attention Ignore Attend Ignore Visual Attention Ignore Ignore Attend

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70 CHAPTER 4 EXPERIMENT 3 SELECTIVE ATTENTION AND FINE STRUCTURE DPOAE IN THE PRESENCE OF CONTRALATERAL NOISE Crossed and uncrossed medial olivocochlear (MOC) efferent neurons extend from the superior olivary complex to the outer hair cells in the mammalian cochlea. These medial olivocochlear efferent neurons primarily receive inputs from the cochlear nucleus and form a part of the reflex arc known as the medial olivocochlear reflex (Guinan et al. , 2003; Guinan, 2006). The MOC neurons synapsing on the OHCs can be subdivided into two groups the ipsilateral reflex, whil e the uncrossed MOC neurons mediate the contralateral reflex. The medial efferent neurons synapse onto the base of the OHC and, when activated, lower the basolateral wall resistance of the OHC, which decreases the drive of the OHC electromechanical motor and lo wers the gain provided by the OHC (Robertson, 2009). The MOC reflex pathways are activated by both acoustical and electrical stimulation, with the acoustical stimulation being either ipsilateral, contralateral or both. While, there are no direct estimate s in humans, studies on non human primates indicate that the ipsilateral and the contralateral MOC reflexes are about equal in strength and direct acoustical stimulation of either the crossed or uncrossed MOC subgroup results in MOC mediated inhibition of OHC activity. While the exact function of the MOC reflex is not fully understood, it is implicated in a variety of phenomena, such as, adaptation of cochlear responses to continuous signals, protection of the inner ear from exposure to loud noise, and im proving the detection of signals in noise (Guinan, 1996; Guinan, 2006; Robertson, 2009). In addition to inputs from the cochlear nucleus, the MOC neurons receive direct descending corticofugal connections from the auditory cortex (Schofield, 2010). This

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71 c orticofugal efferent pathway runs parallel to the ascending pathways and is implicated in a variety of functions involving the MOC reflex arc, such as, frequency selectivity, improvement of signal to noise ratio, mediation of selective attention (Khalfa et al. , 2001; Perrot et al. , 2006; Smith et al. , 2010; Srinivasan et al. , 2012). In addition to forming feedback loops with the different brainstem regions, the descending fibers directly innervate the MOC efferent neurons and the cochlear nuclei (Schofield , 2010). These corticofugal MOC pathways are involved in direct functional control of the peripheral auditory system (Khalfa et al. , 2001; Xiao & Suga, 2002; Perrot et al. , 2006). Perrot and colleagues showed in patients undergoing invasive treatment of epilepsy via direct stimulation of the epileptogenic areas using depth brain electrodes, that stimulation of the contralateral auditory cortex elicited a reduction in the evoked otoacoustic emission (EOAE) amplitudes. In addition, the corticofugal pathway may inhibit OHC activity by an existing firing baseline rate or tone (Xiao & Suga , 2002; Le ó n et al. , 2012). Likewise, Khalfa et al. (2001) reported decreases in MOC mediated inhibition on OHC activity in terms of an equivalent attenuation of OAE measure when the primary and secondary auditory cortices were surgically removed in patients in order to minimize epileptic seizures. Both of these studies indicate that the corticofugal pathways are capable of exerting a significant inhibitory influence over co chlear activity, in addition to local MOC reflex circuits. Otoacoustic emissions, including discrete tone evoked EOAEs and discrete DPOAEs have been used to explore the effects of selective attention on peripheral auditory function (Meric & Collet, 1993; Meric & Collet, 1994; Giard et al. , 1994; Ferber Mart et al. , 1995; Michie et al. , 1996; Maison et al. , 2001; Smith et al. , 2012). Some of

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72 these studies (Meric & Collet 1994; Giard et al. , 1994; Ferber Mart et al. , 1995) reported reductions in OAE amplitude when participants attended to a visual task and ignored the auditory stimuli. Other studies, however reported no effect on OAE amplitude while participants engaged in visual attention (Michie et al. , 1996) and have, in one experiment, reported a n increase in OAE amplitude while attending to visual stimulus (Michie et al. , 1996). Recent studies have reported similar relative increases in DPOAE amplitude when participants ignored the auditory stimulus and engaged in a visual attention task (Smith et al. , 2012; Srinivasan et al. , 2012) compared to when they attended to the auditory stimulus. This lack of agreement on the direction of effects of selective attention on the OAE amplitude is in contrast to results reported in studies looking at effects of selective attention on ERP amplitude at the level of the auditory cortex. Several studies have reported increases in ERP signal amplitude (Picton et al. , 1974; Hillyard et al. et al. , 1993; Kauram ä ki et al. , 2007; Sa upe et al. , 2009) when participants engaged in an auditory attention task compared to when they ignored the auditory stimulus. This discrepancy in the direction of attention effects on OAE amplitude may be explained by a sharpened tuning of the attention filter. Srinivasan et al. (2012) suggested that the DPOAE frequency exists at some cochlear distance away from the primary frequencies and that attending to the primary frequencies might suppress the response at away frequencies similar to the effects rep orted by Dai et al. (1991), Strickland & Viemeister (1995). The tuning of an attention based, MOC mediated influ e nce over cochlear function may be addressed by measuring attention induced changes in DPOAE fine structure (fs DPOAE). DPOAEs are believed to, in part, originate from the nonlinear

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73 transduction properties of OHCs (Shera, 2004) and can be measured in the ear canal with a sensitive microphone. They are believed to be composed of two sources, a nonlinear distortion source originating at the cochle ar position of maximum overlap for the tonal stimulus pair (f1 and f2 Hz), near the f2 frequency cochlear place, and there being a linear reflection source originating from the characteristic cochlear place for the cubic DPOAE frequency 2f1 f2 Hz (Withnell et al. , 2003). When the two sources combine, their respective phases determine whether they interfere constructively or destructively. When the primary DPOAE eliciting tones are swept in frequency, in either ascending or descending series, in fine frequ ency steps along the basilar membrane, the DPOAE amplitude and phase display a quasi periodicity that is referred to as the fine structure (Mauermann et al. , 1999; Talmadge et al. , 1999; Reuter & Hammershøi, 2006). While many studies have characterized th e effects of contralateral noise on otoacoustic emissions (Glattke & Kujawa, 1991; Berlin et al. , 1993; Liberman et al. , 1996; Bassim et al. , 2003; Zhang et al. , 2007; Abdala et al. , 2009), which would be mediated by the uncrossed MOC neural pathway, there are relatively few studies which have explored the effects of selective attention in a task that requires stimulus detection or discrimination in the presence of ipsilateral or con tralateral noise (Michie et al. , 1996; Maison et al. , 2001; de Boer & Thornton, 2007). Many reviews (Robertson, 2009; Ciuman, 2010; Guinan, 2010) suggest that the MOC tracts play an important role in decreasing the cochlear response to background noise, t hereby increasing the salience of a transient target signal. This idea is supported by evidence of firing rate changes at the individual auditory fiber level; Winslow and Sachs (1987) showed that while the

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74 response of the auditory nerve fiber (ANF) to tra nsient signals in quiet situations is attenuated by MOC activation, the transient signal response in a noisy background is amplified if MOC activation is present compared to when it is not. phenomenon is explained by the fact that the bac kground noise increases ANF firing rate above baseline , a nd this reduces the dynamic range of firing rate available for responding to transient signals. The enhanced baseline firing also depletes the supply of the IHC neurotransmitter, thereby increasing the adaptation of the ANF to transient high level tone stimuli. When the MOC system is activated, the ANF firing rate in response to background noise is decreased and the transient signal is released from masking. This phenomenon leads a higher salient s ignal to noise ratio and is thought to aid detection of target stimuli in noisy situations (Guinan, 2006; Robertson, 2009). While many studies have reported the effects of varying levels and bandwidths of contralateral noise on OAEs including SOAE (Zhao & Dhar, 2011), SFOAE (Guinan et al. , 2003; Lilaonitkul & Guinan, 2009) and DPOAE (Moulin et al. , 1993; Williams & Brown, 1997; Bassim et al. , 2003; Purcell et al. , 2008; Deeter et al. , 2009; Henin et al. , 2011), only a few studies have explored the effects of attention on OAEs in the presence of contralateral broadband noise. While Winslow & Sachs (1987) activated the MOC and increased the salience of the transient signal with electrical stimulation, few studies have looked at whether attention can activate these same mechanisms and et al. (2001) explored the effects of selective attention on EOAEs at two frequencies (1 and 2 kHz) with contralateral targets at the two frequencies embedded in co ntralateral noise. Participants engaged in a no task condition where they were presented with

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75 contralateral noise, and contralateral noise with different frequency probes (1, 2 or 4 kHz). In the probe counting condition, they were presented with the same stimuli as in the no task condition and asked to count the probes at different frequencies. In both conditions, tonal stimuli were presented ipsilaterally at 1, 2 or 4 kHz and EOAEs were recorded. When participants counted probes presented in the contral ateral ear at a particular frequency (1 or 2 kHz), the authors reported a relative decrease in ipsilateral ear EOAE amplitudes only at that frequency, compared to when the participants engaged in the no task condition. Based on the increased suppression a t attended frequencies, they concluded during frequency focused selective attention a central process might facilitate OCB activity at the attended frequency in addition to an inhibition of the MOC activity at unattended frequencies. In another selective attention study, de Boer & Thornton (2007) compared the amplitude of click evoked OAEs (CEOAE) when participants were presented with active visual, passive visual, active auditory and a no attention task conditions, in the absence and presence of contralat eral noise at two levels (50 and 60 dB SPL). While there was no difference in the contralateral suppression amplitudes across the four attending conditions, they observed a differential effect across the four conditions on the change in growth of the CEOA E response amplitude as a function of the contralateral noise (defined as the suppression of I/O slope). They reported that there was a significant difference in the I/O slope suppression between the active auditory condition, the passive visual and the n o task conditions. An earlier study by Michie et al. (1996) employed a similar experimental paradigm, instructing participants to attend to tones in the ipsilateral ear at either 1 or 2 kHz, while ignoring the other frequency tone, in the presence of

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76 cont ralateral broadband noise. Michie and colleagues, however, reported no effect of selective attention on EOAE amplitude at the two frequencies. The three studies described above explored the effects of selective attention on tonal stimuli in the presence o f contralateral broadband noise. While the experimental measures by Michie et al. (1996) and Maison et al. (2001) are similar to each other (tone evoked EOAE), a crucial difference in the task design was that Michie and colleagues instructed participant s to attend to tones presented in the ipsilateral ear, while Maison et al. instructed participants to attend to tones embedded in the contralateral noise stimuli. de Boer and Thornton (2007) utilized an auditory attention condition similar to Michie et al . (1996), however, they presented tonal target stimuli while recording CEOAE in response to click stimuli. These methodological and task differences may have contributed to a lack of a clear direction of selective attention effects on the OHC activity via the MOC system. In these studies, the response of the OHC was measured either at discrete frequencies (tone evoked EOAE), or in a lumped manner across the cochlea (CEOAE). A continuous frequency measure of OHC activity by the use of fine structure DPOAE might address the question of how MOC mediated OHC amplification of stimuli varies at attended frequencies vs. unattended frequencies in the presence of contralateral broadband noise. Additionally, t he use of a standard visual attention condition with a phase shifting Gabor patch stimulus, such as one employed in the experiments described in Chapters 2 and 3, might provide an appropriate baseline DPOAE measure. In this experiment a fine structure DPOAE measurement paradigm was employed with broadband no ise presented contralaterally, while participants engaged in

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77 an intermodal attention task similar to the experiment described in Chapter 3. The aim of this experiment was to charact erize the effects of attention on DPOAE amplitude, measured w ith no noise and with contralateral noise DPOAE amplitude was compared across different intermodal attending conditions; when participants performed in an auditory target frequency detection task in the ipsilateral ear, a visual Gabor patch duration discrimination task . The experimental design consisted of three attention conditions. In the first attention condition, the participants attended to the auditory task in the absence of contralateral noise, while ignoring the visual stimuli. In the second attention conditi on, they attended to the auditory task in the presence of continuous contralateral broadband noise, while ignoring the visual stimuli. In the third condition, they attended to the visual task, while ignoring both the auditory stimuli and the contralateral broadband noise. Comparing the fs DPOAE amplitudes and phase in the auditory attend conditions with and without contralateral noise allowed us to verify and quantify the effects of attention on the suppressive effects of contralateral BBN on the fs DPOAE . Comparison of the fs DPOAE measures in the visual and auditory attend conditions with contralateral noise would indicate any effects of selective attention . A fs DPOAE measure would allow us to observe any changes in OHC activity as a result of selecti ve attention over a broad region of the cochlea during the same task. Additionally, the findings may shed any light on the nature of the effects of contralateral BBN on OHC activity. Procedure Participants Nineteen college aged students participated in this experiment. Prior to testing, a brief history was taken from each participant to document ear related complaints, such

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78 as current ear congestion or infection, history of ear infections, ear surgery, noise exposur e, music player and headphone use and ototoxic and chronic medication use. All experiments were approved by the Institutional Review Board at the University of Florida. Instrumentation And Stimulus Parameters Custom software for Mac OS computers developed by C.L.Talmadge (Talmadge et al. , 1999) was used to generate the stimuli and record the ear canal signals (RecordAppX). Two ER2 (Etymotic Research, Elk Grove Village, IL) ear phones were connected to a two port ER10B+ low noise microphone, which was inser ted in the ear canal using an expanding, disposable eartip. Stimuli were presented to the ear via the ER2 ear phones after passing through a headphone buffer (Etymotic Research HF4). The recorded ear responses were conditioned, pre amplified and filtered (Stanford SR650, 300 to 10000 Hz in battery mode) prior to being digitized by a MOTU UltraLite Mk3 hybrid audio interface (24 bit, 44100 samples/sec) under computer control. At the start and end of each session, white noise was played through each ear pho ne in turn, recorded and the levels analyzed to ensure stable probe fit for the duration of the experiment. Auditory stimuli Tone pairs were presented using an up , down frequency sweeping paradigm (Long et al., 2008), an f 2 /f 1 ratio of 1.22 with the f 2 ranging from 1000 11314 Hz (7 s sweep, 2 s per octave), and stimulus levels L 2 = 65 dB and L 1 = 39 dB SPL + 0.4 * L 2 = 65 dB (Kummer et al. , 1998). These intensities were based on the so paradigm, which was developed to ensure that the region of maximum tonotopic overlap remains fixed when stimulus level is changed to optimize the level of the distortion

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79 source (Kummer et al. , 1998). Sweeps (N = 39) were obtained for this primary level and averaged to increase the signal to noise ratio between the measured DPOAE and background noise. Each run consisted of three sessions per attention condition, with 18 trials per session. Each trial consisted of a frequency upsweep and a downsweep pair. The interstimulus interval was 3 s. The audito ry trial presentation was decoupled from the visual target presentation, such that the visual task target would not be perceived concurrently with the tone sweep onset. Swept frequency anomalies were introduced in some of the sweeps (~18%) at two f1 freque ncies, 2kHz and 4kHz, corresponding to ~5 sweeps per session. The anomaly consisted of a reversal in frequency sweep direction for a short duration; a 50Hz traversal duration at 2kHz for the f1 frequency, from 2000 2050 Hz in the downsweep, or from 2050 2000 Hz in the upsweep (~140 ms) and 100Hz traversal duration at 4kHz for f1 frequency from 4000 4100 Hz in the downsweep, or from 4100 4000 Hz in the upsweep (~140 ms). The corresponding f2 frequency ranges were 1.22*f1 = 2439 2500 Hz and 4878 5000 Hz. Contralateral broadband noise was presented using a digital audio player (Sansa ® , 8 GB music player). Broadband noise was generated as a .wav file using a Gaussian Noise function in MATLAB TM . The frequency range was from 10 Hz 10000 Hz. The a udio player volume was calibrated to provide broadband noise at 60 dB SPL. Visual stimuli All visual stimuli were presented on a flat front of the participant. During each trial, after presentation of a fixation cro ss, a Gabor patch, oriented 45 o to the right of vertical, was presented in the center of the screen. Each Gabor patch was composed of seven alternating black/white sinusoidal bars

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80 whose greatest contrast (99.8% Michelson) was at the center of the stimulus , with a Gaussian decline to the edges. The Gabor patch subtended a visual angle of 2.18 o with 3.21 cycles per degree. After a randomized delay of a fe w seconds (mean delay 10 s ), the patch was phase shifted by 180 o . Behavioral Task Participants were instructed to count the occurrence of the instructed target event by making a tally response by pencil when an event was detected. Three different attention conditions were tested. In the attend auditory condition, participants were pre sented with the tone stimuli in the ipsilateral ear and the Gabor patch visual stimuli and were instructed to attend to the tone sweeps and respond to the sweep frequency anomalies. In the attend auditory with noise condition, participants were presente d with the same tone stimuli in the ipsilateral ear and broadband noise in the contralateral ear, along with Gabor patch visual stimuli. The participants were instructed to attend to the tone sweeps and respond to the sweep frequency anomalies, and to ign ore the contralateral noise and the visual stimuli. In the attend visual with noise condition, participants were presented with the same tone stimuli in the ipsilateral ear and broadband noise in the contralateral ear, along with Gabor patch visual stim uli. In this visual attend condition, the participants were instructed to respond to the phase shift in the Gabor patch and ignore the auditory stimuli in both ears; one group (n = 9) of participants was instructed to respond to sweep frequency anomalies at f1 = 2 kHz, f2 = 2.439 kHz during both auditory attention conditions while a second group (n = 10) of participants attended to and detected sweep frequency anomalies at f1 = 4 kHz, f2 = 4.878 kHz during both auditory attention conditions. In all attent ion conditions, participants were instructed to record their responses on paper using a pencil. Two

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81 sessions were run for each attention condition, with the three attention condition sessions were interleaved to remove any confounding effects of fatigue. DPOAE Analysis analysis. Spectrograms of the individual sweeps were visually inspected and sweeps contaminated by noise (eg: Body movement, coughing, loud breathing) in the frequenc y regions of interest were removed before averaging and the remaining sweeps with identical stimulus conditions (i.e., sweep direction and stimulus intensity) were averaged to reduce the noise floor and subtracted to estimate the noise floor. Up and down frequency sweeps were analyzed independently and compared as a cross check. The remaining data analyses were restricted to the up sweep data. The up sweep and down sweep data provide comparable fine structure outcomes (Long et al. , 2008). A least square s fit (LSF) procedure, first introduced by Long & Talmadge (1997), and modified by Long et al. (2008), was used to extract the amplitude and phase of the ear canal DPOAE for each averaged sound file using overlapping Hann windowed segments. The result of the LSF procedure is a band pass filter that changes center frequency as the DPOAE frequency changes. The bandwidth of the filter is determined by the size (i.e. duration) of the analysis window. Once the whole DPOAE was extracted, analysis software deve loped by C. L. Talmadge (Long et al. , 2008) based on the program NIPR (Withnell et al. , 2003; Long et al. , 2008) was used to separate the nonlinear distortion and linear reflection sources. NIPR (Withnell et al. , 2003), a MATLAB TM based (Mathworks, Natick, MA) analysis program, which uses an Inverse Fast Fourier Transfer (IFFT) based algorithm to convert the frequency domain complex valued DPOAE amplitude to the time domain, where a time window filter was then

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82 applied to separate the DPOAE sources based on their phase lag. The nonlinear distortion and linear reflection components arrive at the ear canal with different latencies, with the nonlinear distortion component arriving earlier than the linear reflection component (Talmadge et al. , 1999). Figure 4 1 shows a sample DPOAE across frequency, with the two primary frequencies f1 and f2 in bold. The cubic DPOAE 2f1 f2 is labeled DP. Figure 4 2 shows a sample of the DPOAE fine structure with the nonlinear distortion and the linear reflection components overlaid. The noise floor is shown in gray. An average of the DPOAE nonlinear distortion compon ent was computed for each band and compared across the visual and auditory attention conditions for both groups of participants. The nonlinear distortion sources corresponding to the attended frequency bands were processed for further analysis. In accorda nce with the two source model of DPOAE generation (Shaffer et al. , 2003; Shera , 2004), data from certain frequency bands were extracted and compared. For the f1 = 2 kHz, f2 = 2.439 kHz anomaly the nonlinear distortion source band of interest was calculate d as (2f1/f2 1)* f2 frequency, 1.28 kHz 1.6 kHz. Similarly, for f1 = 4 kHz, f2 = 4.878 kHz, the nonlinear distortion source band of interest was calculated as 2.56 kHz 3.2 kHz. DPOAE peak analysis was performed on the whole DPOAE within 600 6000 Hz . This range was chosen such that the signal to noise ratio was greater than 6 dB and a similar signal in the upsweep and the downsweep. Peak frequencies were calculated using custom MATLAB TM code and compared across the three attention conditions.

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83 Stat istical Analysis Data from both groups of participants were combined. A comparison between attended frequency band DPOAE difference between the DP obtained during visual and auditory attention conditions and unattended frequency band DPOAE difference betw een the DP obtained during visual and auditory attention conditions was performed. The attended frequency bands for the group 1 would be 2 kHz and 4 kHz for group 2. The unattended frequency bands would be 4 kHz for group 1 and 2 kHz for group 2. For s tatistical analysis, the Visual attend DPOAE Auditory attend DPOAE were compared between the nonlinear distortion frequency bands for the unattended and An analysis of variance wa s performed on the peak frequency ratios in the two groups of participants across the attention conditions. The auditory attend and the auditory attend with noise conditions were compared to obtain a ratio of maxima frequency shift. An average maxima fre quency shift was computed for each participant and the two groups were compared using a pooled analysis of variance. alarms. Hits and misses corresponded to the accuracy of the participants in counting the attended band anomalies, while counting an unattended band anomaly was considered a false alarm. The behavioral results obtained from each participant under each attention conditions were compared statistically using a paired t test. Experimental Procedure All experiments were conducted inside of a double walled IAC sound attenuating chamber. After initial screening, participants were seated in a comfortable reclining chair within the sound booth, detailed instructions were provided, eartip recording

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84 probes were inserted and calibrations were performed. At the start and end of each session, white noise was played through each tube phone in turn, recorded and analyzed using a Fast Fourier Transfer (FFT) to evaluate probe fit f or consistency and to ensure that levels near 1000 Hz approximated the required stimul us level for each output. In case s where the response levels varied significantly , the probe position was adjusted and monitored over time to ensure stable positioning. If stable positioning ended . In successful experimental sessions, DPOAE responses were recorded and saved to disk, and the responses were averaged across subjects. Auditory up and down swe pt stimuli were presented in each session with ~18% of the trials consisting of the sweep anomalies. The visual stimuli changed phase with a random interval (mean interval 10 s). During the auditory attention task, the participants were inst ructed to count sweep anomalies at one frequency using a tally measure with a pencil and ignored the visual stimuli. In the auditory attention task with noise condition, participants performed the same task as the auditory attention condition, in the pres ence of contralateral broadband noise. They counted the visual stimulus changes in the visual attention with noise task and ignored the sweep anomalies. The three sessions were interleaved so as to minimize any confounds due to fatigue effects. Results Behavioral Results The behavioral results showing the hit percentage of the responses and the false alarms percentages for all target stimuli are shown in Table 4 2. Hit percentages were comparable in the three attention conditions. False alarms, however , varied significantly across the visual and auditory attention conditions. An analysis of variance

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85 showed no significant variation across the three attention conditions in the hit percentages, F(2,36) = 1.09, p = 0.3438. An analysis of variance of the f alse alarms percentages showed an main effect of attention condition with a F(2,53) = 10.87, p = 0.0001. DPOAE Results An example of a fs DPOAE for one participant is shown in Figure 4 2; DP amplitude, its nonlinear distortion and linear reflection components are plotted for a frequency range from 1000 6000 Hz. As with the fine structure measurement technique described in Chapter 3, t he bold red line represents the composite DPOAE and its quasi periodic fine structure. The non linear distortion component (gDPOAE) is represented by the dotted line and the linear reflection product (rDPOAE) is represented by the thin red line. The nois e floor is shown in gray. The non linear distortion product and the linear reflection product interact constructively and destructively depending on their individual phases, to produce the whole fs DPOAE. included in the dataset o nly if the signal to noise ratio was greater than 6 dB. Individual and group average data gDPOAE amplitude difference values are shown in figure 4 3. For group 1, the gDPOAE amplitude difference between visual and auditory attention conditions is shown in translucent red stars for individual participants. For group 2, the gDPOAE amplitude difference is shown in translucent blue circles for individual participants. The abscissa displays categorical labels for green bands corresponding to the frequency qui rks at f1 = 2 kHz and f1 = 4 kHz. The gDPOAE values joined by translucent lines. The group average gDPOAE amplitude differences are plotted as solid red stars for gr oup 1 and solid blue circles for group 2. The results

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86 indicate that no statistical difference was evident in gDPOAE amplitude in the attended and unattended bands between the two groups of participants during the auditory and visual attention conditions. condition. An example of this ph enomenon is shown in Figure 4 4. This figure also shows an example of shift in peak frequencies in the attention conditions where contralateral noise was presente d. In the current dataset, in both groups of participants, peak frequencies were determined from the fine structure DPOAE curve for each These A ratio of peak frequencies (A/A CN ) in th Figure 4 5 shows the average ratio of peak frequencies for each participant in the two groups. Consistent with other stu dies (Deeter et al. , 2009; Henin et al. , 2011), there An ANOVA of the average peak frequency ratios (A/A CN ) of the two groups of participants indicated a trend towards significant difference between the two groups (F(1,17) = 3.08, p = 0.097 ). Discussion Overall, the present results do not show any statistica lly significant difference in the fs DPAOE amplitude as a function of the attention conditions. The fs DPOAE amplitude difference in the generator components bands corresponding to the

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87 frequency anomalies between the attend auditory with contralateral noise condition and the attend visual with contralateral noise condition was not significantly influenced by which frequency anomalies the participants attended to, in the auditory attend condition. The experimental design incl uded three attention conditions, attend auditory without noise, attend auditory with noise and attend visual with noise. The latter two conditions were similar to the experiment described in Chapter 3; the participants either attended and responded to the auditory task, or attended and responded to the visual task. A contralateral broadband noise stimulus was additionally presented to observe any changes in the DPOAE amplitudes at different frequency bands. As figure 4 3 shows, there was no significant e ffect of attention on DPOAE amplitude difference between the two attention conditions, at either the attended or ignored frequency anomalies. The auditory attend without noise condition was introduced in this experimental design to determine whether there was any change in DPOAE amplitude when participants attended to the auditory stimuli in the ipsilateral ear, with noise presented in the contralateral ear, compared with when no contralateral noise was presented. In this experimental design, fs DPOAE was recorded with a swept tone pair paradigm, with f2 ranging from 1000 Hz to 11314 Hz. The frequency bands for analysis were chosen with a rationale similar to the one described in Chapter 3; attending to the frequency anomalies at primary frequencies f1 an d f2 would results in changes in OHC activity at those frequencies. These changes would manifest in the generator component of the DPOAE arising from the OHC activity corresponding to frequencies between f1 and f2. A crucial difference between this experi mental design and the one described in Chapter 3 was the presence of contralateral broadband noise. Previous studies have

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88 shown that contralateral noise activates the uncrossed MOC reflex (Liberman et al. , 1996). Generally, activation of the MOC by contr alateral noise results in an inhibition of OHC activity in the ipsilateral ear, which can be measured as changes in OAE amplitudes. Because the crossed MOC neurons synapse tonotopically onto ipsilateral OHCs of the same response frequency, their inhibitor y activity on OHCs is frequency specific (Liberman & Brown, 1986; Brown, 1989). Moreover, an increased number of activated MOC fibers results in a larger suppressive (inhibitory) effect on OHC activity (Warr et al. , 1986). This is supported by evidence f rom studies reporting reductions in OAE amplitude as a function of contralateral stimulus bandwidth (Lilaonitkul & Guinan, 2009; Lilaonitkul & Guinan, 2012). In this experiment, we chose to activate the contralateral MOC reflex via a broadband noise stimu lus (100 Hz 10000 Hz) such that suppression of the OHC activity would occur over a relatively large area of the basilar membra n e in the ipsilateral cochlea. The aim was to characterize changes in OHC activity resulting from frequency selective attention in the presence of ongoing inhibition by the contralateral MOC reflex. Additionally, the stimulus conditions were designed to simulate a noisy environment with target signals to which attenti on was directed voluntarily. The absence of a clear pattern of DPOAE amplitude change across the different attention conditions could be explained by a few reasons. A comparison of the behavioral performance in the three attention conditions displays some task dependent differences. While the hit percentages across the three percentages showed differences that might explain the lack of a clear pattern.

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89 Participants had the highest false alarm percentages in the attend auditory wit h no contralateral noise condition (28.42%), followed by the attend auditory with noise condition (16.42 % ) and by the attend visual with noise condition (4.38 % ) . This difference points to a qualitative difference between the auditory and the visual tasks, such that the visual task may not have presented a comparable attentional load as the auditory task. Another reason that might explain a lack of a clear pattern of change involves a possible interaction between the effects of the broadband noise on the O HC activity suppression via the MOC reflex, and the effects of selective attention mediated via descending corticofugal inputs on the OHC activity suppression. Previous studies have reported that manipulation of structures within th e descending corticofug al pathway, for example at the level of the auditory cortex (Xiao & Suga, 2002; Edwards & Palmer, 2010; L é on et al. , 2012) or the inferior colliculus (IC) (Mulders & Robertson, 2000; Groff & Liberman, 2003; Zhang & Dolan, 2006), modulates cochlear sensitiv ity as measured by changes in CAP and CM amplitudes. While some of these studies reported changes in CAP and CM amplitudes different from the classically observed effects such as, decreased CAP, increased CM amplitudes and decreased DPOAE amplitudes (revi ewed by Guinan, 1996) with indirect MOC system activation, Delano et al. (2007) reported a decrease in click evoked CAP amplitude and an increase in tone evoked CM amplitude recorded from chinchillas when they engaged in a visual discrimination task. The other source of suppressive effect on OHC activity via the MOC system arises from the contralateral broadband noise presented during two of the three attention conditions in this experiment. For example, increases in CAP amplitude are observed when

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90 contra lateral noise is presented with ipsilateral noise masked transient signals (Kawase et al. , 1993; Kawase & Takasaka, 1995) leading to greater responses. In the absence of ipsilateral masking noise, however, the CAP amplitude in response to tones is suppres sed in the presence of contralateral noise (Warren & Liberman, 1989; Perry et al. , 1999). Previous studies have reported direct changes in OAE amplitudes as a result of auditory cortex removal or stimulation (Khalfa et al. , 2001; Perrot et al. , 2006), SFO AE amplitude changes (Lilaonitkul & Guinan, 2009), SOAE amplitude changes and frequency shifts (Zhao & Dhar, 2011) and DPOAE amplitude changes (Puria et al. , 1996; Bassim et al. , 2003; Müller et al. , 2005; Wagner et al. , 2007; Purcell et al. , 2008; Abdala et al. , 2009; Henin et al. , 2011) in the presence of contralateral broadband noise. Experimental designs that involve manipulation of selective attention to stimuli in the presence of contralateral noise would likely activate the corticofugal descending p athways in addition to the uncrossed and crossed MOC reflex pathways. The crossed MOC pathways would be activated by the ipsilateral stimuli, while the uncrossed MOC pathways would be activated by the contralateral noise stimuli. There is evidence of ana tomical connections be tween the ipsilateral (IC) and ipsilateral VNTB, from which the uncrossed MOC neurons originate, forming part of the contralateral MOC system (Vetter et al. , 1993; Schofield & Cant, 1999). The ipsilateral VNTB also receives input fro m the contralateral cochlear nucleus (Guinan, 2006; Schofield, 2010). A study by Mulders & Robertson (2002) explored the functional nature of this nexus of the descending inputs from the IC and from the contralateral cochlea. They electrically stimulated the IC of anesthetized guinea pigs and simultaneously presented

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91 contralateral broadband noise. They reported decreases in CAP amplitude to tones at many frequencies (6, 8, 10, 12, 14, 16 and 18 kHz) and increases in CM amplitude to 1 kHz tones as a resul t of the electrical stimulation of the IC, a clear effect of the IC stimulation on the MOC neurons. Additionally, presentation of contralateral noise augmented the effect of the electrical stimulation of the IC on the MOC neurons. When contralateral nois e was presented alone, however, there were no reported changes in the CAP and CM amplitudes. Moreover, the suppressive effects of electrical stimulation of the IC and the contralateral noise on the MOC neuronal activity varied as a function of the frequen cies eliciting the CAP. This suggests that the descending inputs from the IC and the contralateral cochlea might combine non linearly which might help explain the lack of clear pattern of results in the current study. Two of the three attention conditions in the current study involved the presentation of contralateral broadband noise. The participants were instructed to attend and respond to the ipsilateral frequency anomalies at f1 = 2 kHz (4 kHz) in group contralateral auditory stimuli and attend and respond to the phase shifting Gabor patch stimulus. The initial analysis o f the gDPOAE difference across these two attention conditions rested on the assumption that any effect on the DPOAE amplitude as a result of contralateral noise in both conditions would cancel each other. Any difference in the DPOAE amplitude would be cau sed by shifting attention between the auditory and the visual sensory modalities. Our hypothesis was that in both groups, the DPOAE amplitude in the attended band of frequency anomalies in the presence of contralateral

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92 noise at 60 dB SPL would be greater in the auditory attend condition than the visual ANF firing rate reported by many studies (Winslow & Sachs, 1988; Kawase & Liberman , 1993). If the inputs from the contralateral cochlea and the ipsilateral IC combine non linearly as proposed here and by Mulders & Robertson (2002), a linear difference between the DPOAE amplitudes from the two attention conditions may not fully cancel out the eff ect due to contralateral noise. This theory was tested using a different set of two attention conditions in the current study. Comparison of the fs groups showed an effect of the attended frequency band as a main factor. It is well known that there is a suppression of OHC activity, generally resulting in a decrease in DPOAE amplitude in the presence of contralatera l broadband noise (Puria et al. , 1996; Bassim et al. , 1996). As mentioned earlier in this chapter, some studies reported suppression and enhancement of DPOAE amplitudes using a fine structure measure in the presence of contralateral noise ( Sun, 2008; Deet er et al. , 2009; Henin et al. , 2011). These studies also reported significant shifts in maxima and minima frequencies of the fs DPOAE dependent on the intensity of the contralateral noise. In the current study, the frequency shifts were expressed as a pr oportion of the frequency maxima during the Overall, both groups of participants displayed an average fre quency shift in the maxima greater than zero. The ANOVA performed with the groups as a factor showed a significant effect (at alpha =

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93 While this effect was not statisticall y significant at the conventionally accepted alpha of 0.05 , it suggests that attending to different frequency anomalies at f1 = 2 kHz or 4 kHz might interact with the effect of the contralateral noise on the OHC activity mediated by the MOC neurons. An alt attention and contralateral noise, including any possible interactions. The current three condition des ign was chosen as a compromise between the time constraints and robust data collection. Data collection including initial setup was completed within 2 h ou rs and current d esign. An experimental design with a different attention task and intermittent contralateral noise stimuli might accommodate the four attention conditions with a smaller session duration and mitigate the slow effects of the MOC system to sustained contral ateral noise (Larsen & Liberman, 2009). A confounding factor with the present study is the possibility of middle ear muscle reflex (MEM) activation with the levels of the contralateral noise presented. While individual thresholds for the MEM reflex were n ot determined, it is possible that some part of the contralateral noise mediated inhibition was a result of the MEM reflex, in addition to the effect of the MOC system. The levels of contralateral noise presented, however, were comparable to previous stud ies (M ai son et al. , 2001; Bassim et al. , 2003; Mishra & Lutman, 2014) that did not report MEM reflex effects.

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94 Figure 4 1 . Swept frequency paradigm for DPOAE measurement. Tone pairs at geometrically increasing F1 and F2 frequencies are presented to the ear and the response recorded. Shown here is the cubic DPOAE at 2f1 f2 frequency marked DP.

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95 Figure 4 2 . Graph showing a typical DPOAE (bold red line) and its components, the nonlinear distortion source (dotted red line) and the linear reflection source (thin red line). The nonlinear distortion source is produced at the maximal cochlear overlap region of the tone pairs, while the linear reflection source is produced at the 2f1 f2 cochlear place. Both components superimpose to form the DPOA E measur ed at 2f1 f2 frequency. The noise floor is shown in light gray. The narrower vertical gray bars show frequency anomalies at frequencies f1 (f2), 2 kHz (2.439 kHz) and 4 kHz (4.878 kHz). The wider gray vertical bars show the nonlinear distortion source freq uencies presumed to be manipulated by attention.

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96 Figure 4 3 . gDPOAE V A difference between attend visual with noise and attend auditory with noise conditions . band and 4 kHz band joined by thin line. Average gDPOAE V A for group 1 with 2 kHz attention shown in red, group 2 with 4 kHz attention shown in blue.

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97 Figure 4 4 . Example of whole fs DPOAE frequenc y maxima analysis. Fs indicates frequency shift of maxima. Vertical violet arrow indicates suppressi on of DPOAE amplitude.

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98 Figure 4 5 . A NOVA of frequency shift dataset in groups 1 and 2. Group 1 attended to 2 kHz anomalies, group 2 attended to 4 kHz anomalies. The average peak frequency shift A/A CN of the two groups of participants are plotted in this figure.

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99 Table 4 1. Experimental Design for visual and auditory attention with noise 2 kHz frequency anomaly 4 kHz frequency anomaly Gabor patch task Group 1 N = 9 Auditory Attention Attend Ignore Ignore Auditory Attention with Contralateral noise Attend Ignore Ignore Visual Attention with Contralateral noise Ignore Ignore Attend Group 2 N = 10 Auditory Attention Ignore Attend Ignore Auditory Attention with Contralateral noise Ignore Attend Ignore Visual Attention with Contralateral noise Ignore Ignore Attend

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100 Table 4 2. Behavioral Results Attend Auditory without contralateral noise Attend Auditory with contralateral noise Attend Visual with contralateral noise Group 1 (n = 9) Attend 2 kHz Hits: Mean % (SD %) 96.67 (7.07) 97.78 (6.67) 92.89 (7.58) False Alarms: Mean % (SD %) 32.22 (27.73) 23.33 (19.37) 4.64 (6.26) Group 2 (n = 10) Attend 4 kHz Hits: Mean % (SD %) 91.00 (18.53) 97.00 (6.75) 92.36 (10.30) False Alarms: Mean % (SD %) 25.00 (14.34) 11.00 (12.87) 4.15 (5.26) Overall (n = 19) Hits: Mean % (SD %) 93.69 (14.23) 97.37 (6.53) 92.62 (8.87) False Alarms: Mean % (SD %) 28.42 (21.41) 16.84 (17.01) 4.38 (5.60)

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101 CHAPTER 5 GENERAL DISCUSSION AND FUTURE DIRECTIONS The descending corticofugal auditory system consists of an extensive network of connections originating from the auditory cortex , likely including a netwo rk of many corticofugal systems, and terminates within the cochlea (Winer, 2006). The corticofugal pathways converge at many sta ges of the ascending auditory pathway, including the cochlear nucleus (for review, see Schofield, 2010), the superior olivary complex, the inferior colliculus (Mulders & Robertson, 2000; Coomes & Schofield, 2004) and the medial geniculate body (Winer et al . , 2001 ) . These descending connections are implicated in a variety of non auditory and auditory functions (Winer, 2006). The auditory functions include discrimination of signals in noise, protection from loud noise, frequency selectivity and mediation of selective attention (Guinan, 2006; Robles & Delano, 2008; Robertson, 2009). The experiments described in this project report the effects of selective attention on the OHC activity via the corticofugal descending pathways and the MOC efferent reflex pathw ay. As discussed in the introduction and the subsequent chapters, the effects of selective attention have been studied using a variety of electrophysiological and acoustic measures including OAEs (Lukas, 1980; Michie et al. , 1996; de Boer & Thornton, 2007; Smith et al. , 2012). In the first experiment, participants were instructed to attend to visual stimuli, or to auditory stimuli presented to the ipsilateral ear (the ear in which the DPOAEs were recorded), or to auditory stimuli presented to the contralat eral ear of DPOAE recording. A comparison of the recorded DPOAE during the three conditions showed a linear contrast with the ipsilateral attending resulting in the lowest overall DPOAE amplitude and the visual attending resulting in the highest DPOAE

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102 amp litude . The consistently reported decrease in DPOAE amplitude during the auditory attending condition (Smith et al. , 2012; Srinivasan et al. , 2012) then led to the question of whether cochlear sensitivity was modulated (decreased) at frequencies away from attended frequency stimuli with decreased detection accuracy as reported in psychophysical studies (Scharf et al. , 1987; Dai et al. , 1991). The second and third experiments were designed to address the question as to whether or not the MOC systems termin ating at the base of OHCs modulated cochlear sensitivity at a broad range of frequencies when participants engaged in different visual and auditory attention tasks. In the second experiment, monaural swept frequency stimuli were presented simultaneously with visual Gabor patch stimuli. Frequency reversal "objects" we re introduced at two different frequencies. During the auditory attention conditions, one group of p articipants attended to the frequency anomalies at f1 = 2 kHz while the other group attended to the frequency anomalies at f1 = 4 kHz. An analysis window bounded by the f1 and f2 frequencies was chosen to be compared across the different attention conditi ons, with the rationale that the sweeping stimuli were amplified differentially within the range of these frequencies as a result of MOC mediated attentional modulation of OHC activity. DPOAE are considered to be generated at the maximal overlap of the f1 and f2 frequency cochlear place (Shera & Guinan, 1999) and a two component decomposition of the DPOAE based on the latency of arrival at the ear canal (Long et al. , 2008) allowed us to compare the generator (nonlinear distortion) source and reflection sou rce across the different attention conditions. Overall, the reflection components did not exhibit any clear pattern as a factor of the attention components, while the generator DPOAE showed mostly

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103 consistent amplitude changes in attended and unattended fr equencies bands across the visual and auditory attention conditions. The results of the second experiment are in agreement with those reported in the first experiment, with the DPOAE amplitude relatively lower in the auditory attention conditions. The ex perimental design in the third experiment included a similar sweeping DPOAE measure with three attention conditions. This experiment was designed to determine the effect of selective attention on OHC activity in the presence of broadband contralateral noi se. Comparison of the DPOAE generator source amplitude in the visual and auditory attention conditions in the presence of noise didn't result in any systematic pattern of change as a function of the attention conditions. It is possible that continuous pr esentation of contralateral noise may have generated a larger effect of MOC suppression via the uncrossed MOC reflex compared to the attentional effects via the corticofugal pathways. While the unmasking of transient signals in noise phenomenon exhibited by the MOC system is supported by many studies (Winslow & Sachs, 1987; Kawase & Liberman, 1993) these effects are observed in the CAP measured in the primary afferents arising from the IHC. The MOC system may accomplish this unmasking via a non analogous mechanism of suppression of OHC activity. The findings from a recent study by de Boer et al. (2012), that participants with stronger OAE suppression in the presence of contralateral noise showed poorer performance in discrimination of speech in noise perf ormance, support this idea. In addition, other studies (de Boer & Thornton, 2008) show that MOC activity measured via OAE suppression in a speech discrimination task varies considerably with experience. This suggests that the influence of corticofugal pa thways on cochlear

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104 sensitivity via the MOC system may not be straightforward and may be experience and task dependent. While the DPOAE is a measure of outer hair cell activity (Kim, 1980; Kemp, 2002), the interpretation of the frequency selective DPOAE am plitude changes cannot be made in a straightforward manner. The cubic DPOAE is measured at the ear canal at the 2f1 f2 frequency. The DPOAE amplitude and phase is a resultant combination of the non linear distortion source generated at the maximal overla p cochlear place of the primaries and multiple reflections from the basal and the apical ends of the cochlea, including, the primary reflection at the cubic DPOAE characteristic cochlear place (Dhar et al. , 2002). These components may be extracted from th e DPOAE by their latencies at which they arrive at the basal end of the cochlea. While, the data analyses in experiments described in chapters 3 and 4 looked at the generator DPOAE component amplitude alone, the algorithm developed by Long et al. (2008) s eparates the components using their arrival latencies at the ear canal. Different latencies correspond to different points of origin in the cochlea and the separability of these components is dependent on the window size and other filtering parameters spe cified in the algorithm. A cleaner measure of cochlear place dependent OAE amplitude and phase would be obtained by using stimulus frequency otoacoustic emissions (SFOAE) (Guinan, 2011 ). SFOAEs are generated by a simpler linear reflection process in the cochlea (Goodman et al. , 2003; Guinan et al. , 2003; Guinan, 2011) and may reflect better the frequency specific differential OHC activity modulation by selective attention. Tone evoked S FOAEs also reflect changes in OHC activity in response to ipsilateral, contralateral and bilateral narrow and broad band noise stimuli (Lilaonitkul & Guinan, 2009).

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105 While OAE measures reflect changes in cochlear sensitivity, auditory brain stem responses ( ABR) and auditory steady state responses (ASSR) are non invasive measures that may provide more information about the attentional modulations of neural activity at the inferior colliculus, the cochlear nucleus and other brainstem regions innervated by the descending corticofugal pathways. Some neurons in the cochlear nucleus and the inferior colliculus show classic effects to signals in quiet and in background noise similar to those seen in the primary afferents arising from the cochlea (for review, see Ro bertson & Mulders, 2011) and the synchronous far field potential arising from the activity of such ensembles may be recorded at the scalp. Several studies using non invasive electrophysiological measures at the scalp have reported changes in ABR amplitude and latencies (Lukas, 1980; Brix, 1984) and changes in the frequency following response (Galbraith et al. , 2003; Lehmann & Sch önwiesner, 2014) as a result of selective attention. A combined ASSR measure with steady state components reflecting activity ch anges in the auditory cortex ( 40 Hz) and the brainstem ( 80 Hz) may be employed to observe attentional changes in neuronal activity along the brainstem regions along the auditory neuraxis (Herdman et al. , 2002; Slugocki et al. , 2013).

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119 BIOGRAPHICAL SKETCH Sridhar Srinivasan received his Bachelor of Engineering in Electronics and Communication Engineering from Government College of Technology, Coimbatore, India in the spring of 2004. His undergraduate thesis was in object motion detection in a series of images. He continued in a similar line of research in the laboratory of Dr. Clint Slatton in the adaptive signal processing area of the Department of Electrical and Computer Engineering at the University of Florida. After receiving his Master of Science in Electrical and Computer Engineer ing in the summer of 2006, he worked on a gait analysis program suite in the Brain Rehabilitation Research Center at the Veterans Affairs hospital in Gainesville, Florida. In the fall of 2007 he was accepted into the laboratory of Dr. Linda Hermer Vazquez in the behavioral neuroscience area of the Department of Psychology at the University of Florida to study electrophysiological measures of decision making in rodents. This project led to the research for his thesis and he received his Master of Science i n Psychology from the University of Florida in the spring of 2011. After receiving his Master of Science degree, he worked in the laboratory of Dr. David W. Smith in the behavioral and cognitive neuroscience area of the Department of Psychology at the Uni versity of Florida to study effects of selective attention on peripheral auditory function in humans. This project led to research for his dissertation and he received his Doctor of Philosophy in Psychology from the University of Florida in the summer of 2014.