Group Title: relation between whole-nerve and unit responses of the auditory nerve (alligator lizard) /
Title: The Relation between whole-nerve and unit responses of the auditory nerve (alligator lizard)
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Title: The Relation between whole-nerve and unit responses of the auditory nerve (alligator lizard)
Physical Description: viii, 112 leaves : ill. ; 28cm.
Language: English
Creator: Turner, Robert Graham, 1946-
Publication Date: 1975
Copyright Date: 1975
 Subjects
Subject: Acoustic nerve   ( lcsh )
Lizards -- Anatomy   ( lcsh )
Speech thesis Ph. D   ( lcsh )
Dissertations, Academic -- Speech -- UF   ( lcsh )
Genre: bibliography   ( marcgt )
non-fiction   ( marcgt )
 Notes
Thesis: Thesis--University of Florida.
Bibliography: Bibliography: leaves 107-111.
General Note: Typescript.
General Note: Vita.
Statement of Responsibility: by Robert Graham Turner.
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Bibliographic ID: UF00098162
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 - 000165492
oclc - 02804197
notis - AAT1870

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THE RELATION BETWEEN WHOLE-NERVE AND UNIT RESPONSES
OF THE AUDITORY NERVE (ALLIGATOR LIZARD)










By

ROBERT GRAHAM TURNER


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

1975





































To

Pol ly
















ACKNOWLEDGEMENTS


The author wishes to express his gratitude to his supervisory

committee chairman, Dr. Donald C. Teas, for his guidance, support,

and patience. Thanks are due to the other members of the supervisory

committee, Drs. Daniel E. Sellers, William A. Yost, G. Paul Moore and

William E. Brownell. A special thanks is given to Dr. Donald W. Nielsen

for his interest in this research.
















TABLE OF CONTENTS


Page

Acknowledgments iii

Abstract vi

Introduction 1
General Considerations 1
Lizard Anatomy 2
Peripheral Auditory System 2
Fine Structure of the Inner Ear 3
Alligator Lizard 8
Physiology 9
Overview 9
Alligator Lizard 10
Research Implications 11
Whole-Nerve Action Potential 14
Experiments 16

Methods 18
Animal Selection 18
Surgical Procedures 18
Acoustic Stimulation 19
Electrical Recording 20
Data Collection and Analysis 22
Figures 23
Experimental Protocol 24

Results 27
Click Stimuli 27
AP Response 27
Single Unit Activity 38
Contributions of the Two Fiber
Populations to the AP 51
High-Pass (2.0 kHz) Click Stimuli 51
AP Response 51
Single Unit Activity 52
Contributions to the AP for Low Frequency Stimuli 56
Low-Pass Click Stimuli 62
AP Response 62
Single Unit Activity 70
Narrow-Band Click Stimuli 76
Tone and Tone Burst Stimuli 76
AP Response 76
Single Unit Activity 84

















TABLE OF CONTENTS (continued)


Page

Discussion 93
Analysis of the AP Response to Clicks 93
Relationship of the AP to Inner Ear Anatomy 95
Innervation Pattern for Afferent Fibers 97
Basic Neural Response Unit 99
New Technique for Analyzing the AP 100
AP as a Research Tool 103
Filtered Click Stimuli 103
Tone and Tone Burst Stimuli 104

References 107

Biographical Sketch 112












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




THE RELATION BETWEEN WHOLE-NERVE AND UNIT RESPONSES
OF THE AUDITORY NERVE (ALLIGATOR LIZARD)

by

ROBERT GRAHAM TURNER

August, 1975

Chairman: Donald C. Teas
Major Department: Speech

The papilla of the alligator lizard contains two populations

of hair cells: a ventral (apical) population with unidirectional

orientation, associated with a tectorial membrane, and a dorsal

(basal) population with bidirectional orientation and free

standing cilia. Previous work demonstrated that the auditory

nerve fibers could be divided into two populations, low frequency

(CF< 0.8 kHz) and high frequency (CF,0.9 kHz), on the basis of

tuning curve shape. The low frequency fibers were associated

with the ventral papilla and the high frequency fibers with the

dorsal papilla.

In the alligator lizard, the whole-nerve action potential (AP)

response to clicks is characterized by a complex change in the

waveform which occurs when click polarity is reversed. Post-

stimulus-time (PST) histograms of the response of single high









frequency fibers to click stimuli have a single peak of approxi-

mately the same shape and latency for both click polarities. Click

histograms for low frequency fibers show multiple peaks which

shift in time when click polarity is reversed. High pass (2.0 kHz)

clicks significantly reduce the activity of low frequency fibers

resulting in an AP whose waveshape is independent of stimulus

polarity.

On the basis of the response to low-pass (0.6 kHz) clicks,

individual high frequency fibers were classified as Type 1 or

Type 2. The response of both types of fibers depended on stimulus

polarity. The response pattern of a Type I fiber to a stimulus

of one polarity is the same as the response pattern of a Type 2

fiber to that stimulus with opposite polarity. Using similar

criteria, the responses from low frequency fibers could not be

classified into two or more types.

The AP response to tones and tone bursts shows phase-locked

activity at frequencies below 1.0 kHz. This wave activity is

the same frequency as the sinusoidal stimulus. PST histograms

for low and high frequency fibers have peaks which occur once

for every period of the sinusoidal stimulus and which shift in

time as stimulus polarity is reversed. For frequencies above

1.0 kHz, the low frequency fibers have little response; whereas

the high frequency fibers respond significantly, but with little

phase-locked response activity.

Amplitude-latency plots as a function of filter frequency

were derived from the AP responses to low-pass and narrow-band









clicks. These data show a transition from a predominately polarity-

independent response to a polarity-dependent response as filter

frequency is decreased from 2.0 kHz to 1.0 kHz.

It is concluded that low frequency fibers innervate hair cells

in the ventral papilla, and that because of the unidirectional

orientation of these hair cells, the low frequency fiber population

contributes a polarity-dependent component to the AP. A high

frequency fiber innervates hair cells of only one orientation in

the hair cell population of the dorsal papilla. Because of the

bidirectional orientation of these hair cells, the high frequency

fiber population contributes a polarity-independent component to

the AP. The analysis of the AP response to high-pass, low-pass,

and band-pass clicks reflects the tonotopic organization of the

alligator lizard's papilla.















INTRODUCTION


General Considerations


Hearing loss affects more Americans than any other chronic

health problem. Over eight and one-half million Americans suffer

from some degree of loss (Rees, 1973). A hearing loss (conductive

loss) which results from an abnormal condition of the external or

middle ear is usually treatable with medication or surgery. Unfortu-

nately, very few techniques are available for correcting or compen-

sating for a hearing loss (sensorineural loss) which is due to patho-

logies of the inner ear or auditory pathways. It is estimated that

about 120,000 Americans have sensorineural hearing impairments of the

severity classifiable as total handicaps (Carhart, 1974). A much larger

number suffer from less severe hearing losses.

The majority of sensorineural losses result from abnormalities of

the inner ear and/or primary auditory nerve fibers, resulting in the

improper coding of acoustic information by the cochlea, or the inability

of the nerve to correctly transmit the information. The major help

available to individuals with sensorineural hearing losses has been

the hearing aid, which often is of limited value. Recently, a new

technique has been developed, the cochlear implant, but its success

has been minimal (Merzenich et al., 1974). The failure of the hearing









aid and the cochlear implant to adequately compensate for the sensori-

neural loss results, in part, from an insufficient knowledge of

cochlear processes, In particular, the transduction of the motion of

the basilar membrane into the activity of primary auditory nerve fibers.

Certainly, much is known about the auditory system as the result

of anatomical and physiological research on animals and humans. Pri-

marily, mammals have been used for auditory research because of the

similarities among mammals, including humans, in the anatomy of their

auditory systems and thus, the applicability of animal research to

human problems. Occasionally, a non-mammalian animal demonstrates an

anatomy which is advantageous for the study of a particular question.

A classic example is the recording of intracellular potentials from

single hair cells in the lateral-line organ of amphibia and fish.

Recently, researchers have found that the lizard, in particular, the

alligator lizard, is a valuable preparation for the study of the peri-

pheral auditory system because of the unique anatomical structure of

its inner ear.



Lizard Anatomy


Peripheral Auditory System

The following anatomical description is applicable to many species

of lizards: the major exception being the burrowing lizards which have

no external ear openings in the skin. A short external auditory meatus

terminates at the tympanic membrane (for some species, the tympanic

membrane is at the level of the skin). The lizard's middle ear consists









of two bones, the stapes (columella) and the extrastapes (extracolumella),

which join in a rod-like manner to provide a direct connection between

the tympanic membrane and the inner ear (Fig. 1).

The oval window, containing the stapes footplate, opens.into scala

vestibuli. Scala vestibuli is connected to scala tympani by the helico-

trema, and both scalae are part of the perilymphatic system. Unlike

the mamalian cochlea, the inner ear of the lizard is not coiled, nor

are the scalae long narrow chambers similar to an "uncoiled" mammalian

cochlea. They are, instead, chambers of irregular shape, separated

by the cochlear duct (Fig. 2).

The space within the cochlear duct, scala media, is filled with

endolymphatic fluid. The medial boundary is formed by supporting struc-

tures for two sensory organs: the lagena macula and the basilar papilla.

The lateral boundary of the cochlear duct is the vestibular membrane.

The function of the lagena is not clear; the lizard's basilar papilla

is the analog of the mammalian organ of Corti. The papilla sits on

the basilar mambrane which separates scala media from scala tympani.

The papilla is a strip of neuroepithelium which consists of hair

cells, supporting cells, and the unmyelinated portion of the basilar

(auditory) branch of the VIllth nerve fibers which pass between the

supporting cells and terminate at the base of the hair cells.


Fine Structure of the Inner Ear

At a gross level, there is similarity in the anatomy of the peri-

pheral auditory systems of many species of lizards: however, recent

studies by Wever (1965, 1967b, 1967c, 1968, 1970a, 1970b, 1971a, 1971d,










EAR OF "TYPICAL" LIZARD


Fig. 1. Diagram of a section of the head of a "typical"
lizard. This figure is representative for many species of
lizards, including the alligator lizard. (Adapted from Weiss
et al., 1974a)













Dorsal

Medial---


Ganglion-



Basilar Branch
VIII NerveH


Scala Tympani-
Basilar-
Membrane
Basilar--
Papilla
Recess of-
Scala Tympani


INNER EAR OF ALLIGATOR LIZARD

Helicotrema



[ Lagenar
Otolithic Membrane

Vestibular-
Membrane
Scala Vestibuli
Neural Limbus
Tectorial Membrane


Stapes Footplate


Fig. 2. The inner ear of the alligator lizard. The
basic structure of the alligator lizard's inner ear is similar
to that of other species. The dotted line indicates the surgi-
cal removal of the round window to expose the papilla and nerve.
A microelectrode is shown recording from the auditory nerve.
(Adapted from Weiss et al., 1974a)









1974), Miller (1966, 1973a, 1973b, 1974), Mulroy (1968), Baird (1967,

1969) and Bagger-Sjoback and Wersall, (1973), reveal the significant

differences in the fine structure of the inner ears of different lizard

species. There is a remarkable variation in papilla size, number of

hair cells, hair cell orientation, and hair cell associated systems.

Papilla size can vary from less than 200 pm in species of Chamaeleonid

to more than 2 mm in Varanid. Less than one hundred hair cells are

present in some species of Iguanid, while many Gekkonid have over two

thousand. A population of hair cells may be orientated in the same

direction (unidirectional), adjacent regions of hair cells may be

orientated in opposite directions (bidirectional), or hair cells of

both orientations may be intermixed in the same region multidirectionall)

(Fig. 3).

There are four basic hair cell systems found in the inner ears of

lizards. Wever (1971b, 1971c) has proposed that these structures are

involved in the mechanics of hair cell stimulation. The first, a

tectorial membrane system is most similar to the mammalian structure

and is characterized by a thin membrane attached to the neural limbus

and connected at the other end to the cilia of the hair cells. The

second variety, the sallet system, is similar to otolith systems, con-

sisting of a large mass lying free in the cochlear fluid except for

contact with the cilia of one or more hair cells. A variation of the

sallet system is the culmen system, a large mass which contacts the

cilia of all the hair cells of a population. The culmen system is

usually found in association with the sallet system. The free-standing

cilia system, the fourth variation; is actually the lack of any addi-









ANGUID





IGUANID
li


SCINCID






GEKKONID







LACERTID



TEIID

I TITTI


Neural

Ventral -- )Dorsal

Abneural



Tectorial Membrane


Free Standing Cilia

...:..:.. :.'5 :: ...
Sallets : ..


Culmen Ii~!i


VARANID




Fig. 3. The anatomy of the papilla in seven families of lizards.
Hair cell orientation (indicated by direction of the arrows) is shown
for seven families; hair cell associated structures are shown for four
families. The alligator lizard belongs to the Anguid family.
(Adapted from Miller, 1974)









tional structure associated with the hair cell. The cilia of the

hair cell stand free in the cochlear fluid except for, perhaps, a small

amount of tectorial like substance found between the cilia.

The three patterns of hair cell orientation could be combined

with the four types of hair cell systems to produce many possible pat-

terns of inner ear anatomy. Available data indicate that the inner

ear anatomy of species within the same family follows the same basic

pattern of hair cell orientation and hair cell systems; however, there

is great diversity among different families. The basic pattern of inner

ear structure is shown for seven families in Fig. 3.


Alligator Lizard

In the alligator lizard (Gerrhonotus multicarinatus), the papilla

is slightly larger at both ends than in the middle, varying in width

from 50 pm to 75 pm. The basilar membrane is larger than the papilla

and is wider in the middle than at the ends, with a width of 60 pm to

100 pm.

Each hair cell in the papilla of the alligator lizard has a single

kinocilium and from fifty-five to seventy stereocilia arranged in a

random pattern. The length of the stereocilia of a single cell de-

creases in a direction away from the kinocilium. The morphological

polarity (orientation) of a hair cell is defined by the eccentric posi-

tion of its kinocilium. The alligator lizard belongs to the family

Anguid; its pattern of hair cell orientation and hair cell systems is

typical for its family (Fig. 3). In the ventral (apical) region of

its papilla, all the hair cells (about 50) are morphologically polarized









in the same direction (unidirectional orientation). A tectorial mem-

brane is present and the cilia have a length of 5 pm to 9 pm. In the

dorsal (basal) region, the hair cells are divided into two groups of

opposite polarity (bidirectional orientation), with about fifty cells

in each group. The hair cell group closest to the nerve (neural) is

orientated away from the nerve, whereas, the other group (abneural) is

orientated towards the nerve (Fig. 41). No tectorial membrane has been

found in this region, but a small amount of tectorial substance is

found between the cilia (free-standing cilia system). In this region

the cilia are longer and vary from 12 pm to 31 pm (Mulroy, 1968; Weiss

et al., 1974a). In summary, in the ventral region of the papilla, the

alligator lizard has a unidirectional orientation/tectorial membrane

system; in the dorsal region, a bidirectional orientation/free standing

cilia system.

There are 600 to 1000 afferent auditory nerve fibers innervating

the papilla of the alligator lizard. Little is known about the in-

nervation pattern of the afferent fibers in the papilla. Unmyelinated

afferent fibers pass through the habenula perforata and become myelinated.

They form the basilar branch of the VIIIth nerve which passes anteriorly

and medially to enter the brain stem. Efferent fibers have been ob-

served forming synapses on the hair cells in the ventral region but not

the dorsal region of the papilla (Weiss et al., 1975).



Physiology


Overview

The most extensive physiological data have been provided by









Wever (1965, 1967a, 1968, 1970a, 1970b, 1971a, 1971b, 1974) and his

associates (Wever et al., 1964, 1965, 1973; Wever and Hepp-Reymond,

1967). They have studied many species of lizards using the standard

technique of recording extracellular potentials using a metal elec-

trode placed on the round window. Wever considers these potentials

cochlear in origin, and has attempted to answer fundamental question,

such as tonotopic organization of the papilla, using these data.

Others (Campbell, 1969; Crowley, 1964; Hepp-Reymond and Palin, 1968;

Johnstone and Johnstone, 1969b) have used similar techniques in dif-

ferent species of lizards.

Additional physiological data include the measurement of middle

ear frequency response in two species (Saunders and Johnstone, 1972),

and single unit recordings from the auditory nerve in one species

(Johnstone and Johnstone, 1969a) and from higher order auditory fibers

in three species (Manley, 1972, 1975; Suga and Campbell, 1967).


Alligator Lizard

Of all the species of lizards, the most extensive physiological

data are available for the alligator lizard. Wever (1971a), Crowley

(1964), and Campbell (1969) recorded electrical responses to sound

with a round window electrode. Intracellular potentials were recorded

from individual hair cells in the papilla (Weiss et al., 1974a; Mulroy

et al., 1974). The waveshapes of the intracellular responses sug-

gested tonotopic organization of the papilla and appeared related to

the orientations of the hair cells in the two regions of the papilla.

Weiss, Mulroy, Turner and Pike (1974b, 1975) recorded single unit

activity from primary fibers in the auditory nerve. On the basis of the









tuning curves obtained from individual fibers, they divided the fibers

into two populations: a low frequency population (0.2 kHz 4 CF- 0.8 kHz)

and a high frequency population (0.9 kHz CF_ 4.0 kHz) (Fig. 4). This

division was evident on the basis of both tuning curve shape and the

distribution of the characteristic frequencies (CF) of the fibers. As

indicated by dye-marking experiments, the fibers of the low frequency

population enter the ventral end of the papilla, while the fibers of

the high frequency population enter the dorsal end of the papilla.

There appears to be a tonotopic organization of the papilla from the

ventral (low frequency) to the dorsal (high frequency) end of the

papilla. Although the exact innervation pattern is unknown, it is

reasonable to conclude that the low frequency fibers innervate the hair

cells in the ventral papilla while the high frequency fibers innervate

the hair cells of the dorsal papilla. Electrical responses recorded

from individual hair cells in the papilla support this assumption (Weiss

et al., 1974a). They also found that the rate of response of a low

frequency fiber to a tone at CF could be suppressed by the simultaneous

presentation of a second tone whose frequency is above the CF of the

fiber (Weiss et al., 1975). This phenomenon, called two-tone rate

suppression (TTRS), was not found for high frequency fibers.



Research Implications


In the mamalian cochlea, there are two populations of hair cells,

inner and outer hair cells, which differ in anatomical structure and

which are innervated by different afferent auditory nerve fibers









TUNING CURVES


UNIT 30
CF:0.23kHz.


100


80


60


40-


20-


40-


20-


40-


z



WE






0u
2
0
E 0



O "



n-


0 .


I I I *I *


1.0


FREQUENCY (kHz.)


Fig. 4. Typical tuning curves for primary auditory nerve fibers
in the alligator lizard. Note that the two tuning curves with CF
greater than 1.0 kHz have a broader shape than the three tuning curves
with CF less than 1.0 kHz. (From Weiss et al., 1975)


UNIT 26
CF:0.54kHz.



UNIT 52
CF:0.69kHz.


UNIT 12
CF:3.1 kHz.


0.01


0.1









(Spoendlin, 1973). The relationship of these significant anatomical

differences to the discharge patterns of individual nerve fibers is

not clear. There is significant research interest in this question

and the closely related question concerning the relationship between

the motion (displacement and velocity) of the basilar membrane and the

initiation of auditory nerve activity (Dallos et al., 1972; Dallos and

Wang, 1974; Sokolich and Zwislocki, 1974; Zwislocki and Sokolich, 1973;

Konishi and Nielsen, 1972, 1973).

The great diversity in the anatomy of the inner ears of lizards

offers exciting possibilities for the study of cochlear transduction

processes, including questions similar to those discussed above. A

comparative study, involving species from many families, could result

in an understanding of the relationship between particular anatomical

structures in the inner ear and the discharge patterns of individual

auditory nerve fibers. Such a study would require extensive time and

effort and would rely primarily on single fiber recordings. Other

techniques are needed which would permit the quick survey of many

species for the answers to a few fundamental questions. One possibility

is the recording and analysis of the whole-nerve action potential (AP).

There has been only limited interest in recording the AP in the lizard

(Weiss et al., 1974a; Campbell, 1969; Hepp-Reymond and Palin, 1968).

A wire electrode on or near the round window was used to record the AP

response in several species of lizards. For two species, the alligator

lizard and the tokay gecko, the AP response to condensation and rare-

faction clicks is shown. Unlike the mammal where the waveshape of the









AP is independent of click polarity, there is a complex change in the

waveshape of the response in these two species of lizards when click

polarity is changed. It has been proposed that this complex change

in response results from the complex structure of the papilla (Hepp-

Reymond and Palin, 1968). These data strongly imply the potential

value of recording the whole-nerve action potential in the lizard.



Whole-Nerve Action Potential


The whole-nerve action potential (AP), recorded near the inner

ear or auditory nerve, is an electrical signal which reflects the summed

activity of individual auditory nerve fibers. In the mammal, the most

popular recording technique has been the differential electrode pair.

Other techniques include recording from the round window or the auditory

nerve with a wire electrode.

The AP is largest when many fibers fire in a short period of

time. Transient stimuli, such as clicks or tone bursts with fast rise

times, are best for producing the synchronous firing of the individual

nerve fibers needed to produce the AP. Teas, Eldredge and Davis (1962)

proposed that the waveshape of the AP is the convolution of the basic

response unit of an individual fiber with the temporal firing patterns

of the fibers. The shape of the basic response unit is affected by

the recording electrode configuration. For an electrode pair in the

cochlea of a mammal, the basic response unit is diphasic, first negative,

then positive.

In the mammal, the AP response to a click primarily reflects the









activity of the nerve fibers in the basal end of the cochlea (Kiang

et al., 1965). Fibers in the basal turn have similar latencies and

fire synchronously to a transient stimulus. Differences in latency

among fibers become greater closer to the apex causing the activity

of these fibers to cancel. It cannot be concluded that the activity

of the more apical fibers does not affect the waveshape of the AP.

Teas, Eldredge and Davis (1962) used bands of noise to reduce the

synchronous firing of a small group of the more apical fibers and

found that this altered the AP. Legouix and Pierson (1974) also

presented evidence that there was some contribution to the AP from

basal and apical fibers.

Tasaki (1954) proposed that the source of the AP is the nerve

trunk in the basal turn of the cochlea. Teas, Eldredge and Davis

(1962) concluded that the internal auditory meatus forms an insulating

tube and that the origin of the AP is the nerve as it emerges from

the meatus. Dallos (1973) agreed with Teas et al., (1962) and devel-

oped a model to explain the initial negative-positive complex. The

complex results because the nerve is first contained within an in-

sulator and then emerges into a conducting fluid which is in contact

with the recording electrode.

With the development of better techniques for the recording from

individual nerve fibers, the research interest in the AP decreased.

Recently, there has been renewed interest in the AP because of

electrocochleography (ECoG). ECoG is the clinical recording of the

AP in man to determine the condition of the peripheral auditory










system. Much of the work in humans has concentrated on determining

an audiogram using the AP. The AP response to a click reflects, pri-

marily, the condition of the basal turn fibers; other stimuli, such

as filtered clicks and tone bursts, have been used with some success

to measure the condition of fibers from all turns of the cochlea.



Experiments


The potential value of the AP in ECoG and to study the peripheral

auditory system of the lizard has increased the need to better under-

stand and interpret the AP. The objectives of this research are to

analyze the alligator lizard's AP response to clicks and to investi-

gate the use of the AP as a research tool.

The click AP in the alligator lizard demonstrates a complex

change when click polarity is reversed. It has been suggested that

this phenomenon is related to the complex anatomy of the papilla.

The analysis of the click AP will be in terms of the contributions

of the low and high frequency fiber populations and the relationship

of these contributions to the anatomy of the inner ear. The investi-

gation of the AP as a research tool will consist of the analysis of

the AP response to carefully selected stimuli, to determine which

stimuli provide the best information concerning the activity of cer-

tain groups of fibers and the physiological significance of inner

ear anatomical structures.

The parameters of the AP response of interest will be waveshape,

amplitude and latency. Click, filtered click, tone and tone burst

stimuli will be manipulated in terms of polarity, intensity and spectral






17


composition. Other attempts to analyze the AP have relied primarily

on stimulus manipulation (Teas et al., 1962; Eggermont and Odenthal,

1974). Since the AP reflects the activity of individual nerve fibers,

it seems advantageous to record single unit activity simultaneously

with the recording of the AP. A knowledge of single nerve fiber

activity should greatly facilitate the interpretation of the AP.

The basic strategy of the experiments is as follows:

1. Stimulus manipulation (polarity, intensity, spectrum).

2. Record single unit activity.

3. Record the AP.

4. Analyze the AP in terms of waveshape, amplitude and

latency.















METHODS


Animal Selection


The alligator lizards were obtained from Hermosa Reptile, Inc.,

Hermosa Beach, California. There are approximately ten species of

alligator lizards (Genus Gerrhonotus) found in the United States (Smith,

1946). Considering the geographical location in which the lizards were

captured, it can be concluded that the majority of lizards used in the

research were Gerrhonotus multicarinatus webbii (San Diego alligator

lizard) with the possibility of a few Gerrhonotus multicarinatus multi-

carinatus (red-backed alligator lizard). There is no information

available concerning anatomical variations among subspecies of Gerr-

honotus multicarinatus



Surgical Procedures


The experiments were performed on twelve alligator lizards which

weighed from 11 to.31 grams. The animals appeared in good health; the

external auditory meatus and the tympanic membrane were examined under

a Zeiss operating microscope and the ears were cleared of any parasites.

The animals were anesthetized for surgical preparation with sodium

pentobarbital (Nembutal) administered intraperitoneally in doses of

25 mg/Kg of body weight. Additional injections of one-half the initial









dose were administered as needed to maintain the proper anesthetized

state.

The middle ear of the anesthetized lizard was exposed by removing

the skin and muscle of the ventral wall of the pharynx. A cannula was

inserted into the trachea, and the posterior end of the retroarticular

process of the lower jaw, together with the associated muscles, was

removed. Scala tympani was opened by removing the ventral bony edge

of the round window and the round window membrane (Fig. 2). The basilar

papilla and the basilar (auditory) branch of the Villth nerve could be

seen with an operating microscope (Weiss et al., 1974a).



Acoustic Stimulation


The stimuli were normally clicks, filtered clicks, tones and tone

bursts. The standard click was a 100 ps pulse, which was filtered

(Krohn-Hite 3550) to produce a low-pass, high-pass, or narrow-band

click (24 dB per octave slope (s)). The standard tone burst was 50 ms

in duration with no shaping (fast rise time). The tone burst was trig-

gered on the positive-going zero-crossing of the sine wave. Stimulus

presentation rate was 10/sec. The amplitude and polarity of the stimu-

lus could be controlled.

The acoustic stimulus was generated by a 1.0-inch condenser micro-

phone (Bruel and Kjaer 4132) fitted into a speculum which was sealed to

the entrance of the external auditory meatus. Within each speculum there

was a small diameter probe tube that permitted sound pressure measurements

close to the tympanic membrane. Acoustic calibrations were obtained










with the speculum sealed to the external meatus and the probe tube con-

nected to a calibrated 0.5-inch condenser microphone (Bruel and Kjaer 4134)

(Fig. 5).

Click and filtered click intensities are in decibels (dB) relative

to 45 volts-peak into the earphone (1.0-inch condenser microphone), and

tone and tone burst intensities are in dB relative to 27 volts rms into

the earphone.



Electrical Recording


The experiments were conducted in a sound-proofed, electrically-

shielded chamber. The whole-nerve action potential (AP) was recorded

by a nichrome wire electrode placed on the bone near the round window

membrane. This electrode also recorded the cochlear microphonic (CM),

the activity of higher auditory elements and electrical and physiological

artifact. The electrode was connected to the input of a high gain

(20,000 or 50,000), AC-coupled amplifier (Grass P511). The output of

the amplifier was displayed on an oscilloscope, recorded on tape and

led to a LAB-8e computer for on-line averaging.

The spike activity of an individual nerve fiber was recorded using

a glass microelectrode. The reference electrode for the microelectrode

and the wire electrode was a steel wire inserted into a muscle in the

head. The electrodes were filled, using the fiber-fill technique

(Tasaki, 1968), with 3 M KC1. Tip diameter was less than 1 pm, and

resistance at 25 Hz was 20 to 100 MA. Once the inner ear was opened,

visual observation was used to insert the electrode, which was connected













ACOUSTIC CALIBRATION


140-


z

mE

L U 120-
5 0
CD



00-

So

CL'
(a


Lizard 15
Signal: 9v rms Tone


0.01 0.1 1.0 10.0

FREQUENCY (KHz)


Fig. 5. Typical acoustic calibration curve for the sound
pressure near the tympanic membrane. The curve was generated
by a 9 volt rms tone into the one-inch condenser microphone
earphone.









to a hydraulic microdrive (Kopf 1207B), into the fluid above the nerve.

The electrode was lowered into the nerve using the microdrive at a

location peripheral to the internal auditory meatus and most of the cell

bodies of the ganglion. This procedure insured recording from primary

auditory nerve fibers.

The microelectrode was connected to the input of a high impedance,

low gain amplifier with capacitance neutralization (Keithley 605). The

output of this amplifier was connected to an AC-coupled Grass amplifier

with a gain of 500, 1000, or 2000. The output of the Grass was dis-

played on an oscilloscope, passed through a band-pass filter (Krohn-

Hite 3100), fed to an audio monitor, recorded on tape, and led to a

LAB-8e computer for on-line calculations of poststimulus time (PST)

histograms.



Data Collection and Analysis


Most of the data analysis was performed off-line from tape recordings

of the data. The FM channels of the tape recorder (Ampex FR-1300) have

a band-width of DC to 1.6 kHz when recording at 3.75 in/sec. Compari-

sons of averages of the AP computed on-line and off-line from tape

recordings revealed little distortion of the AP waveform due to the

limited high frequency response of the tape recorder. This also had

little effect on the processing of single unit data. The following

signals were recorded on separate channels of the tape recorder:

1. Amplified output of the microelectrode

2. Amplified output of the wire electrode









3. Voice commentary

4. Master timing pulse

5. Electrical stimulus to earphone

A LAB-8e computer was used off-line to produce averages of the AP and

the electrical stimulus and PST histograms of single unit activity.



Figures


Most of the data are presented in the figures as averages of the

AP or PST histograms of single fiber activity. Except were noted, the

rarefaction and the condensation stimulus conditions are shown together

in the figures with rarefaction above condensation. The amplitude of

the response is indicated for the rarefaction condition but also applies

to the condensation condition.

When the AP average is shown by itself or with the stimulus wave-

form, the amplitude of the AP response is indicated by the calibration

bar. When the AP is presented with a PST histogram, the amplitude of

the AP is not indicated; however, the AP average shown was recorded

simultaneously with the single unit activity represented by the histo-

gram. An AP average always represents 100 averages; positive voltage

up.

A PST histogram always represents 300 stimulus presentations. The

number of bins of the histogram is always 200 except for the histograms

with a 10 ms time base; here the number of bins is 100. The bin width

is evident from the number of bins and the time base. The number of

spikes contained within a bin is indicated by the scale shown with the









histogram; note, this number represents the total for 300 stimulus pre-

sentations.

When a stimulus contained very little energy above 1.0 kHz, an

average of that stimulus is shown in the figures with the AP average

and the PST histogram. Stimuli with significant energy above 1.0 kHz

are not shown because their waveshape is distorted by the limited band-

width of the FM tape recorder. The stimuli presented were always re-

corded simultaneously with the corresponding AP response or single unit

activity.

Zero time in the averages and histograms and for the measurement

of latencies corresponds to the master tuning pulse recorded on tape,

not the arrival of the acoustic stimulus at the tympanic membrane. To

facilitate comparison of the AP response and single unit activity to

the stimulus waveform, the stimulus average in a figure is shown de-

layed 2.0 ms. The stimulus actually began at zero on the AP average

or PST histogram time scale, even though, it appears to have begun at

2.0 ms.



Experimental Protocol


The following general procedure was used during the experiments:

1. Initial surgery. The first part of the surgery was

completed so as to expose the middle ear.

2. Acoustic calibration.

3. Recording the AP. The wire electrode was placed on the

bone near the round window membrane and the AP was re-

corded for the stimuli of interest.









4. Final surgery. The surgery was completed so as to expose

the papilla and the auditory nerve.

5. Measurement of the visual detection level. The wire

electrode was again placed on the bone in approximately

the same location as before. A visual detection level

(VDL), the intensity at which a response to clicks is

just visible in the unaveraged AP, was determined. The

VDL was monitored throughout the experiment and served

as an estimate of the condition of the ear. Data were

not used if the corresponding VDL was poorer than -40 dB.

6. Recording from single units. Using clicks as a search

stimulus, the microelectrode was advanced into the nerve

until either spike activity was recorded or the electrode

passed completely through the nerve. Spikes recorded by

the microelectrode were typically monophasic, positive in

voltage, and had a width of about 1 ms. Single unit data

were obtained from 160 units, with the units divided

equally below low and high frequency fibers. Because of

the thinness of the nerve near the papilla, less than four

units were normally encountered during one pass through

the nerve. It was difficult to hold a unit very long;

the time varied from a few seconds to an hour, with the

average about five minutes.

7. Determination of fiber population. Once a unit was en-

countered, a 2.0 kHz tone burst was used to assign the






26


fiber to either the low or high frequency fiber popu-

lation. A high frequency fiber would show obvious re-

sponse to the stimulus; a low frequency fiber would not.

8. Estimation of characteristic frequency. The frequency

of the tone burst was quickly varied from 100 Hz to 4.0

kHz to provide an estimate of the characteristic fre-

quency (CF) of the fiber. A more accurate measure of

CF was not made because of the time required.

9. Data collection. Single unit activity and the AP were

simultaneously recorded for the stimuli of interest.















RESULTS


Click Stimuli


AP Response

Typical whole-nerve action potential (AP) responses to clicks are

shown in Fig. 6A. The most obvious feature is that the waveforms of

the responses depend upon the polarity of the clicks. The response

to both polarities demonstrates a large initial negative-positive

diphasic complex (NI-Pl), which is larger for condensation than rare-

faction. The rarefaction Pl peak is typically broadened, as indicated

by the arrow in Fig. 6A. The initial NI-PI is followed by two smaller

diphasic complexes, N2-P2 and N3-P3. The major activity in the AP

response is over before 10 ms. Also note that there is little cochlear

microphonic (CM) or electrical artifact evident in the average. The

gross response recorded with the wire electrode consists almost entirely

of neural activity.

The waveform of the AP response depends upon click intensity (Fig.7).

At -80 dB, little synchronous activity is evident in the AP. At -70 dB,

the response is not well defined. For -60 dB through -20 dB, the quali-

tative features described above are evident; however, at -10 dB, complex

changes have occurred in the response. Some of the changes present at

-10 dB are evident at -20 dB.


























Fig. 6. AP response to clicks. A: Pictured are typical
AP responses to rarefaction and condensation clicks showing the
large N1 P, and smaller N2 P2 and N P complexes, and
the broadening of the rarefaction PI peak (arrow). The two
responses are shown to the same amplitude scale. B: The AP
responses to rarefaction and condensation clicks are shown
superimposed with the rarefaction N1 P1 amplitude made equal
to the condensation N1 P1 amplitude by adjusting the scale
factor of the AP averages. Even though the two responses ap-
pear very similar in Fig. 10, differences are evident in this
figure. All AP responses represent 100 averages. The stimulus
was a 100 us duration click presented 10/sec.







AP RESPONSE TO CLICKS
Intensity: -30 dB



Lizard 9

P, RAREFACTION
P2 P3
N, N,

NI
PI

P2 P3



NI

0 2 4 6 8 1C


Lizard 14


..As RAREFACTION



CONDENSATION
CONDENSATIT


loms


I I I Im
0 2 4 6 8
time
































Fig. 7. AP response as a function of click intensity.
Note the change in response waveform as intensity is increased
from -30 dB to -10 dB. Click intensity is in decibels relative
to 27 volts peak into the earphone. The amplitude calibration
bar shown for each intensity applies to both the rarefaction
and condensation response.










AP RESPONSE TO CLICKS
Lizard 1S



Intensity (dB)

-80 -40


- /-..... rarefaction -

2 pv

condensation


-70














-60














-50












0 time 10ms


-30














-20









--V--1 -




-10












0 10









Another useful measure of the click AP is the amplitude of the

NI-Pl complex and the latency of the Pi peak. Amplitude-latency plots

as a function of click intensity are shown for four lizards in Fig. 8.

The NI-Pl amplitude increases with intensity up to about -30 dB; at

higher intensities, the amplitude decreases. In the mid-intensity range,

the NI-PI amplitude is larger for condensation than rarefaction; however,

at the lower and higher ends of the intensity range, the amplitudes tend

to be the same for both polarities. The latency of the NI peak decreases

with increasing intensities, reaching a minimum of about 2.0 ms.

There is some variation in the click AP with time during the ex-

periment (Fig. 9). The AP responses shown represent a time period

greater than twelve hours.. The first AP pair pictured was recorded

before the final surgery, where the round window is removed to expose

the nerve. The remaining four AP pairs were recorded simultaneously

with single unit activity, after the completion of all the surgery.

The click AP remains fairly stable with time, always demonstrating the

qualitative features typical of its waveform.

There is some variation in the click AP recorded from different

lizards (Fig. 10). The amplitude of the NI-PI complex varies, for

constant click intensity of -30 dB, within a range of approximately

10 pv to 40 pv. Even with all the variation in waveform evident in

this figure, the qualitative features described for the responses in

Fig. 6A are present in these responses. The condensation Nl-PI ampli-

tude is always greater than the corresponding rarefaction NI-P) ampli-

tude. The amount of broadening of the rarefaction P1 peak varies with

animal. There is significant broadening for lizard 9 and lizard 12,



































Fig. 8. Amplitude-latency plot as a function of click
intensity. The AP amplitude data for each lizard are plotted
in percent of the amplitude of that lizard's AP response to
-30 dB condensation clicks.










AP RESPONSE TO CLICKS


100-




a

c 80-


cu
0
U
-,

I
0 60-

0
0


* 40-






S20-
0 '


1 I '


I I I


-60 -40

INTENSITY (dB)


I I I -
-20 0


i /
I
i'1 I

S I

I
/

Lizard: 8 11 14 15

Rarefaction 0 A o

Condensation a A" B


T


0-


3.5-


E



Z
2.5-

z
I-,


-80
-80


| I








EFFECT OF TIME ON AP
Stimulus: -30dB Click
Lizard 10

Before final surgery
Srarefact ion


1 6pvon
A condensation


After final surgery U10-3


U10-9


0 time 10ms


0 10


Fig. 9. Variation with time of the AP responses to clicks.
The first responses (before final surgery) and the last responses
( U 10-23) were recorded more than twelve hours apart in time.
Click intensity was -30 dB.


U10-17


U10-23


-VI-V---






































Fig. 10. Variation of the AP response to clicks
across animals. Click intensity was -30 dB.










AP VARIATION ACROSS ANIMALS
Stimuli : -30 dB Clicks


Lizard 5 Lz.11


rarefaction


121v1 A

condensation


Lz.12


Lz.6








Lz.
-Lz.--


Lz.13


N


Lz. 9










S time 10m
0) time 10ms


Lz.14


0 10
t ----------
o 10


16


~c~









with little present for lizard 6 and lizard 14. The AP responses for

lizard 14 are superimposed with the NI-PI amplitudes made equal (Fig.

6B). It is clear that the Pi peak is broader for rarefaction than con-

densation, even though this is not obvious in Fig. 10. Other, more

subtle differences with polarity are evident in Fig. 6B.

The effects on the AP waveform of click duration and presentation

rate were examined (Fig. 11, Fig. 12). As click duration was changed,

click intensity was adjusted to maintain equal energy in the electrical

stimulus to the earphone. For these conditions, a variation of click

duration from 25 Vs to 200 ps had little effect on the AP waveform

(Fig. 11). For click presentation rates of I/sec to 20/sec, the AP

waveshape is similar, although the amplitude begins to decrease at rates

greater than 10/sec. At rates of 80/sec and 200/sec, the AP response

is significantly decreased in amplitude and modified in waveform (Fig.

12). Clicks of 100 ps duration presented 10/sec were acceptable stimuli

to use in these experiments since the AP response to this stimulus con-

dition is the same as the AP response to clicks of shorter duration and

slower presentation rates.


Single Unit Activity

The AP in the lizard, as in any animal, can be better understood

by examining the activity of individual auditory nerve fibers since the

AP is the weighted sum of the response of single fibers. The post-

stimulus time (PST) histogram for a high frequency (CF. 0.9 kHz) fiber

is characterized, at a gross level, by a single peak which has the same

shape and latency for both click polarities (See U8-3 in Fig. 13 for a









EFFECT OF CLICK DURATION ON AP
Lizard 14


Duratlon(D): 25ps
Inenslty(l):-28dB
rarelact on

lelYcodesaIon


0:50 1:-34


-Sc-.



-S--


0 time 1ms
time lOms


0:100 1:-40











0:200 1:-46










0 10


Fig. 11. Effect of click duration on the AP. The
intensity of the click was adjusted to maintain equal energy
in the electrical signal to the earphone. Click duration
varied from 25p s to 200p s.









EFFECT OF CLICK RATE ON AP
Intensity: -30dB

Lizard 8

Rate [clicks/sec.]
1 20


Srarefaction




f\j condensation


5



12







10





12



time l-me

o time 0ms


80




12


200











0 Ib
0 lo


Fig. 12. Effect of click presentation rate on the AP.
The click intensity was -30 dB. Click presentation rate varied
from 1/sec to 200/sec.


12 uv





























Fig. 13. Single unit response of high frequency fibers
to clicks. The amplitude of the AP response is not indicated;
however, the AP responses for rarefaction and condensation
clicks are shown to the same amplitude scale. The number
of spikes in a bin for 300 stimulus presentations are indicated
by the scale shown for each pair of histograms. The scale
applies to both the rarefaction and condensation histogram.
The bin width is determined from the number of bins, which is
100 for a 10 ms time scale and 200 for any other time scale,
and the time scale. For these histograms, the bin width is
100 ps. See the text for explanation of the Type 1 and Type
2 categorization of high frequency fibers. AP: whole-nerve
response; PST: post-stimulus-time histogram.









HIGH FREQUENCY UNITS
RESPONSE TO CLICKS
Intensity (I): -30dB (unless noted)


Type 1

U8-3 I:-40dB






1P--------
PST












-r ---
A --P
Vl condensation
PST













U11-28
125]









U10-13
62.








0 time 1orms


Type 2

U9-21
621

rarefaction



condensation



U8-8 1:-40dB
62-









U9-17
125-









U10-11
125





10


0 10









typical PST histogram). The maximum amplitude of the peak occurs in

time between the N, and the Pl peaks of the AP response (Fig. 13).

The response of a fiber depends upon click intensity (Fig. 14).

As click intensity is increased up to -30 dB, the peak in the histogram

for the high frequency fiber increases in amplitude, decreases in latency,

and becomes sharper. At -20 dB, the amplitude of the peak decreases

slightly. Multiple peaks are not present for any of the intensities

shown in the figure, although some spontaneous activity is evident. The

amplitude and latency of the NI-PI complex in the AP appears well cor-

related with the activity of high frequency fibers.

PST histograms of low frequency (CF4 0.8 kHz) fibers responses to

click stimuli are characterized by multiple peaks (Fig. 15, Fig. 16,

Fig. 17). The time between the peaks is equal to 1/CF of the fiber

(Turner and Weiss, unpublished research). For some units, these peaks

can occur greater than 35 ms after the presentation of the click stimuli

(Fig. 15, Fig. 16). The latencies associated with the various peaks

are shown in Fig. 18. For a rarefaction click, intensity equal to -30 dB,

there is an early peak (Ro) which occurs for some units, but not all.,

at about 2.0 ms. There is an additional peak (R,) which is present for

all units. The latency of this peak varies, probably with the CF of the

fiber, but the minimum latency is about 3.0 ms. Additional peaks may

be present; the latencies of these peaks depend upon the CF of the fiber.

For a condensation click, there is an initial peak (C1) at 2.5 ms,

whose latency varies little across fibers. This peak may be followed

by additional peaks whose latencies depend on the CF of the fiber.

The response of low frequency fibers also depends upon stimulus








HIGH FREQUENCY UNIT
RESPONSE TO CLICKS

Unit 15-2


Intensity (dB)

-60
125-1
rarefaction
---------- AP
-- PST

condensation
--------. -- AP
~-- -~.e.r ~PST


-50
125-










-40
1251









0 time 10ms


-30
125
12 1---- "-"---









-20
1251









0 10


Fig. 14. Single unit response of a high frequency fiber
as a function of click intensity.

































Fig. 15. Single unit response of low frequency fibers
to clicks. Click intensity was -30 dB.









LOW FREQUENCY UNITS
RESPONSE TO CLICKS


rarefaCton A

n .AJk r - P:
condensate ion
SAl

._ A...p,


1251










621










62]
-- tie l----





I A
0 time 10msg


U9-7
Intensity: -30dB
P 621


ST M juM h V


P

ST


U9-11 1:-30
1251


A^____



_


U13-4 1:-3(










U9-6 1:-3(


0
125]











125








I0 4


I k lo ....-.,.













Intensity (dB): -20
31 rarefaction
L AP

U10-7 A Jl PST
condensation

---^- AP

JKJ PST


31


U10-3 t _





0 time 1ims


)W FREQUENCY UNITS
RESPONSE TO CLICKS
-30
31 1







... w Al


161









60 10
A^A J .^^]


-30
31-


1}A/V Aui


0 40


Fig. 16. Single unit response of low frequency fibers to clicks.
Click intensity was -20 dB and -30 dB.












Intensity (dB): -10



U9-23


LOW FREQUENCY UNIIS
RESPONSE TO CLICKS


-20
31





mLAvhi-


30
62
AP

PST
AP
- ^ -- PST


250 250
2 ra re fa ctio n
11333 '**-^- 3--"---- 1--- V
U13-22 condensation



0 time l0ms


62


1 ^ ^ ^ ^n .^n -
125 31 161


31


1--ilh>---_a-i


1251__ 31 16 -






0 10 0 10 0


Fig. 17. Single unit response of low frequency fibers as a function of
click intensity.


-40


U8-7


































Fig. 18. Latencies of the peaks in PST histograms
for low frequency fibers. The stimuli was a -30 dB clicks.









LOW FREQUENCY UNITS
RESPONSE TO CLICKS
time (ms)
0 1 2 3 4 5 6 7 8 9
Intensity: -30 dB
Ro RI
Rarefaction 0 0*
6 Condensation 0 0
Cl

U8-19 0 0


U8-20 0


U8-21 0 0
Q 0
S
U8-22 0


U9-6. 0 O


U9-7 0 0



0 0 0
U9-10 0

0

U9-23 O O
o0


U9-29 0 0
o @
UlO- 3 0 0
U10-6. O o
0 0 0


U13-18 0
1-22 0
U13-22
0 0









intensity. In general, the discharge rate of the fiber increases with

increasing click intensity; however, the relative amplitude of the peaks

can change in a complex way. There is a tendency, at higher click in-

tensities, for the early peaks, RO, R1, and C1 to decrease in amplitude

relative to the later peaks (Fig. 16, Fig. 17). This is best illustrated

by U10-3 (Fig. 16) and U8-7 (Fig. 17).


Contributions of the Two Fiber Populations to the AP

The PST histograms of the response of a high frequency fiber to

clicks are similar for both click polarities. The response pattern of

a low frequency fiber changes significantly when click polarity is re-

versed. Since the AP is related to the activity of individual auditory

nerve fibers, the following conclusion can be made. For click stimuli,

the high frequency fiber population contributes a polarity-independent

component to the AP; whereas, the low frequency fiber population con-

tributes a polarity-dependent component to the AP. A more detailed

analysis of the click AP waveform in terms of single fiber activity is

presented in the Discussion.



High-Pass (2.0 kHz) Click Stimuli


AP Response

It is possible to test the conclusions stated above concerning the

contributions of the low and high frequency fiber populations to the

AP. If the low frequency fibers do contribute a polarity-dependent

component to the AP, then elimination of this component from the AP









should produce an AP waveform that is the same for both click polari-

ties. It may be possible to eliminate the polarity-dependent component

by significantly reducing the activity of the low frequency fibers.

The high frequency slope of the tuning curve for a low frequency

fiber is sharp; there is little response area of the tuning curve above

1.0 kHz (Fig. 4). Treating the nerve fiber as a linear filter, a click,

high-pass filtered at 2.0 kHz, should excite the high frequency fibers,

yet have little effect on the low frequency fibers. The AP responses

to high-pass clicks are compared to the responses to clicks (Fig. 19).

The most striking feature of the data is that the waveform of the AP

response to the high-pass click is the same for both polarities; whereas

the response to a click shows the normal changes with polarity. Also,

the broad rarefaction P1 peak has been eliminated by the high-pass

click. The filter used to generate the high-pass clicks did not have

infinite slopes (24 dB/octave), thus the stimulus contained energy in

frequencies below 2.0 kHz. At -10 dB, the high-pass click contained

sufficient energy below 2.0 kHz to significantly excite low frequency

fibers, resulting in the change in AP waveform with stimulus polarity.

Amplitude-latency data as a function of stimulus intensity is

presented in Fig. 20 for click and high-pass click stimuli. Not only

is the waveform the same for both high-pass click polarities, but, as

indicated in the figure, the N1-P, amplitude and the latency of the Nl

peak is the same, except at high intensities.


Single Unit Activity

The AP responses to the high-pass clicks supported the conclusions

concerning the contributions of the low and high frequency fibers to
































Fig. 19. AP response to clicks and high-pass clicks.
Cut-off frequency of the high-pass clicks was 2.0 kHz. The
AP responses to high-pass clicks were recorded from the same
animal immediately following the recording of the AP responses
to clicks. At each click intensity, the calibration bar
applies to all four AP responses.













AP RESPONSE TO CLICKS AND

HIGH-PASS CLICKS (2KHz.)

Lizard 10

CLICK HIGH PASS
CLICK
Cut-Off Frequency: 2.0 KHz.
Intensity (dB)
-70


rarefection



.pr
condensation




-60 -30












-50 -20




16 1
16






-40 -10







16 16 o 7


S--------- ms E ------------
t0 7i
time









AP RESPONSE TO CLICKS

AND HIGH-PASS CLICKS

[2.0 KHz]
Amplitude:
click high-pass
Rare. a 0/
Cond. k ,'

Latency: /
click high-pass / '
Cond. Ma a


100.

o
0

C
0
% 80-
0
a
C,


.. 60-
0

a.




i
60-







L-
a



4 20-
-

S.


n 11


-80


I 60
-60


I I
-40


I -
-20


0


INTENSITY (dB)


Fig. 20. Amplitude-latency plot for clicks and high-pass
clicks as a function of stimulus intensity. The latency data
is plotted only for the condensation stimulus condition since
the latencies for the rarefaction stimulus condition are es-
sentially the same. Cut-off frequency of high-pass click was
2.0 kHz.


Lizard 9


E

z

-2.5 s

u
Z
I.-
-,


5









the AP. That test was based on the assumptions that the low frequency

fibers would not be significantly excited by the high-pass click and

that the response of the high frequency fibers would be similar for

click and high-pass click. The best way to test those assumptions is

to record the responses of single high and low frequency fibers to

high-pass clicks. Even at the relatively high intensity of -20 dB,

the spike activity of a low frequency fiber is significantly reduced

relative to its response to a click (Fig. 21). The single unit response

of a high frequency fiber to a high-pass click is very similar to its

response to a click; although there are small changes in discharge rate

and latency (Fig. 22).



Contributions to the AP for Low Frequency Stimuli


The data presented thus far have shown that the high frequency

fibers contribute a polarity-independent component to the AP and that

the low frequency fibers contribute a polarity-dependent component for

a click stimulus. For low frequency fibers, that result can be extra-

polated to any general, non random stimulus. It is not clear that the

contribution to the AP by high frequency fibers would be polarity-

independent for a low frequency stimulus (a stimulus which has most of

its energy below 1.0 kHz). This is related to the issue, to be dis-

cussed later, of the innervation pattern of afferent fibers in the papilla.

An inspection of the AP response to a low frequency stimulus reveals

little, if any polarity-independent component (Fig. 25). Either the

contribution of the high frequency fibers is polarity-dependent, or it


















click
c AP

.... _.__PST

U high-pass click
---AP
U-b. J .J... ......... PST

U9-23


31,

blz-iii














6 i---m----
_, _A. ,__..__. J. J
310 time 2ms








0 time 20ms


LOW FREQUENCY UNITS
RESPONSE TO CLICKS & HIGH-PASS CLICKS
Cut-Off Frequency: 2.0KHz.
Intensity: 20 dB

Rarefaction Condensation
U8-21


U10-5


0---
4A4---&.














---





o-V-----


Fig. 21. Comparison of the response of low frequency
fibers to click and high-pass click stimuli. The amplitude
scale applies to both the click and high-pass click histogram.
Cut-off frequency of high-pass click was 2.0 kHz. Stimulus
intensity was -20 dB.









HIGH FREQUENCY UNITS
RESPONSE TO CLICKS & HIGH-PASS CLICKS
Cut-Off FreqUency: 2.0KHz.

Intensity: 30dB


Rarefaction


S \ click



S high-pass click






62"


U15-1


125]












62]


U15-2


0 tme 1m
0 time 10ms


1251









0 10


Fig. 22. Comparison of the response of high frequency
fibers to clicks and high-pass clicks. Cut-off frequency of
high-pass click was 2.0 kHz. Stimulus intensity was -30 dB.


Condensation









is polarity-independent, but not evident in this response.

The best way to resolve this question was to record the activity

of individual fibers in response to a low frequency stimulus. A low-

pass (0.6 kHz) click was used as the standard stimulus. PST histo-

grams for a sample of eight high frequency fibers are shown in Fig. 23.

The fibers have been classified as Type I or Type 2. The responses

of both Type I and Type 2 fibers are dependent upon stimulus polarity.

The response of a Type I fiber to a rarefaction low-pass click is simi-

lar to the response of a Type 2 fiber to a condensation low-pass click.

Likewise, the response of a Type 1 fiber to a condensation low-pass

click is similar to the response of a Type 2 fiber to a rarefaction

low-pass click. The histogram of the response of a Type 1 fiber to

a rarefaction low-pass (0.6 kHz) click (or a Type 2 fiber to a conden-

sation low-pass click) is characterized by a single peak; whereas, the

histogram of the response to the opposite polarity shows two peaks.

For most units, the two peaks have approximately the same amplitude;

however, the relative amplitudes of the two peaks can vary, as in U9-21

and Ul0-ll (See arrows in Fig. 23). The effect of polarity on a Type

1 fiber's response is opposite the effect on the response of a Type 2

fiber. A total of 39 fibers were classified as Type I or Type 2 on

the basis of their response to a low-pass (0.6 kHz) click; 22 fibers

were Type 1 and 17 were Type 2.

In Fig. 13, the fibers are separated into Type 1 and Type 2 on

the basis of their response to a low-pass (0.6 kHz) click; however,

the histograms shown represent the responses of those fibers to clicks,

not low-pass clicks. Careful examination reveals small differences in
































Fig. 23. Single unit response of high frequency fibers
to low-pass clicks. Cut-off frequency of low-pass click was
0.6 kHz. See text for explanation of Type 1 and Type 2
categorization of high frequency fibers. St: stimulus wave-
form; AP: whole-nerve response; PST: post-stimulus-time
histogram.










HIGH FREQUENCY UNITS
RESPONSE TO LOW-PASS CLICKS

Cut-Off Frequency: 0.6KHz.


Type 1

U8-3
Intensity(l): -38dB



Srarefaction




condensation
^ -. A .. ______


U8-1 I:-38dB
62-











U11-18 I:-38dB
125





,\^~--------





U10-13 I:-48dB


125








0 time n10ms


Type 2


U9-21 I:-28dB
125-


A_ rarefaction




condensation


U8-8 I:-38dB
62-




-V~----






U9-17 1:-38dB
621












U10-11 I:-48dB
62]










o 10









the latencies of the peaks for rarefaction and condensation clicks.

In general, the response of a Type I fiber to a rarefaction click has

a shorter latency than that of a Type 2 fiber.

The responses of low frequency fibers to low-pass (0.6 kHz) clicks

are shown in Fig. 24. While there are variations in the discharge

patterns of low frequency fibers, most likely related to fiber CF,

the fibers cannot be divided, using the same criteria used for high

frequency fibers, into two or more types.

The response of a Type 1 or a Type 2 high frequency fiber depends

upon stimulus polarity; however, because of the unique symmetry in the

response of the two types, the net contribution to the AP of all the

high frequency fibers is polarity-independent. The response of a low

frequency fiber changes with stimulus polarity; the net contribution

of all the low frequency fibers to the AP is polarity-dependent.



Low-Pass Click Stimuli


AP Response

With the knowledge gained in the analysis of the click AP, and

the understanding that the contribution to the AP of high frequency

fibers is polarity-independent while the contribution of low frequency

fibers is polarity-dependent, it was possible to investigate the use of

the AP as a research tool. AP data for low-pass clicks are shown for

various cut-off frequencies (fc) and intensities in Fig. 25 and Fig. 26.

For fc equal to 1.0 kHz or less, the AP consists almost entirely of a

polarity-dependent component. For fc greater than 1.0 kHz, the AP has

a significant polarity-independent component.































Fig. 24. Single unit response of low frequency fibers
to low-pass clicks. Cut-off frequency was 0.6 kHz. Even
though there are some differences in the response patterns
of the fibers, the fibers cannot be separated into two types
in the same way as the high frequency fibers.










LOW FREQUENCY UNITS
RESPONSE TO LOW-PASS CLICKS

Cut-Ofi Frequency: 0.6 KHz.


Intensity: -38 dB

U9-7
!50 rarefaction St

-- AP

A A PS
condensation
st



PS



U9-6
125












U13-18
1251 \












U9-23





-""-- --
2 I 5








0 time lOms


1:-48dB

U13-4
250-



A. A








U9-11
250- __












U13-7
125-,












U10-7
31










0 10









AP RESPONSE TO LOW-PASS CLICKS
Rarefaction Intensity(dB) Condensation


Cut- Off Frequency 0.2 KHz. Lizard 10

St

IV I AP -68



2 -58 2



8 -48 8



-38





0 20 0 20


Cut-Off FrequencyiO.6


-I





61 -

12


12r


12


0 time 10ms



Fig. 25. AP response to
of stimulus intensity.


Lizard 9


0 10


low-pass clicks as a function
































Fig. 26. AP response to low-pass clicks as a function
of cut-off frequency. Stimulus intensity was -48 dB. Stimulus
not shown for cut-off frequencies greater than 1.0 kHz.










AP RESPONSE TO LOW-PASS CLICKS

Intensity: -48 dB

Lizard 11

Cut-Off Frequency (KHz.)


St
AP
0.21
S --St

AP


rarefaction

(Hz.


condensation


0.4 1.5


0.6
,v--- -6


0.8 4.0


Vu-_


1.I
0 ti -e 1.,





O time 10ms


(click)




0 10


4p I


o'v.



.s-- '


2.0


--- --.









An amplitude-latency plot as a function of cut-off frequency is

shown in Fig. 27. Of particular interest is the discontinuity in the

amplitude plot for the rarefaction stimulus and the obvious change in

the nature of the latency data which occurs between 1.0 kHz and 1.5

kHz. There is a difference in amplitude with polarity for all cut-

off frequencies; however for fc greater than 1.0 kHz, the difference

in amplitude is small relative to the amplitude for a rarefaction stimu-

lus. For fc less than 1.5 kHz, the difference in amplitude is larger

than the amplitude for the rarefaction stimulus. This suggests a large

polarity-independent component for fc greater than 1.0 kHz and a small

component for lower cut-off frequencies. The latency of the response

is independent of stimulus polarity for fc greater than 1.0 kHz, and

dependent upon polarity for lower cut-off frequencies. These data

indicate a transition from a predominately polarity-independent re-

sponse to a polarity-dependent response as fc is decreased. Decreasing

fc to 1.0 kHz should affect the response of the high frequency fibers

more than the low frequency fibers; thus, this amplitude-latency plot

supports the conclusion that the contribution of high frequency fibers

to the AP is polarity-independent while the contribution of low fre-

quency fibers is polarity-dependent. It is clear from the single unit

data in Fig. 23 that high frequency fibers do respond to a low-pass

(0.6 kHz) click. The lack of an obvious polarity-independent component

in the AP is a problem; but, there is an explanation. First, for a low

frequency, low-pass click, the low frequency fibers have a higher dis-

charge rate and may tend to dominate the AP (Fig. 23, Fig. 24). Second,











AP RESPONSE TO LOW-PASS CLICKS


00
(cl ick)


CUT-OFF FREQUENCY (KHz)


Fig. 27. Amplitude-latency plot for low-pass clicks as a function of cut-
off frequency. N1 P, was the first negative-positive complex evident in the AP
responses in Fig. 26.


-3.5



E

-2.5 z

a
u
Z
u
-1.5 .
-i









a comparison of the response of high and low frequency fibers demon-

strates that the response patterns for a low frequency fiber and a Type

2 high frequency fiber are similar (Fig. 28). The total contribution

to the AP of these two fiber groups probably obscures the contribution

from the Type I high frequency fibers. In Fig. 28, the Type 1 fiber

response may be reflected in the small dip (arrow) in the AP response

to the rarefaction low-pass click. At higher low-pass (0,6 kHz) click

intensities, the AP response to the condensation stimulus shows a broad-

ening of the first negative peak and then a decrease in latency until,

at -18 dB, the latency is the same as for the first negative peak in

the AP for the rarefaction stimulus (Fig. 25). This suggests that the

polarity-independent contribution of the high frequency fibers is more

obvious at high intensities.


Single Unit Activity

Single unit data for the response of high frequency fibers to low-

pass clicks of various cut-off frequencies are shown i'n Fig. 29. The

main point to observe is that the classification of a high frequency

fiber as Type 1 or Type 2 is consistent with the response of that fiber

to low-pass clicks with cut-off frequencies other than 0.6 kHz. Low-

frequency fibers cannot be separated into two types even if the cut-

off frequency of the low-pass click is varied (Fig. 30).

As the cut-off frequency of the low-pass click is increased

above 1.0 kHz, the responses of low frequency fibers demonstrate a

pattern that is similar to the phenomenon observed for low frequency

fibers when click intensity is increased above about -30 dB (Fig. 30,





UNIT RESPONSE TO LOW-PASS CLICK
Cut -Off Frequency: 0.6 KHz.
Intensity:- 38 dB
Lizard 10


Stimulus

AP


Low Frequency
U10-7

High Frequency
U10-11 Type 2


High Frequency
U10-1 Type 1


Rarefaction








-~-------


Condensation


~iK





AL_


10ms 0


Fig. 28. Comparison of the
auditory nerve fibers. Stimulus
was 0.6 kHz.


AP response and single unit activity of individual
was a -38 dB low-pass click; cut-off frequency


time


1































Fig. 29. Single unit response of a Type I and a Type 2
high frequency fiber as a function of low-pass click cut-off
frequency. Note the similarity in the response of the two
types of fibers to stimuli of opposite polarity.










HIGH FREQUENCY UNITS
RESPONSE TO LOW-PASS CLICKS


Type 1
Ull-25
125 /-\\rarefaction


7 E I:- 38dB
/ condensation



125-


I:-36dB





125


1=-34dB





62-


1251







tm 1m----
I :- 20dB





0 time 10ms


Cut-Off Type 2
Frequency (KHz.) U11-11
621 rarefaction
P ] r --- -


0.6 |-28dB
iT 0.6 '2--8e d --- --

condensation


iT
--A----
62,


08 l:-26dB
0.8





621


1.0 L -- -4dB


2.0


621



L_ 28dB





10


00
(Click)



































Fig. 30. Single unit response of two low frequency fibers
as a function of low-pass click cut-off frequency. Stimulus
not shown for cut-off frequencies greater than 1.0 kHz.










LOW FREQUENCY UNITS
RESPONSE TO LOW- PASS CLICKS


Cut-Off
Frequency (KHz.)


U13-17


125 rarefaction St




'condensation St

^ AP
l A PST
1251


S-4 o0.6










0.8
-N ^













62



S1.0---
,.,. 1o


62,


1:-30




S--ime i_ s

0 time tOres


00


(Click)


U13-19


/ 1, ,1:-38

0.2 J



-jrr

0 20
125-


I: -48
0.6









r---,
125
1.0








1:-28
2.0





62]




is-20




1 10
. A _/_ .-









Fig. 31). The early peaks are decreased in amplitude relative to the

later peaks. From a linear systems viewpoint, adding energy above 1.0

kHz should not affect the response of low-frequency fibers since their

tuning curves show little response area above 1.0 kHz.



Narrow-Band Click Stimuli


It may be possible to excite a smaller group of fibers with a

narrow-band click than a low-pass click because the energy in a narrow-

band click is contained within a smaller frequency range than the low-

pass click. For narrow-band center frequencies of 2.0 kHz and 4.0 kHz,

the AP response is the same for both polarities and, therefore, it

consists entirely of a polarity-independent component (Fig. 32). For

lower center frequencies, the AP consists largely of a polarity-dependent

component. It is clear in the plot of amplitude-latency as a function

of narrow-band click center frequency that there is a significant

change in the AP response as the center frequency varies from 1.0 kHz

to 2.0 kHz (Fig. 33). Above this frequency region, amplitude and latency

are independent of stimulus polarity; below this region, they are depen-

dent upon polarity.



Tone and Tone Burst Stimuli


AP Response

For low frequency tone bursts, the AP shows synchronous, phase-

locked neural activity in the form of regular, diphasic complexes

(Fig. 34). A comparison with the stimulus waveform reveals that this

































Fig. 31. Comparison of the single unit response of low
frequency fibers to clicks and low-pass clicks. Cut-off fre-
quency of low-pass clicks was 1.0 kHz.











LOW FREQUENCY UNITS
RESPONSE TO CLICKS & LOW-PASS CLICKS
Cut-Off Frequency: 1.0 KHz.


Low-Pass Click


Click


1251 rarefaction




condensation




/d------
P J



1251












1251 A_
-^--


.~a_. J ^-- ^-
31




0 time 10ms


U9-7
Intenslty:-30dB

St 6
AP

PST
St

AP
PST


U9-29 1-20
621











U13-18 1:-20
621








-, I --- __ ^i .. r*


U10-7 1:-20
31]









0 10


~-L--

hNC M_- _CL~-U

































Fig. 32. AP response to narrow-band clicks as a function
of center frequency. Stimulus intensity was -50 dB. Stimulus
is not shown for center-frequencies greater than 1.0 kHz.











AP RESPONSE TO NARROW-BAND CLICKS
Lizard 15

Intensity; -50 dB

Center Frequency (KHz.)

St
rarefaction

8piv ~ "AP 0.2
St

nd AP
condensation


1 -16


0.














St0.





0 time 10mi


0 10


8 J~`











AP RESPONSE TO NARROW -BAND CLICKS


Lizard 15
Rare.
tude e


Intensity: -50 dB
Cond.
M


O 0


s1 5 0 4.0

1E


`O I z
-3.0
`00
v
----------- 0
-~8-`----,,Q-------aU
-' z .


I I i I I 1
1.0


I I I I


CENTER FREQUENCY (KHz)


Fig. 33. Amplitude-latency plot for narrow-band clicks as a function of.
center frequency. Data based upon the AP responses shown in Fig. 32.


Ampli


Latency
































Fig. 34. AP response to tones and tone bursts as a function
of stimulus frequency. Averages were computed by triggering
to the positive zero crossings of the sinusoidal stimuli. Tone
bursts had fast rise times and a duration of 30 ms.












AP RESPONSE TO TONES & TONE BURSTS

Lizard 13
Intensity:--40 dB [unless noted]

Frequency ( KHz.)


1: -30
rarefactio
8 pv Ton

8 i- v--- --.- Ton

0.
condensation
STon

Ton








8

0.










8

21
0














0





0 time 40ms
^^^'m^V^


e Burst

ie

.1

e Burst

ie


7J1------'--I

2 1.0 2














.4 2.0


.6 4.0


0 40
0 40


4


fJAAfAA^kAA&V0AAAAMjv0*Mv^^
^:,A ^ ^ ^





.' AV^AAMA^^hZ~\M-nMAMAVAA.









phase-locked activity occurs once-per-period of the sinusoidal stimu-

lus (Fig. 35), and shifts 180 degrees as stimulus polarity is reversed.

As tone burst frequency is increased, the diphasic complexes move to-

gether in time, until around 0.4 kHz they begin to overlap. As the

frequency is increased, the net result, except at the onset of the

tone burst, is sinusoidal in appearance. For frequencies of 1.0 kHz

or higher, there is little phase-locked activity evident in the AP.

The above description applies as well to the AP response to tones

(Fig. 34).

The sinusoidal component in the AP at frequencies of 0.4 kHz to

0.8 kHz could be cochlear microphonic (CM) or electrical artifact.

If this was true, then for a tone burst stimulus, the CM or artifact

should be evident before the synchronous neural activity begins.

Evaluation of the data in Fig. 35 does not reveal any CM or artifact-

like component in the first 2 ms of the recorded response. An attempt

was made to record electrical artifact. The speculum was removed

from the external auditory meatus and plugged. The earphone was ro-

tated slightly and the speculum placed against the side of the lizard's

head. The "AP" recorded in this condition is compared to the AP re-

corded under normal conditions just prior to this test (Fig. 36).

The electrical artifact is small compared to the recorded neural ac-

tivity.


Single Unit Activity

The spike activity of an individual high frequency fiber is phase-

locked, once-per-period, to the sinusoidal stimulus (Fig. 37). The

phase-locking is present for frequencies as high as 1.0 kHz, but by

2.0 kHz, it is not present in the processed data. The responses of four







AP RESPONSE TO TONE BURSTS
Intensity:-40 dB (unless noted)


Frequency (KHz.)
0.1 1: -30


16-1 St



condenses 1on
k~- AP


0.2


S time 20m


Lizard 13
0.4



-?V/-



1'V



0.6




16 -















V 1


o o


1.0





16 V V


4.0


161


I0 10


Lizard 15
0.4 I -50


8


I



0.6



8







2.0




16






0 6C


Fig. 35. AP response to tone bursts as a function of stimulus frequency. The
right most column illustrates the DC-like positive potential evident in the AP response
for some frequencies.
































Fig. 36. AP response to tone bursts as a function of
stimulus intensity. The electrical artifact was recorded by
the wire electrode after plugging the speculum to significantly
reduce the acoustic stimulus. Stimulus intensity was -20 dB
for frequencies of 0.2 kHz and 0.6 kHz.









AP RESPONSE TO TONE BURSTS
Lizard 14


Frequency(KHz.l
Rarefaction
St
SAl



0.2
8




a2 B
Art -20



2

4 \ .









Art -20


Intensity (dB)


-70
-60

-50

-40

-30

-20


B I\---" -.



8 time 2
0 time 20ms


Condensation

\J \

2










,I,














---v~ 'A^^^^^
4

















,\------^ "2
-^ ---- -.-^ 2

-^------- 4



....-- i -

0 20









HIGH FREQUENCY UNIT
RESPONSE TO TONE BURST

U15-2


Frequency (KHz.)


Intensity- 30 dB
rarefaction

'- St
AP




AP.

PST
0 20
1:-40




0.6







1:-40




0.8





r0 time 10ms


1:-40
1251



1.0







1:-50
621 _N_



2.0





0 10
1:-50
125]



2. 0.--.---
2.0





70


Fig. 37. Single unit response of a high frequency fiber
to tone bursts of different frequencies. The histograms
were triggered on the positive zero-crossings of the stimulus.
Stimulus not shown for frequencies greater than 1.0 kHz.









high frequency fibers are shown in Fig. 38. Careful examination

reveals that the phase-locked activity of U15-4 and U15-5 is 180 degrees

out of phase with the activity of U15-7 and U15-8. While there is in-

sufficient data to relate this result to the Type 1, Type 2 classi-

fication of high frequency fibers, the result does indicate that the

once-per-period, phased-locked activity in the AP is associated with

the low frequency fibers.

Individual low frequency fibers exhibit once-per-period, phased-

locked activity to low frequency sinusoidal stimuli, and little response

to frequencies above 1.0 kHz (Fig. 39). While there is some variation

in the phase of the response of the low frequency fibers to the same

stimulus, this variation is much less than 180 degrees (Fig. 40). This

supports the above statement that the phased-locked AP activity is

contributed primarily by low frequency fibers.










HIGH FREQUENCY UNITS
RESPONSE TO TONE BURSTS
Intensity: -50dB

Frequency: 0.6 KHz.


U15-4
125 ,,\ -\ St
rarefac~ton


S PST
St
condensation
AP

PST



U15-7





12 I 5




0 time -10m
0 tIme 10mB


U15-5
125 1 F --












U15-8










0 10
--J /^Y---


Fig. 38. Single unit response of high frequency fibers
to tone bursts. Tone burst frequency was 0.6 kHz.










LOW FREQUENCY UNIT
RESPONSE TO TONE BURST

U15-12

Intensity :-50 dB (unless noted)

Frequency (KHz.)


62- St
.--v AP
j rareiaction
PST 0.2


i AP

P St

o 20

125 0.



0.4








125]



1, 0.6






0 time 1oms


125










1.0















31










02.


Fig. 39. Single unit response of a low frequency fiber
to tone bursts of different frequencies. Stimulus not shown
for frequencies greater than 1.0 kHz.











LOW FREQUENCY UNITS
RESPONSE TO TONE BURST

Frequency(KHz.)


U9-23
Intensity:-30dB
125 St


AP
PST 0.18
st
AP
PST



U15-17 1-40
125-
125 rarefac ion

SI 0.2

~ nsat cond





U15-11 1:-40
1251



0.2,





Time 20ms


U15-15 1:-50
621n"



0.6 I_








U15-13 V:-50



0.!2s1 25i1








U15-11 I:-50


0.1251

0.6





o i0


Fig. 40. Single unit response of low frequency fibers
to tone bursts.




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