Title: Effects of phonetic processing and stimulus relevance on the auditory evoked response
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Title: Effects of phonetic processing and stimulus relevance on the auditory evoked response
Physical Description: ix, 87 leaves : ill. ; 28 cm.
Language: English
Creator: Silva, Dennis Alfred, 1948-
Copyright Date: 1977
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Subject: Brain -- Localization of functions   ( lcsh )
Evoked potentials (Electrophysiology)   ( lcsh )
Psychology thesis Ph. D   ( lcsh )
Dissertations, Academic -- Psychology -- UF   ( lcsh )
Genre: bibliography   ( marcgt )
non-fiction   ( marcgt )
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Statement of Responsibility: by Dennis Alfred Silva.
Thesis: Thesis--University of Florida.
Bibliography: Bibliography: leaves 79-86.
General Note: Typescript.
General Note: Vita.
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Bibliographic ID: UF00098660
Volume ID: VID00001
Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
Resource Identifier: alephbibnum - 000186419
oclc - 03372619
notis - AAV3009

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EFFECTS OF PHONETIC PROCESSING AND STIMULUS RELEVANCE
ON THE AUDITORY EVOKED RESPONSE

















By

DENNIS ALFRED SILVA


A DISSERTATION PRESENTED TO THE GRADUATE COUNCIL OF
THE UNIVERSITY OF FLORIDA
IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE
DEGREE OF DOCTOR OF PHILOSOPHY



UNIVERSITY OF FLORIDA


1977

































































Copyright by

Dennis Alfred Silva
1977















ACKNOWLEDGMENTS


I wish to express my sincere appreciation to William Yost,

chairman of my committee, not only for his helpful suggestions for

the present investigation, but also for his tremendous contribu-

tions to my graduate education. In addition, I wish to thank

Donald Teas for the use of his laboratory facilities which made

the present research possible, as well as for his useful comments

on the work itself. I would like to express my appreciation also

to my other committee members, Keith Berg, Howard Rothman, Paul

Satz, and Robert Isaacson, for their knowledgeable comments and

suggestions.















TABLE OF CONTENTS


ACKNOWLEDGMENTS.............

LIST OF TABLES...............

LIST OF FIGURES .............

ABSTRACT ....................

INTRODUCTION...............


Summary.....


METHOD. ................. ...................................

Subjects..................................................
Stimuli and Apparatus.....................................
Procedure.............................................. ...

RESULTS .....................................................

Topographical Effects...... ..............................
Phonetic versus Nonphonetic Tape Effects.................
Effects of Stimulus Relevance.............................

DISCUSSION ..................................................

Topographical Effects....................................
Stimulus Relevance...................................... ..

APPENDIX A: Ordering of Stimuli on Tape 1..................


APPENDIX A:


Ordering of Stimuli on Tape 2..


APPENDIX B: Order of Experimental Conditions...............

APPENDIX C: Summary of Partial Amplitude Data (Microvolts)
for Two Additional Subjects....................

REFERENCES ..................................................

BIOGRAPHICAL SKETCH..... .................. .................


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LIST OF TABLES


Table Page

1 Summary of Experimental Conditions.............. 29

2 Mean Amplitudes (microvolts) for Major Com-
ponents of Evoked Responses Recorded from
Three Locations................................. 37

3 Number of Persons Having Greater Left Hemi-
sphere Amplitudes for Various Components........ 39

4 Mean Latencies (msec) for Major Components of
Evoked Responses Recorded from Three Locations.. 43

5 Mean Amplitudes (microvolts) for Major Com-
ponents of Potentials Evoked by Stimuli on
Phonetic and Nonphonetic Tapes.................. 45

6 Mean Latencies (msec) for Major Components of
Potentials Evoked by Stimuli on Phonetic and
Nonphonetic Tapes................................ 48

7 Significant Wilcoxen Tests between Potentials
Evoked by Stimuli on Phonetic and Nonphonetic
Tapes....................................... ... .. 53

8 Mean Amplitudes (microvolts) of Evoked Poten-
tial Components in Conditions of Stimulus
Relevance........................................ 56


















LIST OF FIGURES


Figure Page

1 Averaged evoked potentials to CV stimulus recorded
from vertex, left, and right temporo-parietal sites
during various conditions............................ 34

2 Averaged evoked potentials to T1 stimulus recorded
from vertex, left, and right temporo-parietal sites
during various conditions............................ 35

3 R-values and hemispheric voltage differences for
all subjects across all conditions................... 41

4 Latencies for N1 component of potentials evoked by
CV1 during phonetic and nonphonetic processing
tasks............................................... 50

5 Averaged evoked potentials recorded from left and
right hemispheres during phonetic and nonphonetic
tasks............................................ ..... 52

6 Averaged evoked potentials recorded from the vertex
to CV1 and T1 evoking stimuli during conditions of
stimulus relevance................................... 60

7 Averaged evoked potentials recorded from Subjects
1 4 during conditions of stimulus relevance......... 61

8 Averaged evoked potentials recorded from Subjects
5 8 during conditions of stimulus relevance......... 62












Abstract of Dissertation Presented to the Graduate Council
of the University of Florida in Partial Fulfillment of the Require-
ments for the Degree of Doctor of Philosophy



EFFECTS OF PHONETIC PROCESSING AND STIMULUS RELEVANCE
ON THE AUDITORY EVOKED RESPONSE

By

Dennis Alfred Silva

March 1977

Chairman: William A. Yost
Major Department: Psychology

There is a substantial body of research data, gathered from

both clinical and nonclinical behavioral investigations, which sup-

port the phenomenon of specialization of function in the human

brain. In addition, recent findings suggest that there are structural

asymmetries in the cortex which parallel the functional differences.

Results of recent electrophysiological investigations suggest

that the left hemisphere's specialization for language is reflected

in auditory evoked responses to speech and nonspeech stimuli and

during phonetic and nonphonetic processing of speech stimuli.

Because of several inconsistencies in the existing data, however, the

precise nature of the effect as well as the actual processing mechan-

isms reflected in the reported asymmetries remain obscure. For

example, the evoked potential asymmetries might be reflecting the

activity of a lateralized neural center responsible for phonetic

analysis of speech stimuli or they might be due to a lateralized

cortical activation occurring in preparation for phonetic processing.













One purpose of the present investigation was to investigate

further the evoked potential correlates of hemispheric specializa-

tion for language by recording auditory evoked responses to a

speech and a nonspeech stimulus from left and right temporo-

parietal locations (referred to linked mastoids).

Eight subjects participated in a series of phonetic and

nonphonetic discrimination tasks. During each condition two

speech (CV syllables) and two nonspeech (tones) stimuli were pre-

sented to subjects and evoked potentials were averaged to one of

each class of stimuli. In one type of task the speech stimuli

were phonetically different (/ba/ and /da/); in the other type

they were phonetically similar (/ba/) but differed in fundamental

frequency. By presenting both speech and nonspeech stimuli within

a single run while subjects are engaged in tasks requiring either a

"phonetic" or "nonphonetic" cognitive set, it is possible to

determine if an evoked potential asymmetry is due to a lateralized

preparatory activation or to the activity of an actual phonetic

analyzer localized in the left hemisphere. If the former explana-

tion is correct, then asymmetrical cortical responses should be

manifest in the evoked potential to a stimulus whenever a subject

expects to make a phonetic discrimination, regardless of the

actual nature of the stimulus presented. If the latter explanation

is correct then an asymmetrical response should occur only to a

speech stimulus and not to a nonspeech stimulus in spite of a

"phonetic set."


viii













Analysis of the data demonstrated no significant differences

between the left and right hemispheres for mean amplitudes or mean

latencies of the evoked potential components. The majority of

subjects, however, did have slightly larger left hemisphere

responses to both evoking stimuli.

Regarding potentials obtained during phonetic and nonphonetic

processing of stimuli, the data confirm a previously reported

finding of a differential left hemisphere response to a speech

stimulus during phonetic processing and extend this observation to

the use of natural speech syllables. No differences in potentials

occurred to the nonspeech stimulus even when subjects were engaged

in the phonetic processing task. This suggests that the dif-

ferential left hemisphere response is not simply one of a prepara-

tory nature but rather it may reflect the activity of a phonemic

analyzer. The possibility of two left hemisphere mechanisms was

discussed -- one for speech detection and one for phonetic analysis.

Finally, vertex potentials were analyzed for effects of

stimulus relevance, with the stimulus which subjects were discrimi-

nating being the relevant stimulus and the other three stimuli

being irrelevant stimuli. It was found that the P3 component of

the evoked potential was most sensitive to various conditions of

stimulus relevance. The amplitude of the P3 component was largest

when the evoking stimulus was most like the relevant stimulus, and

smallest when the evoking stimulus was of the other class (i.e.

speech or nonspeech) than the relevant stimulus. These findirigs

support a "neural template" model of P3.

















INTRODUCTION


Since the nineteenth century when anatomists and neurologists

first noted a relationship between sensory-motor disorders of the

right half of the body and speech disturbances, mounting scientific

evidence attests to the functional disparity in humans between the

two cerebral hemispheres with respect to certain cognitive capacities.

Over the years the understanding of cerebral specialization in the

human brain has grown, and investigation of the phenomenon has

expanded from predominantly clinical observations to research with

non-brain damaged persons in a variety of experimental paradigms.

Novel approaches and more sophisticated apparatus have resulted in

an expansion of methodologies for investigating functional locali-

zation in the brain. Not only have behavioral experiments flour-

ished, but, during the last eight years or so, reports of electro-

physiological investigations of the phenomenon have begun to appear

in the literature. The purpose of the present report is to expand

further our knowledge of evoked potential correlates of language

localization in the brain while attempting to integrate some of the

existing data on hemispheric asymmetries. The effects of phonetic

processing and stimulus relevance on the auditory evoked response to

speech and nonspeech stimuli are addressed in the present investigation.












With respect to their localization in the brain, speech and

language functions are perhaps the most extensively studied and

documented of human cognitive processes. A substantial amount of

the clinical evidence for the localization of language in the left

hemisphere comes from the work of Penfield and Roberts (1959).

Those authors reported that, excluding persons with cerebral injury

occurring prior to age two, over 70 percent of their patients who

had surgery on the left hemisphere showed transient dysphasia. This

was so for both right and left handers. In contrast, they found

that fewer than 1 percent of right and fewer than 7 percent of left

handers showed dysphasic symptoms following operation on the right

hemisphere.

Penfield and Roberts (1959) showed also that electrical stimula-

tion of certain areas of the left hemisphere can cause an arrest or a

hesitation of speech or can result in a variety of dysphasic responses

such as misnaming, word distortion or repetition, and perseveration.

They reported that stimulation of three regions of the brain in par-

ticular could alter speech processes: (1) Broca's area (the three

gyri anterior to the lower precentral gyrus); (2) the supplementary

motor area (on the medial surface of the brain anterior to the

precentral leg area); and (3) the posterior temporo-parietal area

(the posterior part of the first, second, and third temporal convolu-

tions; the supramarginal gyrus; and the angular gyrus).

Lateralized cortical lesions have been shown to result in

significant deficits in performance on verbal intelligence tests












when these lesions occur on the left side but not on the right

(Heilbrun, 1956; Klove, 1959; McFie & Piercy, 1952; Reitan, 1955;

Satz, Richard, & Daniels, 1967), although Milner (1958) noted

that if the disorder is restricted to the left temporal lobe this

deficit does not occur. She did report, however, as had others

previously, that learning capacities and memory for verbal material

is impaired following left temporal lobe lesion (Meyer & Yates,

1955; Milner, 1958).

Additional clinical evidence for lateralization of language

functions in the left hemisphere comes from persons having undergone

the Wada sodium amytal test (Branch, Milner, & Rasmussen, 1964;

Wada & Rasmussen, 1960) or the surgical sectioning of the forebrain

commissures.

The Wada test was developed as an alternative to using manifest

handedness as the sole means for identifying cerebral dominance in

patients about to have surgery. The technique consists of injecting

a barbiturate, sodium amytal, into the right and left carotid

arteries on separate occasions, thereby influencing the functioning

of the right and left hemispheres individually. The Wada treatment

results in a temporary interference with normal hemisphere functions

and thus allows for comparison of the roles of each side of the brain

in speech and language. Treatment on the side of the language domi-

nant hemisphere, therefore, typically results in a transient inter-

ference of expressive and receptive language functions (Branch et al.,












1964; Perria, Rosadini, & Rossi, 1961; Terzian, 1964; Wada &

Rasmussen, 1960). In contrast, injection of the barbiturate on

the nondominant side has been reported to impair musical ability

(Bogen & Gordon, 1971; Gordon & Bogen, 1974).

Sectioning of the forebrain commissures for the control of

epileptic seizures has provided a yet different means to evaluate

the capabilities of each hemisphere, independent of the influence

of the other. In the human, commissural sectioning has been success-

ful clinically, but as a research resource human studies have been

somewhat inconclusive. In most of these cases the left hemisphere

is clearly the dominant one for language functions although there is

some evidence that the right hemisphere does possess at least minimal

receptive and expressive capabilities (Gazzaniga & Hillyard, 1971;

Gazzaniga & Sperry, 1967; Levy, Nebes, & Sperry, 1971). Because of

factors such as preexisting brain damage and cross-cueing strategies

between the two hemispheres, however, more sophisticated capabilities

of the nondominant right hemisphere remain unresolved.

Over the years, investigators have searched for structural and

physiological differences between the hemispheres that might account

for observed differences in function. Recently, several studies

have reported left-right asymmetries in humans in the region

posterior to Heschl's gyrus on the superior aspect of the temporal

lobe (Geschwind & Levitsky, 1968; Wada, Clarke, & Hamm, 1975;

Witelson & Pallie, 1973). This area, the planum temporale, comprises













part of the region classically known as Wernicke's area and has

been shown to be of major importance in language function (Penfield

& Roberts, 1959). This left-right anatomical difference has been

reported to be present as early as the 29th gestational week (Wada

et al., 1975). Other asymmetries have been reported to exist in

humans: the length of the Sylvian fissure, with the left being

approximately 10 mm longer than the right (Yeni-Komshian & Benson,

1976); and the pathway of the middle cerebral artery in the region

of the Sylvian fissure which suggests a larger parietal operculum on

the left (LeMay & Culebras, 1972).

In addition to the research with clinical populations described

earlier, a vast number of behavioral investigations have been con-

ducted with nonclinical subjects. Experimental paradigms in both

the visual and auditory modalities have provided a means for investi-

gating laterality of function in the cerebral hemispheres.

In man approximately half of the fibers originating from each

eye are contralateral. Fibers from the nasal half of the retina

cross at the optic chiasm and higher order neurons eventually termi-

nate in the opposite striate cortex. Fibers from the temporal

halves remain uncrossed and their higher order neurons eventually

terminate in the ipsilateral receiving area. Consequently, percep-

tion of visual stimuli in the left visual half-field (impinging on

the right hemi-retinae) occurs in the right occipital lobe, and

perception of visual stimuli in the right visual half-field (imping-

ing on the left hemi-retinae) occurs in the left hemisphere.











6

Experimenters have found that when a stimulus is presented to

an observer so that it is confined to a single visual half-field

(i.e. the left or right), the field of presentation which results

in the superior recognition is dependent on the nature of the

stimulus. For example, a right half-field superiority is obtained

most frequently with words (Kaufer, Morais, & Bertelson, 1975;

McKeever & Huling, 1971; Mishkin & Forgays, 1952; Terrace, 1959),

letters (Bryden, 1965, 1966; Bryden & Rainey, 1963; Heron, 1957;

Kimura, 1966), and digits (Hines & Satz, 1971; White, 1969),

whereas no significant field differences are usually found when

stimuli consist of nonalphabetic material such as geometric shapes

or nonsense forms (Bryden, 1960; Bryden & Rainey, 1963; Heron, 1957;

Kimura, 1966; Terrace, 1959). Recognition of faces is superior when

they are presented to the left visual half-field (Ellis & Shepherd,

1975).

Because words and letters, but not geometric or nonsense forms,

are identified more accurately in the right visual field, Kimura

(1966) suggested that the more direct pathway from that field to the

language dominant left hemisphere is the basis for the visual field

differences. Reaction time data employing the visual half-field

paradigm also support a cerebral dominance model: observers respond

faster to verbally coded stimuli when they are presented in the right

visual half-field (Cohen, 1972; Geffen, Bradshaw, & Nettleson, 1972;

Geffen, Bradshaw, & Wallace, 1971; Seamon & Gazzaniga, 1973), while












the use of nonverbal stimuli results in faster reaction lines

when they are presented in the left visual half-field (Rizzolatti,

Umilta, & Berlucci, 1971).

In the auditory modality a dichotic listening paradigm has

been employed to study hemispheric differences in the processing

of stimuli. This technique, introduced by Broadbent (1954), con-

sists of presenting conflicting stimuli simultaneously to the two

ears. The listener's task is to report the stimuli which he

perceives.

Using the dichotic technique, Kimura (1961) found that both

normal and temporal lobe brain damaged persons usually had higher

scores for reporting verbal material presented to the right ear

than to the left ear, with the exceptions being persons who had

speech functions localized in the right hemisphere as determined by

the Wada test. Since the left hemisphere is the language dominant

hemisphere in the vast majority of persons (Penfield & Roberts, 1959),

the ear asymmetry which occurs with dichotic stimulation is usually

referred to as a right ear advantage. Since Kimura's early studies,

the right ear advantage has been replicated with a variety of

verbal stimuli including meaningful and nonsense words (Curry, 1967;

Curry & Rutherford, 1967); digits (Broadbent & Gregory, 1964; Dirks,

1964); and backwards speech (Kimura & Folb, 1968). Shankweiler and

Studdert-Kennedy (1967) have shown that a right ear advantage is

obtained with consonants but not with vowels.












To explain her findings, Kumura suggested that the crossed audi-

tory pathway in man is more efficient than the uncrossed pathway,

thus providing the ear contralateral to the language hemisphere with

a superior channel to the speech processor. Unlike the visual path-

ways, the fibers in the auditory system are bilateral. By proposing

a greater efficiency for the contralateral pathway and a suppres-

sion of ipsilateral fibers during dichotic stimulation, however,

Kimura (1967) was able to interpret the right ear advantage in

audition in the same vein as the perceptual asymmetries in visual

half-field investigations, i.e. superior transmission to the language

dominant left hemisphere. Support for her model is derived from

electrophysiological recordings in lower mammals which suggest that

the contralateral auditory pathways are superior to the ipsilateral

pathways (Benson & Teas, 1976; Hall & Goldstein, 1968; Rosenzweig,

1951; Tunturi, 1946).

Kinsbourne (1970) has proposed a model which attempts to account

for laterality differences in perceptual experiments on the basis of

an attentional bias toward stimuli coming from either the left or the

right rather than on a perceptual asymmetry resulting from differences

in the neural pathways from sensory receptors to the language process-

ing hemisphere. He suggested that the use of verbal stimuli "induces

preparatory left hemisphere activation, and thus biases attention"

(Kinsbourne, 1970, p. 196) to stimuli on the contralateral right side.

Because of this selective attention to the right side, verbal stimuli













to the right ear are more accurately recognized than verbal stimuli

to the left ear. Likewise, since the right hemisphere is special-

ized to process nonverbal stimuli, their use activates the right

hemisphere which in turn facilitates perception of stimuli presented

to the contralateral left side. To support his model Kinsbourne

presented data which showed that subvocal rehearsal of verbal

stimuli (a process which should activate the language dominant left

hemisphere) results in an increased perception of stimuli on the

right side but not on the left.

At present it remains to be determined whether Kimura's

perceptual asymmetry model or Kinsbourne's attention model is the

better one, since both can account for the majority of the existing

data. Both, however, rely on the left hemisphere's specialization

for speech and language and thus support a cerebral dominance model

for explaining performance asymmetries in visual and auditory experi-

ments.

In the early 1970's researchers began to investigate the possi-

bility that lateralization of function in the cerebral hemispheres

is reflected in cortical potentials recorded from the human scalp.

Some researchers looked at ongoing electroencephalograms (EEG's)

and found relatively less alpha activity on the left side when

subjects were engaged in linguistic tasks (Galin & Ellis, 1975;

Galin & Ornstein, 1972; McKee, Humphrey, & McAdam, 1973; Morgan,

MacDonald, & Hilgard, 1974; Robbins & McAdam, 1974; Wilson, Vieth,












& Darrow, 1957). In contrast, if a task is spatial in nature, a

greater decrease in alpha is seen over the right hemisphere

(Galin & Ellis, 1975; Galin & Ornstein, 1972; Morgan et al., 1974;

Morgan, McDonald, & MacDonald, 1971; Robbins & McAdam, 1974).

A second electrophysiological approach to studying hemispheric

processing of information has been to compare sensory evoked cortical

potentials recorded from the left and right hemispheres to linguistic

and nonlinguistic stimuli. The sensory evoked response reflects

electrical activity of the brain which is time-locked to a change

in sensory input. When recorded from the human scalp the evoked

response is typically represented as an average of many responses

so as to minimize the random background activity of the EEG. In

the auditory modality, the evoked response waveform has been sub-

divided into early components which occur within 8 msec, middle

latency components occurring between 8 and 40 msec, and late com-

ponents with latencies greater than 40 msec (Picton, Hillyard,

Krausz, & Galambos, 1974). It is the late components, reflecting

cortical activity, which have been investigated with respect to

hemispheric differences.

When recorded from the vertex the late potentials include a

series of alternating positive and negative waves traditionally

labelled as P1, N1, and P2' with latencies of approximately 50,

100, and 180 msec, respectively (Davis, Mast, Yoshie, & Zerlin,

1966; Davis & Zerlin, 1966; Rothman, Davis, & Hay, 1970).












Depending on the experimental conditions, a third positive com-

ponent may occur, typically at 300 msec or longer following stimulus

onset (Sutton, Braren, Zubin, & John, 1965; Sutton, Tueting, Zubin,

& John, 1967). This late positive wave is usually referred to as

P3 or, because of latency, P300. The negative peak between P2 and

P3 is called N2 and usually appears between 250 and 300 msec post-

stimulus onset.

Although the N1 and P2 components can be affected by changes in

the stimulus parameters of intensity and duration (Butler, Keidel,

& Spreng, 1969; Davis et al., 1966; Onishi & Davis, 1968; Rapin,

Schimmel, Tourk, Krasnegor, & Pollack, 1966), they are also sensitive

to certain psychological variables. For example, the NI-P2 peak-

to-peak amplitude is increased when attention is enhanced through

counting (Gross, Begleiter, Tobin, & Kissin, 1965; Mast & Watson,

1968) and in discrimination/detection tasks (Davis, 1964; Hirsch,

1971). When subjects are told to listen selectively to stimuli

arriving through a particular channel (e.g., sense modality), the

N1-P2 amplitude is larger than the corresponding amplitude evoked

by the same stimuli arriving through a nonattended channel

(Debecker & Desmedt, 1966; Satterfield, 1965; Spong, Haider, &

Lindsley, 1965). Naatanen (1975) has chosen to explain this effect

as being due to an increase in a subject's general arousal as

opposed to his selective attention, but recent data presented by

Schwent and Hillyard (1975) conflict with Naatanen interpretation.













By presenting stimuli rapidly through different auditory "channels"

(i.e., lateralized to the left or right) they were able to keep

the subjects' arousal levels from fluctuating from stimulus presen-

tation to stimulus presentation.

The P3 component of the evoked potential was shown by Sutton

and colleagues to be dependent on psychological variables (Sutton

et al., 1965, 1967). Since then, several investigations have been

conducted in an effort to deliniate precisely what these variables

are. It has been shown that the P3 wave can be evoked by stimuli

which deliver information relevant to the task of a subject

(Donchin & Cohen, 1967; Sutton et al., 1965, 1967), by stimuli

about which a decision must be made (Rohrbaugh, Donchin, & Eriksen,

1974; Smith, Donchin, Cohen, & Starr, 1970), and by novel or

unexpected stimuli (Ritter & Vaughan, 1969; Ritter, Vaughan, &

Costa, 1968).

In 1971 reports began to appear in the literature describing

hemispheric differences in auditory evoked responses to speech and

nonspeech stimuli. Unfortunately, it is not possible to integrate

all the data presently available because neither methodologies nor

dependent measures have been consistent among different investi-

gators.

Morrell and Salamy (1971) measured responses from frontal leads

(over Broca's area), from Rolandic leads (over the sensory-motor

field of vocalization muscles), and from temporo-parietal leads












(over Wernicke's area) to verbal nonsense words. They compared

the peak amplitudes of the N1 and P2 components and the peak-to-

peak N1-P2 amplitude. They found that the N1 wave was signifi-

cantly larger from the left hemisphere than from the right hemis-

phere, particularly in the temporoparietal region. No hemisphere

difference was found when the amplitudes of the P2 waves were

compared. In addition, those authors reported that the peak-to-

peak measurements tended to obscure the hemisphere differences

that did occur. Since nonverbal stimuli were not employed in

that study it is not known whether the hemispheric asymmetries

obtained by Morrell and Salamy were specific to the use of speech

or speech-like signals or if they might have occurred also with

other stimuli.

Cohn (1971) compared the left and right hemisphere responses

to monosyllabic speech stimuli and to click stimuli. He reported

that in about half of his subjects the verbal stimuli produced an

initial negative wave, largest in the temporo-parietal region,

which peaked between 30 and 50 msec. The remaining subjects

showed no hemisphere differences with these stimuli. The signifi-

cance of Cohn's findings is questionable, however, because the

stimuli he employed were not equated on the basis of duration,

rise-decay time, frequency composition, and other physical

parameters.

Molfese, Freeman, and Palermo (1975) presented noise stimuli,

nonsense syllables, and monosyllabic words to babies, children,












and adults. In all three groups they found greater left hemi-

sphere N1-P2 amplitudes for both classes of verbal stimuli and

greater right hemisphere N1-P2 amplitudes for the noise stimuli.

Studies looking at hemispheric differences to speech and

nonspeech stimuli when subjects were actively engaged in discrimi-

nation or detection tasks have been reported also. Friedman,

Simson, Ritter, and Rapin (1975a) averaged potentials to human

speech and human sounds when they comprised a "no task" condition,

when they were "signal," and when they were "nonsignal" stimuli in

a vigilance task. They reported that the amplitude of the left

hemisphere N1 component of the evoked response to signal words

was greater than the corresponding wave recorded from the right

hemisphere. The asymmetry did not occur to signal sounds. No

differences in N1 or P2 waves were obtained in the nonsignal or

the no task conditions. Those authors did not report peak-to-peak

N1-P2 amplitudes. In a second study, Friedman and his colleagues

found larger left hemisphere N1 amplitudes to words which delivered

task-relevant phonemic information (Friedman, Simson, Ritter, &

Rapin, 1975b). In both of Friedman's studies hemispheric asymme-

tries in the P3 waveform were inconsistent.

Matsumiya, Tagliasco, Lombroso, and Goodglass (1972) investi-

gated the effect of the "meaningfulness" or "significance" of

verbal and nonverbal stimuli to determine if this factor, rather

than the verbal nature of the stimuli per se, could be responsible












for the differential hemisphere response. As a measure of

asymmetry they employed the ratio R = W1 / (W1 + Wr) where "W"

refers to the peak-to-peak amplitude of the N1-P2 components and

"1" and "r" refer to the left and right hemisphere responses,

respectively. They reported that the response to meaningful words

showed the greatest asymmetry, followed by that to meaningful

noverbal stimuli. The latter, in turn, was larger than the asymmetry

occurring in the two nonmeaningful stimulus conditions. Even though

greater asymmetries were observed during the meaningful conditions

than in the others, most subjects did show a larger left hemisphere

response in all conditions.

Galambos, Benson, Smith, Schulman-Galambos and Osier (1975)

recently investigated the auditory evoked response to speech (/ba/;

/pa/) and tonal (250 Hz; 600 Hz) stimuli and found no significant

hemispheric differences in the mean amplitudes of the major com-

ponents. In analyzing the output voltage at each digitized point

along the time dimension of the waveforms, the general shape of the

left hemisphere's response to the speech stimuli was found to differ

substantially from that same hemisphere's response to the tones,

particularly in the region of the N1 and P3 components. Those

authors noted that although the group differences were not par-

ticularly large, some of the subjects had marked hemispheric

asymmetries. The right hemisphere's responses to the speech and

tone stimuli, however, were found to be quite similar to each other.











Wood, Goff, and Day (1971) designed an experiment that

allowed them to compare each hemisphere's response to a speech-

like signal when the signal was part of a "linguistic" and a

"nonlincuistic" discrimination task. In the former condition a

subject had to make a phonemic discrimination between /ba/ and

/da/; in the latter the stimuli differed in fundamental frequency,

and thus required only a pitch discrimination (/ba/-low versus

/ba/-high). The stimuli were such that /ba/ in the linguistic

task was identical to the /ba/-low in the nonlinguistic discrimina-

tion task. Their data analysis consisted of a point by point

comparison of the digitized output as described above. They found

no difference between the shapes of the right hemisphere's

responses to the common /ba/ in the two tasks, but the response

obtained from the left side when subjects made the phonetic

discrimination was significantly different at several time points

from that hemisphere's response when only a pitch discrimination

was made. Those authors suggested, therefore, that the left hemi-

sphere responds differently to a stimulus during linguistic analysis

than it does during nonlinguistic processing.

In 1975, Wood replicated the findings of his earlier study

using different steady state formants and phonemes with different

places of articulation. In addition, he included a condition in

which subjects had to discriminate between two second formants iso-

lated from acoustically different, but linguistucally similar

stimuli. Since he found no differential left hemispheric response












to the formants, he concluded that the previous finding with

actual speech stimuli was related to the processing of the

linguistic aspect of the stimuli and not simply the acoustic

processing of the formant transitions of the stop consonants.

At the present time there remain several inconsistencies in

the existing data on hemispheric asymmetries of auditory evoked

potentials. Because of this, the cortical functions which under-

lie these asymmetries remain obscure. The fact that some investi-

gators (e.g. Molfese et al., 1975) have observed a hemispheric

asymmetry in the amplitudes of the evoked potential components

when subjects were passive participants in an experiment suggests

that the differential left hemisphere activity might reflect a

physiological "linguistic detector." This interpretation would

be consistent with the report of hemispheric asymmetries in

babies (Molfese et al., 1975). The existence of such a detector

which responds to phonemic transitions in a speech signal would

explain the fact that asymmetries occur as early as 100 or 200

msec following stimulus onset, when much of the stimulus would

not yet have been heard.

Other data, however, are inconsistent with the notion of a

lateralized linguistic detector. For example, if such a device

does exist then Friedman and his colleagues (Friedman et al.,

1975a) should have observed significant hemispheric asymmetries

to words when they were of a nonsignal nature and when they com-

prised the no task condition just as they did when the words

were signal stimuli.













Galambos et al. (1975) showed that the waveforms recorded

from the left hemisphere in response to speech and tonal stimuli

were different while those recorded from the right hemisphere

were not. That finding by itself supports a model for a lateralized

detector in the left hemisphere. What is inconsistent, however,

is that the asymmetry reported by Molfese et al. (1975) was mani-

fest in the amplitudes of the major components (N -P2 amplitude)

while in the Galambos study the hemispheric differences were not

in amplitude measurements but in the differential shapes of their

responses to speech and nonspeech stimuli.

In Wood's investigations (Wood, 1975; Wood et al., 1971),

hemispheric responses were not compared for speech and nonspeech

stimuli. Because of this, the data can be used neither to support

nor refute the existence of a lateralized phonemic detector. The

left hemisphere's response to a speech stimulus did differ, however,

between phoneme discrimination and pitch discrimination. The

data do suggest, therefore, the possibility of a lateralized

center for phonemic discrimination as opposed to simple phonemic

detection.

For an alternative explanation of Wood's data, one can borrow

from Kinsbourne's (1970) model of selective attention. If, as

Kinsbourne suggests, perceptual asymmetries result from a prepara-

tory activation of the hemisphere responsible for analysis of an

expected stimulus, then most of the evoked potential asymmetry














data are confounded with this factor. For example, Friedman et al.

(1975), employing human words and human sounds, presented these

two types of stimuli to the subject in separate runs. Because

the subjects were aware of the type of processing demanded by the

task, differential activation of the hemispheres might have occurred.

Similarly, Galambos and his colleagues employed speech and tones

in their experiment but these, also, were presented in separate

listings (Galambos et al., 1975).

The present investigation is designed to investigate further

the phenomenon of evoked potential asymmetries as related to the

left hemisphere's functional specialization for language processes.

In addition, the present set of experiments is designed to investi-

gate the role of expectancy or "cognitive set" on the auditory

evoked potentials recorded from the two hemispheres. This can be

done by presenting both speech and nonspeech stimuli within a

single run while subjects are engaged in various tasks demanding

phonetic and nonphonetic processing. If a lateralized cortical

activation in preparation for verbal analysis of a stimulus does

occur, then this effect should be manifest in the evoked response

to a stimulus whenever a subject expects to make a phonetic discrimi-

nation, regardless of the actual nature of the stimulus presented.

On the other hand, if subjects are involved in a discrimination

task requiring nonphonetic (e.g. acoustic) analysis of speech or













nonspeech stimuli, a hemispheric asymmetry should not be obtained,

even if the evoking stimulus is verbal in nature. Thus, by averaging

potentials to a speech and a nonspeech stimulus within the same

series of stimuli, it can be determined if the data reported by

Wood (Wood, 1975; Wood et al., 1971) were due to a differential

left hemisphere activation in preparation for phonetic analysis or

to the actual processing itself.

Finally, since speech and nonspeech stimuli will be employed

and subjects will be discriminating stimuli belonging to one class

(i.e. speech or nonspeech) from other stimuli of both classes,

these experiments will allow for investigating possible differences

among potentials when evoked by a relevant stimulus, an irrelevant

stimulus belonging to the same class as the relevant stimulus, and

an irrelevant stimulus belonging to the irrelevant class.

Ford, Roth, Dirks and Kopell (1973) reported that an irrelevant

stimulus in the same sensory modality as a relevant one results in

a smaller amplitude P3 than does the relevant stimulus, but that

no difference in N2 occurs between the two stimuli. In contrast,

waveforms evoked by stimuli in a modality other than the one of

the relevant stimulus exhibit a small, if any, N2 and P3. Because

of these findings, those authors suggested that the N2 component

reflects preliminary processing of stimuli based on sensory modality

and that the P3 component reflects the final decision process.

Interpreting their data in terms of a neural template model (Hillyard,

Squires, Bauer, & Lindsay, 1971), they suggested that an irrelevant












stimulus of the same modality caused a P3 wave because of a

partial match to the template of the relevant stimulus.

The intramodality auditory stimuli employed by Ford and her

colleagues were not clearly described in their article. They used

a click as one stimulus and a change in the level of a background

noise as the other stimulus, but they did not report the duration

of the click or the duration of the new noise level before it

returned (if it did return) to the original "prestimulus" level.

Also, they did not report the transition time from one noise level

to the other. These stimulus parameters are important to the

perception of the stimuli and without them it is difficult to

evaluate the degree of "match" of one stimulus to a neural template

of the other.

The present work involves procedures which use two speech

stimuli and two nonspeech stimuli. Since these four auditory

stimuli will be presented to subjects while they are engaged in

tasks of stimulus relevance, the present study should serve as a

more thorough test of the template mode. Since irrelevant stimuli

will occur in both a relevant and an irrelevant class, a template

model would predict an ordering of the P3 amplitudes as follows:

the relevant stimulus should give the largest P3, followed by the

irrelevant stimulus in the same class as the relevant stimulus,

which, in turn, should give a larger P3 than an irrelevant stimulus

in an irrelevant class.












If the N component reflects a decision of sensory modality

as suggested by Ford, then no difference in the N2 amplitude

should occur in the various conditions of stimulus relevance

because all stimuli are auditory.

Summary

There is a substantial body of research data, gathered from

both clinical and nonclinical behavioral investigations, which

support the phenomenon of specialization of function in the human

brain. In addition, recent findings suggest that there are

structural asymmetries in the cortex which parallel the functional

differences.

Results of recent electrophysiological investigations suggest

that the left hemisphere's specialization for language is reflected

in the auditory evoked potentials elicited by speech and nonspeech

stimuli, and during linguistic and nonlinguistic processing of

speech stimuli. Because of several inconsistencies in the data,

however, the precise nature of the effect as well as the actual

processing mechanisms reflected in the asymmetries remain obscure.

One purpose of the present work is to further investigate the evoked

potential correlates of hemispheric specialization for language by

recording auditory evoked potentials to speech and nonspeech stimuli

from left and right temporo-parietal recording sites. The results

of the present investigation should clarify certain issues regarding

hemispheric asymmetries, with the following questions being addressed:

How do left hemisphere responses compare to right hemisphere













responses when the evoking stimulus is a speech signal and when

it is a nonspeech signal? Are individual subjects' relative

left-right asymmetries influenced by the verbal nature of a

stimulus? Do subjects' responses to stimuli differ if they are

evoked during a phonetic processing task as compared to a non-

phonetic processing task? Finally, can the reported hemispheric

asymmetries be explained as a preparatory activation of the left

cortex due to the anticipation of phonetic processing of stimuli?

A second issue to which the present work is addressed is one

of electrophysiological correlates of stimulus relevance, and the

results should broaden our understanding of the N2 and P3 components

of the evoked potential with respect to their psychological corre-

lates. For example, how do they relate to stimulus modality or

stimulus similarity? Data from the present work should support

or refute the interpretations that N2 reflects a decision regarding

sensory modality of a stimulus and that P3 reflects a higher level

match-mismatch detector.

















METHOD


Subjects

Subjects were six males and two females who were paid $2.00 per

hour for their participation in the study. Six subjects were

recruited through an ad in the University of Florida student news-

paper and two subjects were acquaintances of the investigator.

Subjects'ages ranged from 19 years to 27 years, with a mean of

approximately 23 years. All subjects had normal hearing and no

history of hearing problems. All subjects were self-reported right-

handers, with six of the eight having two parents who were also

right-handed. The remaining two subjects had one right-handed and

one left-handed parent.

Stimuli and Apparatus

Stimuli consisted of three natural speech consonant-vowel

syllables and two nonspeech signals. The speech stimuli consisted

of a low-pitched /ba/ having a fundamental frequency of 110 Hz; a

low-pitched /da/ also with a fundamental frequency of 110 Hz; and

a high-pitched /ba/ with a fundamental frequency of 144 Hz. Each

stimulus was uttered by an adult male and was 260 msec in duration

(+ 4 msec). Fundamental frequency was measured from displays of the

waveforms on a storage oscilloscope.










25

The nonspeech stimuli were two square wave signals generated

by a General Radio oscillator, Type 1313-A. One signal had a

frequency of 110 Hz and the other a frequency of 144 Hz. The

nonspeech signals were shaped with an electronic switch (built at

the Communication Sciences Laboratory of the University of Florida)

to have a rise-fall time of 10 msec and a duration of 260 msec.

Each stimulus was recorded onto a Sony two-channel stereo tape

recorder, Model TC-353D and then dubbed onto an identical tape

recorder in a prescribed order to make four series of stimuli, each

on a separate tape.

Each of the four audiotapes presented a total of 256 stimuli:

64 presentations each of two speech stimuli and two nonspeech stimuli.

The order of the four types of stimuli on each tape was randomized

with two restrictions: first, each quarter of the tape (64 stimuli)

contained an equal number (16) of the four stimuli; second, no

more than two consecutive presentations of a given stimulus were

allowed. These restrictions served to eliminate extreme sequences

of stimuli which might have occurred by chance. The stimuli were

recorded to have an interstimulus interval ranging from 3 sees to

6 secs. The orderings of the stimuli on the tapes are shown in

Appendix A.

Two of the audio tapes contained the following four stimuli:

the two nonspeech stimuli described above and the two low-pitched

speech stimuli, /ba/ and /da/. These two tapes are referred to as












phonetic tapes. Two phonetic tapes having different orderings of

stimuli were employed to reduce the possibility of the subjects

becoming familiar with the order of the stimuli.

The remaining two audio tapes contained the same two nonverbal

stimuli, but the speech stimuli were the low-pitched /ba/ and the

high-pitched /ba/. These tapes are referred to as the nonphonetic

tapes. Again, two tapes were prepared to reduce subjects' familiari-

zation with the order of the stimuli.

The low frequency and high frequency nonspeech stimuli are

referred to throughout this paper as T1 and T2, respectively. The

low /ba/ is referred to as CV1. CV2 refers to the remaining

consonant-vowel stimuli, i.e., the low /da/ on the phonetic tapes

and the high /ba/ on the nonphonetic tapes. Responses were averaged,

using Digital's Basic Averager, to two evoking stimuli--CV1 and T .

All stimuli were played on a Sony stereo tape recorder, Model

TC-353D and presented to the subjects binaurally at 35 dB SL through

Grason-Stadler TDH-39 earphones.

The evoked potentials were recorded using Grass silver-silver

chloride electrodes and Grass electrode paste. The signals were

amplified by calibrated Grass P511 amplifiers with a gain of 20,000

and half-amplitudes of .1 Hz and 100 Hz. Responses, stimuli, and

synchronized pulses were all recorded onto separate channels of an

Ampex FM tape recorder, Model Fr-100A. Averaging to CV1 was on

line; averaging to Tl was off line, without further filtering.

Averages were plotted on a Houston Omnigraphic X-Y plotter.












Procedure

Subjects were tested individually over three separate sessions.

They were seated comfortably in a reclining chair in a sound-proof

IAC room. Recordings were taken from the vertex (Cz) and the left

and right temporo-parietal scalp locations (midway between T3 and

P3 on the left and T4 and P4 on the right) after carefully prepar-

ing the site and applying the electrodes. The active sites were

prepared by removing the hair at the location, rubbing briskly

with acetone, rubbing the area with a small amount of paste, apply-

ing the electrodes and securing them with collodion. The removal

of a few strands of hair at each site not only lowered the resist-

ance but also guaranteed precise repositioning of the electrodes

in subsequent sessions. The scalp locations were referenced to

linked mastoids which were prepared similarly but with the elec-

trodes secured with tape. The resistance between each pair of

electrodes was kept below 5000 ohms and approximately equal (within

1000 ohms) for each pair. The subject was grounded with a mid-

forehead electrode.

The same amplifier was used for the vertex recordings for all

subjects. However, to control for any slight differences between

the amplifiers used for the hemispheric recordings the leads from

half the subjects were reversed so that each of the two remaining

amplifiers received signals from the left hemisphere for half the

subjects and the right hemisphere for the other half of the subjects.












Each subject participated in eight conditions of stimulus

relevance (four with the phonetic tapes and four with the non-

phonetic tapes) and two control conditions (one with each type of

tape). The ten conditions are summarized in Table 1.

Prior to each condition of stimulus relevance, subjects were

instructed as to the identity of the relevant stimulus for that

trial. Subjects were told that they should listen selectively for

the relevant stimulus and ignore all other stimuli. Half of the

subjects responded, following the presentation of a relevant

stimulus, by depressing with their index fingers a small button

mounted on a box on the arm of their chair. The remaining half of

the subjects responded with a similar button-push following all

irrelevant stimuli and withheld a response following each presen-

tation of the relevant stimulus. To avoid the contamination of

the evoked response by muscle activity or motor potentials, subjects

were instructed to wait from 1 to 2 sees following the stimulus

before responding. In order to equalize the responses between the

left and right hands, midway through each trial (following presen-

tation of 128 stimuli) the experimenter switched the response box

from one arm of the subject's chair to the other. Also during

midtrial, the experimenter reversed the subject's headphones so

that the effect of any possible difference between the earphones

would be equalized.

In communicating to the subjects, the experimenter identified

the relevant stimulus for a given trial as the "low tone," "high












Table 1

Summary of Experimental Conditions


Task

Relevant
Stimulus Phonetic Nonphonetic


Evoking Stimulus: CV1


Control


Evoking Stimulus: T1


T

T2

CV

CV
2
Control


Note: Dependent variables in all conditions were amplitude and
latency measures for N1, P2, N2, and P3 components of the evoked
potential recorded from vertex, left temporo-parietal, and right
temporo-parietal sites. In every condition the subject was
presented with a series of 256 stimuli (64 each of CV1, CV2, T1,
and T2) and potentials were recorded to CV1 and T1.








30

tone," "low /ba/," "low /da/" (on phonetic tapes), or "high /ba/"

(on nonphonetic tapes).

During the control conditions subjects were instructed to

read and not attend to any of the stimuli. No response was

required of the subjects during the control conditions.

Half the subjects were presented first with the phonetic

tapes and second with the nonphonetic tapes. This order was

reversed for the other subjects. To minimize the effects of

fatigue or habituation on the evoked potentials, the temporal order

of the stimulus relevance conditions was controlled. For a given

subject the temporal position of the "CV1 relevant" condition, the

"T1 relevant" condition, and the control condition were identical

for phonetic and nonphonetic tape presentations and the CV2

relevant" and "T2 relevant" conditions were never more than one

position removed for the two types of tapes. The actual orderings

of conditions for the subjects are shown in Appendix B.

Each experimental session lasted from two to three hours:

approximately 30 minutes for application of electrodes and about

90 minutes to 2 hours of actual recording. Subjects were given a

15 to 20 minute break following the first 60 minutes of recording.

Prior to each recording session subjects were instructed to

remain very still, not to swallow, and not to blink during the

recording sessions. If it was absolutely necessary for them to do

one of these things they were to do it at the same time they pushed









31


the button (between the presentation of stimuli). During the

conditions of stimulus relevance subjects fixated on a picture or

an object so that eye potentials would not contaminate the evoked

responses.
















RESULTS


Auditory evoked potentials to a speech stimulus (CV1) and a

nonspeech stimulus (TI) were recorded from vertex, left temporo-

parietal, and right temporo-parietal locations. The data were

analyzed for topographical differences, effects of phonetic process-

ing, and effects of stimulus relevance. Peak amplitudes of the

NI' P2' N2, and P3 components, the NI-P2 peak-to-peak amplitude,

and the latencies of the various components were measured. The

6 msec following stimulus onset was used as the baseline from

which the peak amplitudes were measured. Components were defined

as follows: N1 was the most negative point occurring between 80

and 140 msec following stimulus onset; P2 was the most positive

point occurring between 170 and 240 msec following stimulus onset;

P3 was the most positive point between 260 and 420 msec post-

stimulus onset; and N2 was the most negative point occurring

between P2 and P3.

To determine the significance of the differences among mean

amplitudes or latencies of the components recorded from the three

locations during phonetic and nonphonetic processing tasks, repeated

measures analyses of variance were computed for various condition

of stimulus relevance. Because of the design of the study, for each

evoking stimulus there is one condition in which the evoking stimulus












is the relevant stimulus (referred to as R/R); one condition in

which the evoking stimulus is an irrelevant stimulus but of the

same class (i.e. speech or nonspeech) as the relevant stimulus

(I/R); and two conditions in which the evoking stimulus is irrele-

vant and of the irrelevant class (I/I). The two I/I conditions

for the CV1 evoking stimulus are the T1 relevant and the T2 rele-

vant conditions; for the T1 evoking stimulus the two I/I conditions

are the CV1 relevant and CV2 relevant conditions. In order to

simplify the analyses and eliminate redundant information, the

second I/I condition of stimulus relevance was excluded. This

leaves a total of four conditions: R/R, I/R, I/I, and Control.

For the CV1 evoking stimulus these are the CV1 relevant, CV2

relevant, T1 relevant, and Control; for the T1 evoking stimulus

these are the T1 relevant, T2 relevant, CV1 relevant, and Control

conditions.

When appropriate, the Sign test (Siegel, 1956) was used in

addition, to determine the significance of the number of persons

showing a particular response characteristic. The findings of

these analyses are reported below.

Topographical Effects

Amplitude measures. Peak amplitudes of the N1 and P2 waves,

as well as the peak-to-peak N1-P2 amplitudes were substantially

larger at the vertex than they were at the lateral sites. This

effect is illustrated in Figures 1 and 2, which show the evoked









Relevant
Stimulus


L -1NZ


V

CV R

L





V
CV2 R ... -- --









T R

L









Control R

L


100 msecs


Figure 1. Averaged evoked potentials to CV1 stimulus recorded
from vertex (V), left (L), and right (R) temporo-parietal
sites during various conditions. Each potential is a com-
posite of eight subjects. Calibration = 10 microvolts.











Relevant
Stimulus V IS


T R

L






V


T2 R

L







V


CV R


L







V


Control R

L



100 msecs

Figure 2. Averaged evoked potentials to T1 stimulus recorded
from vertex (V), left (L), and right (R), temporo-parietal
sites during various conditions. Each potential is a com-
posite of eight subjects. Calibration = 10 microvolts.












potentials recorded from vertex, left temporo-parietal and right

temporo-parietal locations during each condition of stimulus

relevance. Figures 1 and 2 show the potentials evoked by CV1 and

TI, respectively. Each waveform in the figures represents a

composite of responses from all subjects in both phonetic and non-

phonetic conditions. The actual values of the amplitudes of the

various components for each location are presented in Table 2.

The differences between the vertex responses and the temporo-

parietal responses are significant at the .01 level of probability

for every condition of stimulus relevance as well as for the control

conditions. Results of Tukey's a posteriori test (Kirk, 1968) com-

paring the mean amplitudes of the N1, P2, or N -P2 components

recorded from the left location with those of the right location

were all nonsignificant. No significant topographical differences

were obtained among the three recording sites for the N2 or P3 com-

ponents in any condition.

Although the overall mean differences between the N1, P2, and

N-P2 amplitudes between the left and right hemispheres did not

differ significantly, in most conditions the majority of persons

did show asymmetries with larger amplitudes on the left. Table 3

presents the number of persons in each condition whose left

hemisphere amplitudes were larger than the right hemisphere ampli-

tudes. Although the majority of persons did show a larger left

hemisphere response, particularly when CV1 was the relevant












Table 2

Mean Amplitudes (microvolts) for Major Components
of Evoked Responses Recorded from Three Locations


Evoking Stimulus



CV1 T1
Com- Relevant Relevant
ponent Stimulus Va L R Stimulus V L R


N CV1 10.5 8.5 8.1 T1 12.3 9.7 9.5
11 1

CV2 9.4 8.3 7.4 T2 13.4 10.4 10.1

T1 9.6 7.7 7.7 CV1 13.2 10.3 9.5

Control 9.6 8.2 7.9 Control 12.1 10.0 8.9


P CV 10.1 6.9 6.5 T1 13.1 8.3 7.7

CV2 9.1 6.2 6.3 T2 12.5 8.7 7.7

T 10.5 6.7 6.6 CV1 11.9 7.6 7.3

Control 6.2 4.1 4.3 Control 9.0 5.7 5.7


Nl-P2 CV1 20.6 15.4 14.6 T1 25.4 18.0 17.2

CV2 18.5 14.5 13.7 T2 25.9 19.1 17.8

T1 20.1 14.4 14.3 CV1 25.1 17.9 16.8

Control 15.8 12.3 12.2 Control 21.1 15.7 14.6













Table 2 continued


Evoking Stimulus


CV1 T1
Com- Relevant Relevant
ponent Stimulus Va L R Stimulus V L R


N2 CV1 .7 .5 .8 T1 .7 -.1 -1.5

CV2 -.2 -.2 .1 T2 -1.9 -1.1 -1.0

T1 .9 .7 .7 CV1 -1.1 -1.0 -.8


P3 CV1 4.2 4.5 4.9 T1 4.8 3.4 4.4

CV2 2.1 2.3 3.2 T2 3.3 2.4 2.5

T1 3.5 3.6 3.9 CV1 1.4 2.2 3.4


Note: A positive amplitude for N2 means that the most negative
peak between P2 and P3 was above the baseline.

aV=Vertex; L=Left Temporo-Parietal; R=Right Temporo-Parietal.














Table 3

Number of Persons Having Greater Left Hemisphere
Amplitudes for Various Components


Component

Relevant
Stimulus N1 P2 N-P2


Evoking Stimulus: CV1


CV1 6 6 6

CV2 6 3 5

T1 5 5 4

Control 5 4 4


Evoking Stimulus: T1


T1 4 5 6

T2 5 7 6

CV1 6 6 6

Control 5 3 6












stimulus, in only one condition did the Sign test on this frequency

data actually reach significance. This condition is the one in

which T2 was relevant and the evoking stimulus was T In this

condition a significant number of persons (seven out of eight) had

a larger P2 amplitude on the left than on the right.

To provide an indication of the degree of asymmetry which

occurred for each subject, the procedure described by Matsumiya

et al. (1972) and later employed by others (Friedman et al., 1975a;

Molfese et al., 1975) was used. This procedure entails calculating

R-values for each subject by dividing the left hemisphere amplitude

by the sum of the amplitudes of the left and right hemispheres.

Thus, an R-value greater than .5 indicates a larger left hemisphere

N1-P2 response while a value less than .5 indicates a larger right

hemisphere response. R-values for each subject are shown in the

upper half of Figure 3. Each R-value shown is a composite of all

conditions.

Since R-values are simply ratios, however, they do not provide

a clear indication of the degree of asymmetry. For example, a

2 microvolt difference between the left and right amplitudes would

yield substantially larger R-values if the amplitudes were around

5 microvolts than if they were around 20 microvolts. Also, a

person whose left hemisphere amplitude is 3 microvolts larger

than the right but whose amplitudes are 20 and 17 microvolts for

the left and right hemispheres, respectively, would have an R-value

of .55 while a second person having only a 1 microvolt difference







41

EVOKING
STIMULUS

.55 CV

.54

.53

.52 L>R

.51

.50




S49.4 R>L

.47

.46




1 2 3 4 5 6 7 8 X

SUBJECT
2.4

2.0

1.6

u 1.2

.8
1 L>R


Ez O

W -.4 R>L

, -.8

-1.2

-1.4

1 2 3 4 5 6 7 8 X

SUBJECT

Figure 3. R-values (upper) and hemispheric voltage differences
(lower) for all subjects across all conditions. A larger left
hemisphere amplitude is represented by an R-value .5 or a posi-
tive voltage difference.













would have the same ratio if his left and right amplitudes were

about 7 and 6 microvolts, respectively. Because of this fact,

it was desirable to calculate actual magnitudes of the hemisphere

differences as indices of asymmetries in addition to the R-values.

Thus, shown in the lower half of Figure 3 are the differences between

the mean left hemisphere N -P2 amplitude across all conditions

and the mean right hemisphere N1-P2 amplitude across all conditions.

It is comforting to see that the two halves of the figure agree

fairly well with respect to individual subjects' asymmetries, but

it is the lower half which provides information regarding the abso-

lute magnitude of each person's asymmetry.

Latency measures. In general, the latencies for the N1 and P2

components of the responses recorded from the vertex were slightly

shorter than those recorded from the lateral sites. Across all

conditions the mean N1 latency for vertex, left, and right temporo-

parietal locations are 107, 108, and 109 msec, respectively, when

CV1 was the evoking stimulus; and 106, 109, and 110 msec when T1

was the evoking stimulus. The corresponding latencies for P2 are

200, 204, and 205 msec for CV ; and 190, 194, and 195 msec for T1.

Latencies for individual conditions of stimulus relevance are

presented in Table 4. The analyses of variance failed to show

any significant differences in the mean latencies for the three

recording sites, but it is notable that in no condition were the

mean N1 or P2 latencies for either of the temporo-parietal sites

less than that for the vertex.














Table 4

Mean Latencies (msec) for Major Components
of Evoked Responses Recorded from Three Locations


Evoking Stimulus


CV1 T1
Com- Relevant Relevant
ponent Stimulus Va L R Stimulus V L R


N CV1 107 107 109 T1 106 108 107

CV2 108 110 110 T2 107 109 110

T1 106 107 108 CV1 106 109 110

Control 106 107 107 Control 106 108 111


P2 CV1 201 203 204 T1 189 192 190

CV2 199 201 204 T2 188 193 196

T1 201 205 208 CV1 190 198 196

Control 200 208 204 Control 192 194 197


N2 CV1 278 274 272 T1 276 278 279

CV2 290 287 282 T2 285 279 282

T1 292 288 283 CV1 284 277 276


P3 CV1 325 335 338 T1 328 336 335

CV2 320 323 326 T2 334 334 339

T1 326 327 326 CV1 324 329 329


aV=Vertex; L=Left Temporo-Parietal; R=Right Temporo-Parietal.













The mean latencies for the P3 component of the evoked potentials

were also shorter for the vertex than for the left or right

locations, but these differences also failed to reach statistical

significance. In contrast to the other components, the latencies

of the N2 wave were generally longer when recorded from the vertex.

The latency values for N and P are shown also in Table 4.
2 3
No consistent latency differences occurred between the left and

right recording sites for any of the components.

Phonetic versus Nonphonetic Tape Effects

Amplitude measures. The mean amplitude of the various compo-

nents of potentials evoked by CV1 and T1 on the phonetic tapes and

the nonphonetic tapes are shown in Table 5 for each condition of

stimulus relevance. The values in that table represent the mean

amplitudes for the three recording sites across all eight subjects.

The only differences that proved to be significant occurred during

the condition in which subjects were discriminating the CV1 stimulus

from the other three stimuli. In that condition, the amplitude

of the N1 component was significantly larger during phonetic

processing of the speech stimuli (10.3 microvolts) than during non-

phonetic processing of them (7.8 microvolts). This difference is

significant at the .05 level of probability, F (1,7) = 10.90.

As described under the procedure section of this paper, the

evoked response to the nonspeech stimulus, Tl, was averaged during

the same series of stimulus presentations as was the response to













Table 5

Mean Amplitudes (microvolts) for Major Components of Potentials
Evoked by Stimuli on Phonetic and Nonphonetic Tapes




Evoking Stimulus


CV T1
Com- Relevant Relevant
Ponent Stimulus Pa N Stimulus P N


N1










P2









N -P
1 2


CV1

CV2

T1

Control


CV1

CV2

T1

Control


CV1

CV2

T1

Control


10.3

8.1

8.3

9.7


6.6

7.4

8.2

3.9


16.9

15.5

16.5

13.6


7.8

8.7

8.5

7.3


9.0

7.0

7.7

5.8


16.8

15.7

16.2

13.1


T1

T2

CV1

Control



T1

T2

CV1

Control


T
1

T2

CV1

Control


10.5

12.0

11.0

10.6


10.1

9.1

9.1

6.2


20.6

21.1

20.1

16.8


10.5

10.5

10.9

10.1


9.6

10.2

8.8

7.4


20.1

20.7

19.7

17.5













Table 5 continued


Evoking Stimulus


CV1 T

Com- Relevant Relevant
ponent Stimulus pa N Stimulus P N


N2 CV1 0.0 1.3 T1 1.0 -1.6

CV2 0.3 -0.5 T2 -2.3 -0.4

T 1.4 0.1 CV -0.8 -1.2


P3 CV1 4.0 5.0 T 4.9 3.8

CV2 2.4 2.6 T2 2.7 2.8

T1 4.5 2.8 CV1 2.8 1.9


Note: A positive amplitude for N2 means that the most negative
peak between P2 and P was above the baseline.

ap=Phonetic Tape; N=Nonphonetic Tape.












CV1; yet there was virtually no difference between the N1

amplitudes of potentials evoked by T1 on the phonetic tapes and

on the nonphonetic tapes. The mean amplitudes are 11.0 and 10.9

microvolts, respectively, F (1, 7) = .02, p>.05.

In the same condition, a similar effect occurred with the

amplitude measure of the P2 component, but in the opposite direc-

tion. During phonetic processing the mean peak amplitude of the

response to CV1 was 6.6 microvolts and during nonphonetic proces-

sing it was 9.0 microvolts,F (1, 7) = 10.51, p4.05. The cor-

responding amplitudes of the potentials evoked by Ti are 9.1 and

8.8 microvolts,F (1, 7) = .15, p>.05.

None of the other components of the potentials were found to

be significantly different when evoked by stimuli on the phonetic

tapes as compared to when evoked by stimuli on the nonphonetic

tapes.

Latency measures. Latencies of components elicited by stimuli

on the two types of tapes are shown in Table 6. The only signifi-

cant difference between the phonetic and nonphonetic processing

tasks was in N latency and occurred for the CV evoking stimulus
1 1
when CV1 was the relevant stimulus. Across all subjects, the mean

latencies for N1 are 110 and 105 msecs for the two tasks, respectively,

F (1, 7) = 6.23, p<.05. In contrast, the corresponding latencies

for responses evoked by T1 were not significantly different from

each other, F (1, 7) = .44, p>.05. These values are 109 msecs and

108 msecs for phonetic and nonphonetic tapes, respectively.













Table 6

Mean Latencies (msec) for Major Components of Potentials Evoked
by Stimuli on Phonetic and Nonphonetic Tapes


Evoking Stimulus


CV1 T

Com- Relevant Relevant
ponent Stimulus pa N Stimulus P N


CV1

CV
2

T

Control


CV1

CV2

T1

Control


CV1




T1


CV1

CV2

T
1


T1

T
2
CV1

Control



T1

T2

CV1

Control



T1

T2

CV1



T1

T2

CV1


106

110

108

107


188

194

194

193


274

284

278


339

337

327


a=Phonetic Tape; N=Nonphonetic Tape.











49

A significant interaction occurred between processing require-

ments and recording locations during the condition in which CV1

was relevant and CV1 was the evoking stimulus, F (2,14) = 5.19,

p<<05. This interaction is illustrated in Figure 4. It can be

seen from that figure that the latency difference between phonetic

and nonphonetic processing was greatest when recorded from the left

hemisphere, moderate from the vertex, and least from the right hemi-

sphere.

No significant latency differences were obtained for the other

evoked potential components in that condition or for any of the

components in other conditions of stimulus relevance.

Other measures. The procedure described by Wood et al. (1971)

and subsequently by Wood (1975) and by Galambos et al. (1975), was

employed in the present study to test for differences in the general

shape of the waveforms obtained during phonetic and nonphonetic

processing of stimuli. For each hemisphere, a Wilcoxen matched-

pairs signed-ranks test (Siegel, 1956) was performed on the digi-

tized output voltages at each of 180 time points over a duration

of 540 msecs. Each time point'represents an interval of 3 msecs.

To reduce the probability of a Type I error, a .02 probability

level was chosen for the Wilcoxen tests. Since 180 tests were

performed for each pair of averages, it would be expected that

approximately four of the tests for each pair would reach signifi-

cance on the basis of chance.


















TASK

PHONETIC

NONPHONETIC


Il- r -, '


RIGHT
RECORDING SITE


LEFT


Figure 4. Latencies for N1 component of potentials
during phonetic and nonphonetic processing tasks.


evoked by CV1


112



110


108,

106
106'
S


102


100


VERTEX













The pairs of averages on which the Wilcoxen tests were per-

formed are shown in Figure 5. The upper tracings in each of the

four sections of that figure are averages of potentials recorded

from the left hemisphere; the lower tracings are from the right

hemisphere. The solid lines are averages of potentials evoked by

stimuli on the phonetic tapes; the dotted lines by stimuli on

the nonphonetic tapes. Each pair of averages is anchored at the

6 msec baseline.

Figure 5a and 5b show averages recorded during the conditions

in which CV was the relevant stimulus. In Figure 5a are potentials

evoked by CV1; in Figure 5b are potentials evoked by T1. Figure 5c

and 5d show corresponding averages obtained during the conditions

when T1 was the relevant stimulus.

It can be seen in Figure 5a that, while the right hemisphere's

responses to CV1 are virtually identical when evoked by stimuli on

phonetic and nonphonetic tapes, the left hemisphere's responses

to those stimuli are relatively dissimilar. The absolute difference

obtained is not a large one, only about 2 microvolts. But considering

that the mean peak-to-peak amplitudes are about 13 microvolts, this

represents a 15 percent difference. This difference suggests that

the left hemisphere's activity in response to a speech sound differs

when the stimulus is processed phonetically. There is little differ-

ence in either hemisphere's response to T1 in that condition.

Table 7 shows the number of time points at which differences

between the potentials evoked by phonetic and nonphonetic tapes










EVOKING


LUS

a. L rc 4t


R *.

0*


RELEVANT
STIMULUS


CV


b. L



R


c. L



R "' "' -,-








d. L



R

1


phonetic
nonphonetic .....


100 msecs


Figure 5. Averaged evoked potentials recorded from left and
right hemispheres during phonetic and nonphonetic tasks.
Calibration = 10 microvolts.


ST'IML


CV1












T













Table 7

Significant Wilcoxen Tests between Potentials Evoked by Stimuli
on Phonetic and Nonphonetic Tapes


Hemisphere

Evoking
Stimulus Left Right


Relevant Stimulus: CV



CV1 35 6


T 0 0



Relevant Stimulus: T



CV1 0 1


T1 4 0












were found to be significant. It can be seen that the left hemi-

sphere's response to the CV1 stimulus on the two tapes differed

significantly at several points along the time continuum during

the condition when CV1 was relevant. This is particularly true

in the region of the P2 component--18 significant differences

occurred between 170 msecs and 240 msecs following stimulus onset.

Not only is 35 significant points out of 180 substantially above

the chance number of four, but because the significant points tend

to be clustered in certain regions of the waveform and not distri-

buted randomly buttresses the validity of the difference.

In contrast, the number of points which were significantly

different for the right hemisphere's averages in the CV1 relevant

condition for the CV1 evoking stimulus is six, very close to chance

expectation. There were no significantly different points for

either the left or the right hemisphere responses evoked by T1.

Comparable tests were computed when T1 was the relevant

stimulus. The results of these tests are shown in Table 6 also.

It can be seen that neither hemisphere responded differentially to

the CV1 stimulus when T1 was relevant. In other words, when

T1 was the stimulus to be discriminated it made no difference

whether the speech stimuli on the two tapes were phonetically dif-

ferent or only acoustically different. Again, few differences

occurred between the averages evoked by T1. Figure 5c and 5d

illustrate these findings.












Effects of Stimulus Relevance

Amplitude measures. Amplitudes of vertex potentials were

analyzed for the general effects of stimulus relevance after com-

bining the data for phonetic and nonphonetic conditions. Amplitudes

of the components are presented in Table 8.

Significant main effects due to stimulus relevance were obtained

in the analyses of the N1-P2 peak-to-peak amplitudes and of the P2'

N2 and P3 peak amplitudes. The values are shown in Table 8. In

that table the column labelled R/R shows data obtained when the

evoking stimulus, either CV1 or TI, was the relevant stimulus; I/R

shows data obtained when the evoking stimulus was an irrelevant

stimulus but of the same class (i.e. speech or nonspeech) as the

relevant stimulus; I/I shows data when the evoking stimulus was

irrelevant and of the irrelevant class.

Results of statistical tests for the various components of the

potentials are summarized below.

1. N -P2 components. Results of Tukey's a posteriori tests

showed that the mean amplitude of the N1-P2 component was signifi-

cantly smaller in the control conditions than in all of the conditions

of stimulus relevance, p<.01. The three stimulus relevance condi-

tions did not differ significantly from one another. Since no

difference among conditions was obtained for Nl, the differences in

N -P2 are due to changes in P2 in the various conditions.

2. P component. The P component of the control conditions
was significantly smaller thanin the other conditions, .01, but
was significantly smaller than in the other conditions, p<.01, but













Table 8

Mean Amplitudes (microvolts) of Evoked Potential Components in Condi-
tions of Stimulus Relevance


Condition
Com-
ponent R/Ra I/Rb I/Ic Control F(3, 21) p


N1 11.4 11.5 11.4 10.8 .49 n.s.

P2 11.6 10.8 11.1 7.6 9.10 e.001

N1-P2 23.0 22.3 22.5 18.4 8.94 <.001

N2 .7 -1.0 -1.2 -2.7 6.26 <.01

P3 4.5 2.7 2.4 1.1 9.78 <.001



Note: A positive amplitude for N2 means that the most negative
peak between P2 and P3 was above the baseline.

aRelevant stimulus, relevant class.
bIrrelevant stimulus, relevant class.
CIrrelevant stimulus, irrelevant class.












still no differences among the three conditions of stimulus rele-

vance were obtained. Results of Sign tests on the number of persons

having larger P2 amplitudes during the stimulus relevance conditions

than during the control conditions confirm the findings of the

analyses of variance: all eight persons had larger P2 amplitudes

in the R/R and the I/I conditions than in the control, p<.01;

seven of the eight subjects had larger P2 amplitudes in the I/R

condition than in the control, p<.05.

3. N2 component. Although no clear N2 or P3 components were

evoked during control conditions, for the sake of demonstrating

the differences between the control and other conditions, values

for these components were determined according to the procedure

defined earlier in this section of the paper. Tukey's test showed

the mean amplitude of N2 to be significantly different when evoked

by a relevant stimulus than in the control condition, p<.01. N2

potentials elicited by stimuli in the I/I condition differed from

the control at the .05 level, but those in the I/R condition did

not reach significance at that probability level.

All eight persons had more negative N2 values in the control

condition than in the R/R condition (p<.01) and seven out of eight

persons had more negative N2 values in the control than in the I/R

or the I/I conditions (p405).

4. P3 component. The amplitude of the P3 component showed

the greatest differential effect across the conditions of stimulus

relevance. The mean amplitudes are 4.5, 2.7, 2.4, and .1 for R/R,












I/R, I/I, and control conditions, respectively. Tukey's tests

showed that the mean P3 amplitude in Condition R/R was significantly

different from the control (p<.01); mean amplitude in Condition I/R

was significantly different from the control (p<.05); but the dif-

ference between I/I amplitude and the control failed to reach

significance. Differences among means of the stimulus relevance

conditions failed to reach significance.

The group averages to CV1 and T1 evoking stimuli are shown in

Figure 6. Because of differences in latency of P3 for different

subjects, however, the group averages do not provide accurate repre-

sentations of the individual subjects' responses, which are shown

in Figures 7 and 8. It can be seen from these last two figures

that some subjects (e.g. Subject 2) gave relatively large P3

responses while others exhibited small, if any, P3's (e.g. Subject

4).

In evaluating the number of persons whose P3 amplitudes

showed certain effects, it was found that all eight had larger P3

amplitudes in Conditions R/R and I/I than in the control condition

(p<.01), and seven out of eight persons had larger P3 amplitudes

in the I/R condition than in the control condition (p<.05). Also,

seven out of the eight persons had larger P3 amplitudes in the

R/R condition than in the I/I condition (p<.05). Six persons had

larger P3 amplitudes in the R/R condition than in the I/R condi-

tion, but this frequency does not quite reach significance.












Latency measures. The only significant latency difference

occurred with the N2 component. It was found that the mean latency

was significantly shorter during conditions when the evoking

stimuli were relevant (277 msecs) than in the control condition

(294 msecs),p .05. The N2 latency in the R/R condition did not

differ significantly from the latency in the I/R conditions or

the I/I conditions, both of which were 288 msec.

In addition to all the data reported above, partial data were

collected on two other subjects. These data are summarized in

Appendix C. Though the data are incomplete they are consistent

with the effects reported above.















LATENCY (msec)


300


RELEVANT
STIMULUS


CV1


CONTROL






T1


CV1


CONTROL


EVOKING
STIMULUS






CV1


Figure 6. Averaged evoked potentials recorded from the vertex
to CV1 (upper) and T1 (lower) evoking stimuli during conditions
of stimulus relevance. Calibration = 10 microvolts.








LATENCY (msec)

150 / \ 300 45


RELEVANT
SUBJECT STIMULUS
R/R

I/R

I/I
Control


0


R/R

I/R

I/I
Control


R/R

I/R

I/I

Control


R/R


I/I
Control


Figure 7. Averaged evoked potentials recorded from Subjects 1 4
during conditions of stimulus relevance. Calibration = 10 microvolts.


0O


~---z


rx, I


~---



~-c~









LATENCY (msec)


RELEVANT
SUBJECT STIMULUS
R/R
5 I/R
I/I

Control


R/R
I/R


I/I

Control



R/R


I/R


I/I

Control







R/R
I/R


I/I

Control


450


Figure 8. Averaged evoked potentials recorded from Subjects 5 8
during conditions of stimulus relevance. Calibration = 10 microvolts.


150 3( 3


]Z~



Clhl~-


:\\Jj

















DISCUSSION


This study was designed to investigate the effects of phonetic

and nonphonetic processing of stimuli on the averaged evoked poten-

tial recorded from the left and right hemispheres. A procedure was

chosen to allow the following:

1. a comparison of the right hemisphere's responses with the

left hemisphere's responses when the evoking stimulus was a speech

signal and when it was a nonspeech signal;

2. an estimation of the relative degree of the hemispheric

asymmetries within subjects (i.e. R-values; voltage differences)

when potentials are evoked by speech and nonspeech stimuli;

3. a comparison of the responses to stimuli presented during

a phonetic processing task and a nonphonetic processing task; and

4. the investigation of a possible lateralized cortical acti-

vation in preparation for phonetic processing.

The procedures employed to effect these four objectives

entailed presenting both speech and nonspeech stimuli, intermixed

in a quasi-random order, to subjects engaged in a series of dis-

crimination tasks. Given this procedure, it was desirable also to

evaluate the effects of stimulus relevance and irrelevance on the

components of the evoked potential.













The data pertaining to these issues are discussed below.

Topographical Effects

Hemispheric Asymmetries. The mean amplitudes of the various

components of the evoked responses recorded from the left and right

hemispheres did not differ significantly from each other when either

the speech or the nonspeech stimulus was the evoking stimulus. In

this respect, the present data agree with those reported by Galambos

et al. (1975). On the other hand, the finding of no left-right dif-

ferences disagrees with data reported by Morrell and Salamy (1971)

who obtained significantly larger mean N1 amplitudes from the left

than from the right. It is unclear why the latter investigators'

data are discrepant with those from the study by Galambos and from

the present study. In the last two investigations, the experi-

menters reported taking extreme precautions in calibrating apparatus

and controlling for extraneous factors such as balancing the use of

amplifiers, etc. Morrell and Salamy did not report any such controls.

Although no significant differences occurred between the two

lateral recording sites, the mean amplitudes of the N1 and P2 waves

recorded from the vertex were substantially larger than those of

the left and right hemispheres. This finding substantiates an

effect that has been reported several times previously (e.g. Vaughan

& Ritter, 1970).

Degree of Asymmetry. With respect to the R-values reported in

the present investigation, there seems to be a discrepancy with the












data reported by Molfese et al (1975). In both studies, most sub-

jects had R-values greater than .5 (indicating larger left hemi-

sphere N1-P2 responses) when a speech signal was the evoking stimulus.

However, Molfese reported smaller R-values (less than .5) to non-

speech stimuli whereas this effect was not seen in the present study.

The data reported here show that most subjects had slightly larger

left hemisphere responses to all stimuli, although this effect

failed to reach significance.

The finding of a larger left hemisphere response to nonspeech

stimuli is not unprecedented, however. The data presented by

Matsumiya et al. (1972) showed that more than half their subjects

had R-values greater than .5 in nonspeech conditions.

Again, the reason for the discrepancy among the results of

investigations by Molfese, Matsumiya, and the present author are

unclear. However, it is possible that it could be due to the fact

that the subjects in Molfese's study listened passively throughout

the experimental session while those in the other two studies were

involved in tasks. If the meaningfulness of a stimulus influences

hemispheric asymmetry (as Matsumiya has suggested), then it would

seem that the data from the three studies do not necessarily

conflict. In fact, the finding reported by Friedman et al. (1975a)

that a significantly larger number of persons showed greater left

hemisphere N1 responses than right hemisphere responses only when

words were "signal" stimuli and not during "non-signal" or "no

task" conditions supports this explanation.













If meaningfulness does influence hemispheric asymmetry, one

would expect to find significantly more subjects in the present

study showing larger R-values in the conditions of stimulus rele-

vance where stimuli are meaningful, then in the control conditions

where they are not attended. The N -P data presented in Table 2

do show such a trend: the mean number of persons (across both

evoking stimuli) having larger left hemisphere responses in the

R/R, I/R, I/I, and Control conditions are 6.0, 5.5, 5.0, and 5.0,

respectively. Though the differences are small, their direction is

in accord with what would be predicted if meaningfulness is a

variable influencing degree of hemispheric asymmetry.

To accommodate the data from previous studies as well as the

present one, it is necessary to postulate that two factors, stimulus

class and stimulus relevance, interact in determining the direction

and degree of asymmetry. The nature of a speech stimulus may be

sufficient by itself to invoke the specialized activity of the left

hemisphere in a passive listening situation as demonstrated by

Molfese et al. (1975), but increases in the significance or meaning-

fulness of a stimulus, even a nonspeech stimulus, also seems to

enhance left brain activity.

Although the majority of persons in this and other studies have

been found to exhibit larger left hemisphere responses to speech

stimuli as compared to right hemisphere responses, it should be

noted that the asymmetrical effect can be lost in the evaluation of













group data. Since the magnitude of the asymmetry is quite small,

the inclusion in an analysis of variance of data from even a

couple of subjects who fail to show the effects may be sufficient

to prohibit the rejection of the null hypothesis, particularly when

the number of subjects in the analysis is small. Galambos et al.

(1975), who also found little differences in group data, noted

some substantial hemispheric asymmetries in certain individuals.

Phonetic versus Nonphonetic Effects. Regarding the differences

obtained during phonetic and nonphonetic processing of stimuli,

there appears to be a shift toward negativity in the potential of

the left hemisphere when elicited by a speech stimulus during

phonetic processing. This finding confirms the data reported by

Wood and his colleagues (Wood, 1975; Wood et al., 1971), and extends

this observation to the use of natural speech syllables. Since the

two tasks differed only in their processing requirements, the

differential response of the left hemisphere may reflect phonetic

analysis. Because a comparable shift does not occur in the right

hemisphere potential, Wood (1975) proposed that the left hemisphere's

potential change reflects the activity of a specialized neural center

lateralized on that side. This issue is addressed in greater detail

below.

Evaluation of the Lateralized Preparatory Activation Hypothesis.

There is an alternative explanation for Wood's data. If, as proposed

by Kinsbourne (1970), there is a preparatory arousal of the left

hemisphere for processing verbal stimuli, then this might be reflected












in the evoked potential and could account for the results of Wood's

studies. In the present investigation, however, the subjects

had to develop a cognitive set in order to make the appropriate dis-

criminations and responses. In the condition requiring phonetic

processing of speech stimuli subjects needed to maintain a "verbal"

or "linguistic" set, yet not all stimuli were speech. However, if

there is a preliminary cortical arousal lateralized on the left

and if this were responsible for the differential left hemisphere

potential, then there should have been a differential response to

the nonspeech stimulus as well as to the speech stimulus in the

present study. Since this did not occur, it seems that Wood's

interpretation is a valid one.

Though it can be concluded that the asymmetric activity which

occurs between the two tasks cannot be explained by Kinsbourne's

selective activation hypothesis, it is not possible to eliminate

that theory as a viable explanation for perceptual asymmetries

which occur in visual half-field or dichotic listening paradigms.

It simply shows that if such a functional asymmetry does exist, it

is not manifest in the auditory evoked response.

In addition to the negative shift in the potential of the

left hemisphere, other task differences occurred. These include

a larger N1 amplitude and smaller P2 amplitude evoked by the

speech stimulus in the phonetic task. Both of these differences

can be accounted for by the negative shift in the waveform.













Finally, there was a longer mean latency of the N1 component in

the linguistic task which perhaps is due to a brief delay in the

processing of stimuli requiring phonetic analysis.

Conclusions. Considering all existing data, two processes

seem to be reflected in the auditory evoked potential to speech

sounds. First, in most subjects there is a slight enhancement of

the N1-P2 amplitude on the left and it is elicited simply by the

occurance of a speech stimulus. Though there may be a corresponding

asymmetry for the right hemisphere for nonspeech stimuli (Molfese

et al., 1975), this asymmetry seems to be smaller or reversed when

subjects are engaged in discrimination or detection tasks. Since

the left hemisphere asymmetry occurs when subjects are listening

passively to speech stimuli (Morrell & Salamy, 1971; Molfese,

et al., 1975) it perhaps reflects the activity of a lateralized

neural center specialized for the detection of a speech signal.

The activity of a second mechanism is reflected by a negativity

in the evoked potential recorded from the left hemisphere which

occurs during phonetic processing of a speech stimulus. This per-

haps is due to a more sophisticated level of linguistic analysis

rather than the simple detection of a speech stimulus.

Recently, on the basis of dichotic listening data, it has

been suggested that there may be two neural mechanisms lateralized

in the left hemisphere (Cutting, 1974). In a series of experiments,

Cutting found that employing stimuli with frequency transitions













results in a significantly larger right ear advantage than using

stimuli without transitions. This was true for both speech

(syllables) and nonspeech (frequency modulated sine waves) stimuli.

It was also the case, however, that the speech stimuli gave larger

right ear advantages than nonspeech stimuli for both transitioned

and steady-state signals. Thus, Cutting concluded that there is

one left hemisphere mechanism which responds to acoustic changes

(i.e. frequency transitions) which are characteristic of speech

sounds, and a second mechanism which is responsible for phoneme

identification. It would seem that the former mechanism, providing

an acoustic analysis, would require no active linguistic processing,

whereas there would be no functional reason for the latter mechanism

to be activated unless speech processing does occur.

The two effects which have been seen in the evoked potential

data complement the dichotic listening data quite well in their sug-

gestion of two left hemisphere mechanisms. The NI-P2 amplitude

asymmetry may relate to specific auditory analysis, since it seems

to be related to conditions of passive listening. The negative

shift, also in the region of N1 and P2, is related to phonemic

analysis. Wood (1975) has demonstrated that a comparable shift

does not occur during various types of acoustic processing.

Since the two asymmetrical effects occur at similar regions

of the potential, it seems that both levels of processing, audi-

tory and phonetic, are able to occur simultaneously. This, too,

is consistent with current models of speech perception (e.g.

Wood, 1974).













In conclusion, given the assumption that phonetic analysis

was required when the "low /ba/" (CVl) was relevant and had to be

discriminated from the "low /da/" and from the nonspeech stimuli,

and given the assumption that phonetic analysis was not required

when the nonspeech stimuli were relevant or when the "low /ba/"

was relevant but had to be discriminated from the "high /ba/" and

the nonspeech stimuli, the data from the present study clearly sup-

port the contention that phonetic processing function of the left

hemisphere is reflected in the evoked potentials recorded from

the two hemispheres. The existence of the other mechanism,

influencing the N -P2 amplitude, was not directly supported by

data in the present study, but may be responsible for preliminary

acoustic analysis of incoming signals. At present the available

electrophysiological data on hemispheric asymmetries are not suffi-

cient for proposal of a definitive model of specific neural events

which occur during linguistic processing. Because of this the

present discussion has been restricted primarily to the findings

of the present study and their relation to existing data. Hope-

fully such a model will be possible in the future.

Stimulus Relevance

Evoked potentials were recorded to a speech and a nonspeech

stimulus during four conditions of relevancy: (a) when the evoking

stimulus was the relevant stimulus; (b) when it was an irrelevant

stimulus but of the same class as the relevant stimulus; (c) when

it was an irrelevant stimulus of the irrelevant class; and (d)













during the reading controls. Participation in the discrimination

tasks was found to enhance the N -P2 amplitude, primarily through

enhancing P2. The enhancement of these components during discrimi-

nation or detection tasks is a previously established finding (Davis,

1964; Hirsch, 1971).

The only component showing a difference among the three condi-

tions of stimulus relevance is P3. The finding that the amplitude

of N2 was not affected by the degree of relevance is in accord

with the hypothesis proposed by Ford et al. (1973) that N2 is related

to stimulus modality. Since all stimuli in the present study were

auditory, no difference in amplitudes is consistent with that

hypothesis.

Finally, the finding that the size of P3 seems to be ordered

the way they are supports a neural template model proposed by

Hillyard et al. (1971). In Hillyard's study, P waves were elicited

by "hits" in a signal detection paradigm (i.e. when a subject cor-

rectly reported the presentation of a signal in a noise background),

but not by "correct rejections" (when a subject correctly

reported the nonpresentation of the signal). In a later experi-

ment, Squires, Squires and Hillyard (1975) demonstrated that a P3

potential can be obtained to "correct rejections" under certain

circumstances, but if the probability of a signal being present in

a signal detection paradigm is .5, then the P3 is larger to the

"hits" than to the "correct rejections." Those authors attributed













this to the match of sensory input (which occurs during "hit"

trials) to a neural template for the signal.

In the present study, if subjects matched incoming sensory

signals to a stored template of the relevant stimulus, then the

best match would occur when the evoking stimulus was the relevant

stimulus, followed by the case where the evoking stimulus was at

least of the same class as the relevant stimulus, and finally by

the situation where the evoking stimulus was irrelevant and of the

irrelevant class. This is precisely the ordering of P3 amplitudes

obtained in this investigation, providing further support for a

template model of P3.













APPENDIX A

Table 9


Ordering of Stimuli on Tape 1


First Second Third Fourth
Quarter Quarter Quarter Quarter


CVI
CVl1
Tl
T2
CV2
CV1
T1
T2
CV2
T2
T2
CV2
CV1
T2
Tl
Tl
CV2
CV2
T2
Tl
T2
Tl
CV1
T1
CV2
CV1
T2
CV1
CV2
Tl
CV1
CV2


CV1
T1
T2
T2
T1
CV2
CV2
T1
CVI
CV1
T2
CV1
T1
CV2
CV2
T2
T2
CV2
Tl
T2
CV1
CV2
CV1
CV1
Tl
CV2
Tl
CV2
T2
Tl
T2
CV1


Tl
CV1
CV2
CV1
CV2
T2
T2
CV2
T2
CV1
CV2
Tl
CV1
Tl
T2
Tl
T2
CV2
CV2
CV1
Tl
CV2
T2
T1
Tl
T2
CVI
CV2
Tl
CV1
T2
CV1


CV2
Tl
CVI
Tl
T2
T2
T1
CV1
CVI
CV2
CV2
Tl
T2
T2
CV1
CV2
CV1
Tl
T2
T2
CV2
CVI
T1
T2
CV2
CVI
Tl
CV2
T1
CV1
CV2
T2


CV2
CV1
T2
CV2
CV1
Tl
CV1
CV2
T2
CV1
Tl
T2
Tl
CV2
Tl
T2
CV1
CV1
CV2
T2
Tl
Tl
T2
CV1
Tl
T2
CVI
CV2
Tl
T2
CV2
CV2


CV1
CV2
CV2
CVI
Tl
CV2
T2
T2
CV2
T2
CV1
Tl
Tl
CV1
T2
Tl
CV1
CV2
T1
T2
T2
Tl
CV1
CV2
T1
CV2
CV1
T2
CV2
CV1
T1
T2


CV2
CVI
Tl
CV2
T2
T2
CV2
CV1
T2
CV1
CV2.
T2
T1
T1
CV1
T1
T1
CV2
Tl
T2
CV1
T2
T1
CV2
CV2
CV1
Tl
T2
CV2
CV1
CV1
T2


CV2
T2
CV1
TI
CV2
T2
CV2
CV1
Tl
CV1
T2
CV2
T1
T2
CV1
Tl
T2
Tl
CV2
T2
T1
CV2
Tl
CV1
CVi
T2
T2
CV1
CV2
Tl
CV2
CV1


Note: A phonetic and a
ordering of stimuli.


nonphonetic tape was made with the above
















Table 10



Ordering of Stimuli on Tape 2


First Second Third Fourth
Quarter Quarter Quarter Quarter


T1 CV1 T2 T2 CV2 CV2 T1 CV1
CV2 Tl Tl CV1 Tl CV1 CV1 CV2
T2 CV2 CV2 Tl T2 T1 T2 CV1
CVI T2 T2 T2 T1 Tl CV2 T2
T1 T1 CVl CV2 T2 T2 CV2 Tl
T2 Tl CVl CV2 CV1 CV1 CV1 T2
T2 T2 CV2 T2 CV2 Tl CV2 CVl
CV2 CV2 Tl CVl CV2 T2 Tl CV1
CV1 T1 T2 CV2 Tl CV1 T2 Tl
CV2 T2 CVl CV1 Tl CV2 CV1 CV1
T2 CV1 T2 T2 CV2 T2 CV2 Tl
CVi Tl CVl CV2 T2 T2 T1 T2
T1 T2 CV2 Tl CV2 CV2 T2 T2
CV2 CV2 Tl CV1 CV1 Tl CV2 CV1
T2 T2 CV2 Tl CVl CV1 T2 CV2
Tl CVl CV1 T2 T2 T2 CV1 T1
CVl T1 T2 CV2 CV1 CV2 Tl CV1
T1 CV1 T2 Tl T2 Tl Tl CV2
CV2 CV2 CV2 CV2 CV2 T1 CV1 T2
T2 CV2 T1 Tl T2 CV1 CV2 CV2
CV2 CV1 CV1 T1 T2 CV2 T2 CV1
CV2 T2 CV1 CV2 Tl Tl Tl Tl
T1 CV2 Tl CV1 CV1 T2 Tl Tl
CV1 CV1 T2 T2 CV1 CV2 CV2 CV2
CV1 T1 TI CV1 CV2 T2 T2 T2
Tl T2 CV1 CV2 CV1 CV2 Tl T2
T2 CV1 CV2 CVl T2 T1 T2 CV1
T1 T2 T2 Tl CVl CV2 T2 Tl
CV2 CV1 T1 T2 CV2 T1 CV1 CV1
CV1 CV2 CV1 T1 CV1 Tl CV2 CV2
CV2 Tl CV2 CV2 CVl CV1 CV2 CV2
T2 CV1 T2 Tl Tl T2 T1 T2














APPENDIX B

Table 11

Order of Experimental Conditions


Subject Session


P: CV1
N: CVl
N: CV2


N: CV1
P: CV2
P: CV1

P: CV2
N: Tl
P: Tl

P: CV1
N: CV2
N: CV1


N: Tl
P: Tl
P: CV2


P: CV1
N: CV1
N: T2


P: T2



N: CV2



N: CV2





P: CV2



P: T2






N: CV2




N: CV2



P: CV2


Note: Dependent variables in all conditions were amplitude and
latency measures for NI, P2, N2 and P3 components of the evoked
potential recorded from vertex, left temporo-parietal, and right
temporo-parietal sites. In every condition the subject was pre-
sented with a series of 256 stimuli (64 each of CV1, CV2, T1, and
T2) and potentials were recorded to CV1 and Tl. P = Phonetic tape;
N = Nonphonetic tape.














Appendix C

Table 12

Summary of Partial Amplitude Data
(Microvolts) for two Additional Subjects


Component

Relevant Evoking Recording
Subject Stimulusa Stimulus Siteb N1 P2 Nl-P2 N2 P3


P:CV1














P:T1


CV1


8.9

8.4

8.8

12.3

9.0

11.0

7.9

9.1

9.7

13.5

10.1

12.5

8.8

8.3

7.5

9.2

8.5


P:Control CV1


7.3

7.1

5.2

6.2

6.6

5.9

12.6

8.8

7.7

10.8

8.7

6.1

1.8

1.1

1.1

4.5

4.2


16.2

15.5

14.0

18.5

15.6

16.9

20.5

17.9

17.4

24.3

18.8

16.6

10.6

9.4

8.6

13.7

12.7


2.3

3.4

1.0

-1.9

.6

-1.4

1.5

2.4

.1

4.0

4.9

5.0


2.9

3.8

1.3

1.9

1.8

1.5

2.1

3.8

1.5

4.1

5.7

6.4


R 7.9 3.0 10.9













Table 12 (continued)


Component

Relevant Evoking Recording
Subject Stimulusa Stimulus Siteb N1 P2 N -P2 N2 P3


2 P:CV1 CV1 V 7.1 4.0 11.1 -1.6 2.6

L 5.9 3.3 9.2 .1 3.6

R 5.5 1.6 7.1 1.0 4.5

P:T CV1 V 3.7 5.6 9.3 -2.6 2.2

L 3.3 2.7 6.0 -1.3 3.0

R 2.9 .8 3.7 -.3 1.5

P:Control CV1 V 7.0 1.1 8.1

L 9.3 .1 9.4

R 8.4 .2 8.6


ap = Phonetic tapes; N = Nonphonetic Tapes
bV = Vertex; L = Left Temporo-Parietal; R =


Right Temporo-Parietal.


















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Wood, C. C. Auditory and phonetic levels of processing in speech
perception: neurophysiological and information processing
analyses. Journal of Experimental Psychology: Human Perception
and Performance, 1975, 104, 3-20.

Wood, C. C., Goff, W. R., & Day, R. S. Auditory evoked potentials
during speech perception. Science, 1971, 173, 1248-1251.

Yeni-Komshian, G. H., & Benson, D. A. Anatomical study of cerebral
asymmetry in the temporal lobe of humans, chimpanzees, and
rhesus monkeys. Science, 1976, 192, 287-389

















BIOGRAPHICAL SKETCH


Dennis A. Silva was born on December 9, 1948, in Palmer,

Massachusetts. He is the son of Ermindo J. and Lucille M. Leroux

Silva. He attended St. Matthew's Grade School and Cathedral High

School in Springfield, Massachusetts. He received a B.A. in

psychology from the University of Massachusetts in Amherst in

1970 and an M.A. in experimental psychology from the University

of Hartford, Connecticut, in 1974. During his master's program,

he worked as a research assistant to Bernard Z. Friedlander in

the Infant/Child Language Research Laboratory. While at the

University of Florida Mr. Silva worked as a research assistant

under the direction of William A. Yost, and as a teaching assistant

to W. Keith Berg.













I certify that I have read this study and that in my opinion it
conforms to acceptable standards of scholarly presentation and is fully
adequate, in scope and quality, as a dissertation for the degree of
Doctor of Philosophy.





William A. Yost, Chairman
Associate Profesor of Psychology




I certify that I have read this study and that in my opinion it
conforms to acceptable standards of scholarly presentation and is fully
adequate, in scope and quality, as a dissertation for the degree of
Doctor of Philosophy.





Donald C. Teas
Professor of Psychology




I certify that I have read this study and that in my opinion it
conforms to acceptable standards of scholarly presentation and is fully
adequate, in scope and quality, as a dissertation for the degree of
Doctor of Philosophy.





W. Keith Berg /
Associate Professor of Psychology




I certify that I have read this study and that in my opinion it
conforms to acceptable standards of scholarly presentation and is fully
adequate, in scope and quality, as a dissertation for the degree of
Doctor of Philosophy.





Paul Satz
Professor of Psychology













I certify that I have read this study and that in my opinion it
conforms to acceptable standards of scholarly presentation and is fully
adequate, in scope and quality, as a dissertation for the degree of
Doctor of Philosophy.





Robert L. Isaacson
Professor of Psychology




I certify that I have read this study and that in my opinion it
conforms to acceptable standards of scholarly presentation and is fully
adequate, in scope and quality, as a dissertation for the degree of
Doctor of Philosophy.





Howard B. Rothman
Associate Professor of Speech




This dissertation was submitted to the Graduate Faculty of the Department
of Psychology in the College of Arts and Sciences and to the Graduate
Council, and was accepted as partial fulfillment of the requirements for
the degree of Doctor of Philosophy.

March, 1977


Dean, Graduate School




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