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260 L a u r ie r A venue O tta w a , O n ta r io ’ KIN 6 P 4 ,

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V f

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.

t

STUDIES IN EVOKED POTENTIAL AUDIOMETRY'

University — Universite .UNIVERSITY OF OTTAWA D e g r e e (or which thesis w as p r e s e n te d — G rade p o u r lequel c e tte th e s e fut p r e se n te e P h .D .

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1984

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I

o STUDIES IN EVOKED POTENTIAL AUDIOMETRY

by

David Richard Stapells O

r A dissertation submitted to the School of Graduate Studies of the University of Ottawa in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Psychology. Ottawa, Canada ° December 1983

©

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c ' U N I V E R S I T E D ' O T T A W A / U N I V E R S I T Y OF O T T A W A E c o le des Etudes s u p £ r ie u r e s / S c h o o l of G r a du a te S tu di es et de la r e c h e r c h e aS^d R e s e a r c h

NAME OF AUTHOR

STAPELLS,

--------- 41— TITLE OF THESIS

DEGREE

P h .D .

D a v id

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>

STUDIES IN EVOKED POTENTIAL AUDIOMETRY

(P sy c h o lo g y )

1984

YEAR GRANTED

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STUDIES IN EVOKED POTENTIAL AUDIOMETRY , ' » STAPELLS, D a v i d R i c h a r d

Name of candidate < Degree

P h .D .

P sT e o f defence

Department January 1 8 ,

PSYCHOLOGY

1 9 8 4 ..- . ✓

k

.

Tim i Iicms jtrcjMrcit l i mh-i t h e s u p e r v i s i o n o f

T .W .

P ic to n

has been approved 6y a jury

c o m p o s e d o f th e f o l l o w i n g e x a m i n e r s :

.

K.

CAMPBELL

R.

GALAMBOS

K.

MARSHALL

D.

STU SS

A

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.

A

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(Dean of Graduatr Studies)

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Copyright by David Richard Stapells 1983

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ACKNOWLEDGEMENTS

J

I

would

unfailing

like

guidance

to

thank

Dr.

Terence

Picton

for

and encouragement over the past four

his

years.

His impatience to "Jcnow" was always a challenge, but his patience in the achieving of this goal I

would

made the process enjoyable. ■* ,• also like to thank Dr. Andr£e Durieux-Smith,

who

^provided the setting for' several of the studies completed in this thesis.

Her

advice

and

enthusiasm

for

these

studies

were

essential. Finally,

I gratefully acknowledge the financial support

of

4

the Natural Sciences and Engineering Research Council (NSERC) of * x Canada.

ill


VITA

David the

Richard

Stapells was born February 11, 1955,

amidst

mountains and the sea in Vancouver, B.C., Canada*

-Om he _ graduated receiving

from

Simon Fraser University

in

In %. Burnaby,

1979, B.C.,

a Bachelor of Arts (Honours) degree in Psychology with

a minor in Kinesioldgy. *■

,

He of

Philosophy at the University of Ottawa in January, 1980.

research his

N. r “ V began work on the requirements for the degree of Doctor

supervisor was Terence W. Picton, MD, PhD.

postgraduate

studies

His’

Throughout

he wjis financially supported

by

the

Natural Sciences and Engineering Research Council 6f Canada.

THESIS AND PUBLICATIONS

Stapells, D.R. (1979). Effect of hyperventilation and breathhold on the brainstem evoked response. Unpublished Honours thesis. Department of Psychology, Simon Fraser University, Burnaby, B.C. Picton, T.W., Seguin, J.F., Hamel, G., Talajic, M., & Stapells, D.R'. (1981). Somatosensory potentials. Sensus, 1_, 9 - 20. Stapells, D.R. & Picton, T.W. (1981). Technical aspects-of brain stem evoked potential audiometry using tones'. Ear and Hearing, 2 , 20 - 29. Picton, T.W., Stapells, “D.R., & Campbell, K.B. (1981). Auditory evoked potentials from the human cochlea and brainstem. Journal of Otolaryngology Supplement, 9 , 1 - 4 1 . Stapells, D.R., Picton, T.W., & Smith, A.D. (1982). Normal hearing thresholds^for clicks. Journal'of the Acoustical "Society of America, 72, 75 - 79. Stapells, T.W., Picton, T.W., & Smith, A.D. The calibration of click intensity. Sensus (in press). •i iv


Stapells, D.R., Suffield, J.B., & Picton, T.W. (1982). Effects of stimulus presentation rate on the middle-latency auditory evoked potentials. Journal of the Acoustical Society of America Supplement, 72, S54. (Abstract) Picton, T.W., Stapells, D.R., Perrault, N., Baribeau-Br&un, J. and Stuss, D.T. Human event related potentials. Current perspectives. In R.H. Nodar & C. Barber (Eds.), Evoked Potentials II. New York: Butterworth. (in press) Stapells, D.R., Linden, R.D., Suffield, J.B., Hamel, G.,’ & Picton, T.W. (1984). Human auditory steady state potentials. Ear and Hearing, (in press) Galambos, R., Kileny, P., Stapells, D.R., & Thornton, A.R.(1983). The 40-Hz Event Related Potential (ERP): Theory and • application. ASHA, 25, 154. (Abstract) Linden, R.D., Hamel, G..,, Stapells, D.R., & Picton, T.W. (1983). Human auditory steady state responses analyzed with Fourier analysis: The zoom technique. Journal of the Acoustical Society of America Supplement, 74, S65. (Abstract). Stapells, D.R., Per^z-Abalo, M., Read, D., & Picton, T.W. (1984). Frequency. specificity: Problems and solutions. In J.T. Jacobson (Ed.) The Auditory Brainstem Response. San Diego: College-Hill. (in press) ? Stapells, D.R., Picton, T.W., & Durleux-Smith, A. Estimation of threshold in normal and hearing-impaired individuals using *■auditory evoked potentials. In preparation.

V


I

’■'sN

.ABSTRACT OF THE DISSERTAT: IV T J O j

N '.

Studies in Evoked Potential Audiometry..

David Richard Stapells

Doctor of Philosophy (Psychology) University of Ottawa, 1983

Four

sets' of- experiments were performed to evaluate

psychophysical behavioral

thresholds

to

brief

.the

and

how

these

thresholds may be estimated using the auditory evoked

potentials. The

stimuli

human

1

normal hearing thresholds for the clicks used to elicit

brainstem .auditory evoked potentials were evaluated, and the

effects " oh these thresholds of varying the polarity and symmetry of

the

click assessed. Threshold decreases 4.5 dB

change young

in

per

rate. The average threshold obtained from

40

tenfold normal

adults using 100-us square-wave clicks presented through a

TDH-49

earphone

equivalent

at

10/s was 36.4 dB peak SPL or 29.9

dB

peak

SPL. A root-mean-square measure of the pressure

over

the initial millisecond — measure

SPL(1ms) —

piovides a more consistent

of threshold with clicks of differing symmetry than

peak or peak equivalent measuresv

VI


the

*

The

brainstem

vertex-positive Changes when

so

response

component.

The

morphology

that a vertex-negative component

high-pass 'filter

slopes

contain^ a large V__ of this response

to brief tones

are

used.

settings above 20 Hz

Tones

frequency-specificity,

becomes

with longer

and

prominent '

high

rolloff

rise-time^, have

greater

but rise-times longer than 5 ms result in

brainstem

responses

with 'smaller

amplitudes.

responses

to high-intensity -stimuli are not

The

brainsteA

frequency-specific,

and notched noise masking should be used. Stimulus

rates of 40-45/s result- in a 40-Hz 9 response which is about twice the amplitude of the 10

sinusoidal and

,

presentation

60/s responses. The 40/s response shows a linear decrease in

amplitude

and

intensity

is

recordable

to

High-frequency Signal

a

linear

functions.

in

latency

when

decreased from 90 to 20 dB nHL. This within stimuli

averaging

amplitude/rate,

increase

a

few

dB

resylt

in

of

stimulus

response

behavioral

threshold.

lower-amplitude• responses.

and Fourier analysis provide nearly

. amplitude/intensity,

Fourier

analysis,

is

and

identical

latency/intensity

however, may be

the

faster

and

evaluated

for

t

less-expensive method. Eight

evoked

frequency-specific

potential

techniques

objective audiometry

were

at 500, 1000, 2000, and

\

4000 tests

Hz

in normal-hearing

were

Response

Slow Response, the Transient

(MLR), the

brainstem Notched

the

response

and hearing-impaired

40-Hz

Steady State

techniques

(Derived

subjects. Middle

Potential, Responses,

The

Latency and

Clicks

Noise, unmasked Tone responses, Tones in Notched vii


five in

Noise,

and^rones in White Noise).

The high noise intensities- required

to mask clicks may result in small and variable responses and may cause

temporary threshold shifts. The ABR/Derived Responses

ABR/Clicks useful

in Notched Noise tests do not therefore,appear to

variable

not

appear to be useful for EP audiometry.

and

be

for EP audiometry. The Transient Middle Latency Responses

are

that

and

the

and thresholds are difficult'to-'deêrmine and

The results indicate

auditory brainstem responses to tonal stimuli

unmasked) are the best for audiometry.

thresholds thresholds. prediction

were

within

Noise in

the

4 - 6

masking

of

presence

of

(masked

On average, response

'dB

of

the

tones

steep

do

pure

tone

behavioral

improves

high-frequency

losses.

viii


threshold hearing

TABLE OF CONTENTS page ACKNOWLEDGEMENTS .................................... iii VITA AND PUBLICATIONS ............................

. iv

ABSTRACT OF THE DISSERTATION.........................

vi

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

1

PAPER I. NORMAL HEARING THRESHOLDS FOR CLICKS .......

4

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

5

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

6

METHODS .......................................

9

A*

Subjects

............................

9

B • Stimuli ..............................

9

C . Experimental procedure ................

11

D.

12

Data analysis......

V RESULTS .......................................

13

A. Experiment 1: Normal thresholds for 100-us clicks ...................

13

B. Experiment 2: The rate of stimulus presentation .............

13

C. Experiment 3: Duration of listening period...................

14

D. Experiment 4: Click symmetry ..........

14

DISCUSSION.......

16

FOOTNOTES ........................ ............

22

ACKNOWLEDGEMENTS ..............................

23

TABLES .......................................

24

ix


I

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

27

FIGURE LEGENDS ................................

31

FIGURES ................................. '.....

32

PAPER II. TECHNICAL ASPECTS OF BRAINSTEM EVOKED POTENTIAL AUDIOMETRY USING TONES ..........

38


39

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

40

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

41

METHODOLOGY ............................

44

RESULTS AND DISCUSSION .........................

47

Experiment 1: High-pass filter settings ....

47

Experiment 2: Stimulus presentation rate ....

50

Experiment 3: Location of the reference electrode ...................

52

Experiment 4: Intensity effects .••••••.....

53

Experiment 5: Stimulus rise-times .........

56

Experiment 6: The effects of notched noise ..

59

CONCLUSIONS ...................................

62

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

64

FIGURE LEGENDS ................................

69

FIGURES .................................

74

PAPER III. HUMAN AUDITORY STEADY STATE POTENTIALS ....

85


86

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

87

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

88

GENERAL METHODS .......

91

Subjects ................................. X.


91

I

Stimulus generation .......................

91

EGG recording^S^fcedures ......... ........

92

...... .

93

RESULTS AND DISCUSSION ........................

95

Response analysis

Experiment

1: Stimulus presentationrate ...» 95

Experiment

2: Fourier analysis andaveraging*

97

Experiment

3: Stimulus intensity .........

99

Experiment

4: The zoom technique .........

101

CONCLUSIONS .....

105

REFERENCES .................................... 109 TABLE 1 ....................................... 113 FIGURE LEGENDS ................................ 114 FIGURES ...................................... C, PAPER IV. ESTIMATION OF THRESHOLD IN NORMAL AND

119

I ■

HEARING-IMPAIRED INDIVIDUALS USING AUDITORY EVOKED POTENTIALS •,....................... 132 ACKNOWLEDGEMENTS ..............................

133

ABSTRACT ...................................... 134 INTRODUCTION .................................. 136 Slow Responses ...........................

137

Middle Latency Response ................... 139 Auditory Brainstem Response ...............

142

The 40-Hz Response

149

Summary .................................. 151 M ETHOD ....................................... Subjects

..........................

152 152

Stimuli .................................. 154 xi


EP recordings ..............

159

Experimental procedures ........

160

Data analysis:Response criteria ............ 162 Evoked potential measurements. 164 Statistical analyses .......

166

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

167

(A) Normal subjects:

~

ABR/Derived Responses ........... . 167 ABR/Clicks in notched noise........ 168 ABR/Tones (masked and unmasked)..... 169 i• Transient MLR (10/s) . 171 The 40-Hz steady state potential.... 171 Slow Responses

........

172

(B) Hearing-Impaired subjects .............

173

(C) Test evaluation.....

181

(i)

Test accuracy ..............

181

(ii) Inter-subject consistency .... 184 (iii) Response clarity

....... 185-

(iv) Inter-rater reliability .....

186

(v)

186

Combined results ...........

DISCUSSION ..

187

(A) Normative d a t a ........................ 187 Effects of stimulus intensity

...... 188

Effects of stimulus frequency

...... 188

Effects of stimulus masking .......

189

(B) Audiometric usefulness ................

191

Tests with poor scores ...........

192

xii


I

Tests withintermediate scores ..... 195 Tests with

bestscores .............197

(C) General comments ..............

199

(D) Recommendations ...............

• 201

REFERENCES ..................................... 203 TABLES .........................................220 FIGURE LEGENDS ................................ 230 FIGURES ........................................250

\ S.

Cj

xiii


I

1

^

U

'

;

INTRODUCTION

I This

thesis

consists of four papers, each representing

attempt to provide some answers to questions in audiometry.

The

format

an

evoked potential

of each paper is that required

by

the

journal in which it has been (or will be) published. The first paper investigates a topic which is fundamental to —

the recording of auditory '

and

physical

audiometry. of America, clicks,

calibration

of the acoustic stimuli

Published in the

used

in

this paper provides a normal reference threshold for

stimuli.- The studies in this paper also lay

groundwork

EP

Journal of the Acoustical Society

and compares techniques for the physical calibration

acoustic

for

evoked potentials (EP): the behavioral

for

the calibration of normal behavioral

down

of the

thresholds

tonal stimuli. These thresholds are presented in the

fourth

paper. The

goal of the second paper, published in Ear and Hearing,

was to determine the optimal stimulation and recording techniques for this and

brainstem.EP audiometry using tonal stimuli. In

particular,

paper demonstrates the effects of amplifier filter settings rolloff slopes, stimulus frequency, stimulus rise-time,

notched

noise masking on these responses.

experiments

The results of

and these

are summarized as a set of recommendations for using

this technique. The

third

paper,

to

be

published

in

Ear and Hearing,


I 2

summarizes experiments investigating the recently-described 40-Hz Steady State Potential. Recent response

might

studies suggested that this "new"

prove to be very useful for EP audiometry.

The

results of the studies presented in this paper indicate that this steady

state

potential

I

audiometry.

does

Furthermore,

show

the

promise

results

of

for this

objective

paper

also

indicate that the use of the frequency-based technique of Fourier analysis more

to

record the 40-Hz response may

quickly,

more

objectively,

and

provide

less

information

expensively

than

paper of this thesis presents the results

of . a

conventional signal averaging techniques. The study

final

which evaluates the usefulness of

frequency-specific determine optimal

objective-

eight EP techniques for

audiometry.

The objective was

the best test. Each technique was evaluated using stimulus and recording protocols to provide

to the

information

within a specified time. All techniques were evaluated in each of ten normal-hearing and ten. hearing-impaired subjects. The results presented from

in this"V>aper clearly delineate the poor EP techniques

the better techniques, and demonstrate their strengths

weaknesses. j,,

recommendations

for the practice of evoked potential audiometry. The

*:

The paper concludes with a set of

and

four

papers

to answer some of the potential

in

thesis

represent

an

attempt

technical and practical problems of evoked

audiometry.

providing, an

this

accurate

audiometric information

They

are united by the overall

and efficient

technique

for

goal

of

obtaining

as early as possible in a child's life.

-a


-

StapelIs e t a l .

Page

4

J . Acoust. Soc. Amer.

NORMAL HEARING THRESHOLDS FOR CLICKS

David R. S t a p e ll s

_

;-f

School of Psychology, University of Ottawa, 651 Cumberland, Ottawa Canada

KIN 6N5

I Terence W. Picton

Department of Medicine, University of Ottawa, Ottawa General Hospital 501 Smyth Road, Ottawa, Cartada r K1H 8L6

Andr§e D. -Smith,Department of Audiology, Chi 1dren1s"Hospital of Eastern Ontario, 401 Smyth Road, Ottawa, Canada

K1H 8L1


/

Stapells et a l .

Page

J. Acoust. Soc. Amer.

ABSTRACT This

paper

evaluates the normal hearing

thresholds fo r c l i c k s

and as se ss e s the e f f e c t s on these thresholds of varying the duration of the l i s t e n i n g period and the pr e se n t ati on - ra te , of the c l i c k s . \

There were no

po la r it y

and symmetry

s i g n i f i c a n t changes in threshold a s t h e

\

l is t e n i n g - p e r i o d decreased from 2 s to 300 ms.

There was,

however, a

2 .5 dBNincrease in threshold as the li s t e n i n g - p e r i o d decreased from 300

\

t o 100 ms., creased

Increasing stimulus pre se nt atio n- ra te from

threshold 4 .5 dB per tenf old change in

s i g n i f i c a n t d if f e r e n c e s tion (

i'—

adults

The average \

using

100 ; js

\

ra te .

to 80/s de

There

were no

in threshold between rarefa ction and condensa-

\

clicks.

5

threshold

square-wave

obtained from 40 normal

clicks

presented

young

through a TDH-49

\ earphone at 10/s was 3 6 . 4 peak SPL or 29.9 Neither peak SPL-nor peak equivalent thresholds f o r c l i c k s \

mean-square - SPL(lms)

measure

with of

the

dB peak equivalent

SPL measurements

different

\ pressure

-

degrees over

of

SPL.

gave c o n s is t en t

symmetry.

the i n i t i a l

A root-

millisec ond

gave a threshold of 2 5 . 6 \ d B . This SPL(lms) measure of \ threshold proved to be far more c o n s is t e n t "for c l i c k s with d i f f e r e n t \

degrees of symmetry than e i t h e r the peak SPL or the peak equivalent SPL measures. \ PACS numbers: 43.66.C, 4 3 .6 6. S , 43.63.R


5

S t a p e l l s et a l .

Page

6


INTRODUCTION Brainstem auditory

evoked

A po t en ti a ls are e x t e n s iv e ly used

to

eva luate human auditory and neurological function (Jerger e t a l . , 1980; Stockard

et

a l . , 1980; Glasscock

et

a l . , 1981; Rowe,

la bo r ato r ie s use broad-band c l i c k s generated by (10-250 ,us) standard

Most

passing short-duration

e l e c t r i c a l pulses through an earphone.

technique for

1981).

c a lib r atin g the i n t e n s i t y

There i s as y e t no of

these

stimuli.

Both behavioral and acoustical c a li b r a ti o n s are presently employed. There are two methods of obtaining intensity.

a behavioral

c a li b r a t io n of

In the f i r s t method stimulus i n t e n s i t y i s measured r e l a t i v e

t o the behavioral ("sensation level" cannot be used as

threshold

of

or SL).

the p a r tic ul ar

This i s

subject

an important

a standard because

it

%

will

being te s ted

measurement but i t

vary with

the ambient

n o i s e , the abjîty of the subject to respond a c cu ra te ly , and the degree ■s

of

hearing

loss.

Most

labora tor ies

there for e

obtain

the average

threshold o f ten normally-hearing

young adults (Picton et a l . ,

This second

c a li b r a t io n gives

level"

method of behavioral

or nHL reference.

This

thresholds ("hearing level" 1969)

but

duration,

is

specific

is

or HL)

1977).

a "normal hearing

s im ila r to the standard reference fo r pure

to the stimulus and

frequency-spectra and rate of

tones (ISO,

1'964; ANSI,

to the laboratory.

presentation

of

w i l l vary among la b o r a to r ie s.


The

the stimuli

is c l i c k s of

different

polarities

when presented binaurally at

rates

from

20 to

102/s. t

Increasing the rate of decreased the threshold

stimulus presentation

lev el

by

about

5

from

dB. Perceptual

5 / s to 80/s processes

i n t e g r a t e acoustic energy over several hundred m il lisec ond s (Zwis\ocki, 1969; Pedersen and Salomon, stimuli are s im il a r

1977).

presented probably

manner

to

increasing

Increasing the rate at which b r ie f

invokes the

t h i s temporal summation

duration

of

a continuous


in a sound

St a p e ll s et a l .

Page

17

J . Acoust. Soc. Amer.

(Zwicker,

1975).

There are,

however,

some d if f e r e n c e s

summation processes for the two types of s t i m u l i .

between the

Experiment 2 demon

strated a smaller summation e f f e c t (4 .5 dB per te nf ol d increase in rate per second) f o r c l i c k s than has been reported f o r continuous pure tones (8-10 dB p e r ,te n fo ld inc r ea se in duration). tonal

stimuli have reported e f f e c t s of stimulus presentation rate that

are si mi lar to the present study a l.,

Other s tu d ie s using br ie f

(Zerlin and Naunton,

1979; Yost and Klein, 1979).

The smaller summation e f f e c t may be

relat ed to the wide frequency-content of b r i e f s t i m u l i , and Garner

(1947)

f o r broad-band

having

1975; Picton e t

Zwicker (1975)

reported smaller temporal summation e f f e c t s

stimuli.

The actual

high-frequency

content

of

the

stimulus may also play a part since there i s l e s s temporal summation at higher frequencies

fo r pure

tones (Watson and Gengel,

1969; Pedersen

and Salomon, 1977; ..Chung, 1981) and f o r th ird -o ct av e c l i c k s (Zerlin and 1 Naunton, 1975). Zwislocki (1960, Figure 9) reported a 10 dB summation e f f e c t per te n f o ld change in Flanagan (1961),

It

is

rate of

0.5 ms c l i c k s .

on the other hand, reported very l i t t l e change in the

threshold fo r 100 |is c l i c k s 102/s.

the presentation

presented at

rates

increasing frOT20J>o

po s s ib le that these d if f e r e n c e s

may be related to the

d i f f e r e n t frequency spectra of the d i f f e r e n t s t i m u l i ,

the 500)

jjs

pulse

as

the

of Zwislocki containing r e l a t i v e l y more low-frequency energy: We

observed

a

2.5

dB

decrease

in

threshold

l i s t e n i n g - p e r i o d was increased from 100 ms to 300 ms.

This


indicates^

Stapells et a l .

Page

18


a the

temporal

summation

period

200

period

reported

1,975).

ms

The data

shown in

2.0 ms.

by

our

p lo tt e d by Zwislocki (1960, of 0 .2 -

within

these

comparable

to

others (Zwislocki, 1969; Zwicker,

Figure 5

Figure 7)

Both s e t s

lim its,

are

qui te si m il a r to those

f o r 100/s c l i c k s with durations

of data show a decrease

in threshold of

about 5 dB as the l i s t e n i n g duration increased f \ slopes. Again there was ai^ interaction

I R e p ro d u c e d with perm ission of the copyright owner. Further reproduction prohibited without permission.

S ta p e lls and P ic to n B ra in s te m re s p o n s e s to to n e s

between the two effects such that the effect of rolloff slope was not

significant

differences less

at

in

70

at 10 Hz and increased from 20 to 70

dB as compared to 110 dB but the

7 where,

plotted. same V*

The

were

much

differences

were

amplitude related to filter settings

still -significant at 70 dB. Figure

Hz.

for

The amplitude effects are shown

clarity, only the

results

at

in

35/s

are

The morphology of the V-V* complex changed in much the

manner

at 90 and 70 dB as at 110 dB in Experiment 1.

The

component was most prominent at filter settings of^40 and

70

Hz. The

effects

of changing the rate of stimulus

were

similar to those obtained in experiment 2.

rate

of

There low a

presentation

Increasing

tone presentation caused an increased.wave 'V

the

latency.

was no effect on V' latency except that the combination of intensity, high filter setting and high rolloff slope caused

decrease

rate.

in V' latency with increasing stimulus

There

amplitude

was

no

effect of increasing

The

on

the

V-V*

except at high filter settings and low Intensity where

the amplitude were smaller at 35/s rates. j

rate

presentation

results

»

of this experiment show quite clearly that

at

all intensities larger brainstem responses are recorded using the lower

high-pass filter settings and rolloff slopes.

settings and

20

The optimal

appear to be 10 Hz at either 6 or 24 dB/octave Hz

at 6 dB/octave rolloff.

The recognizability

rolloff of

an

average waveform, however, depends upon both the amplitude of the response

and

the amount of

background

noise remaining after averaging.

electroencephalographic

It is possible, therefore, that

j


56

S ta p e lls and P ic to n B ra in s te m r e s p o n s e s t o

higher

filter

assitance noise

certain clinical conditions where in

the

who • are

activity* filter

settings and/or higher rolloff -slopes may

in

level

patients

to n e s

The

will

10-40 Hz range is high -

sedated

use

or who have high

of

response. it

is

of

background

for

example

levels

of these higher settings on

of

the

in

muscle

high-pass

alter the brainstem response morphology to

prominent vertex-negative V' component. effects

the

be

give

a

There were no consistent

stimulus presentation rate on the amplitude

of

the

Because of the decreased timp necessary for averaging,

therefore

preferable

to

use

the

faster

stimulus

presentation rates.

< Experiment 5 - Stimulus Rise-times This experiment evaluated the effect of different rise-times on

the

Tones t

; *

,

brainstem

with

response to tones of. different

rise-times

of

1,

2, 5 and

8

ms

frequencies.

and

equivalent

fall-times

were presented at frequencies of 500, 1000, 2000 and t « 4000 Hz and aj i an intensity of 100 dB peak SPL at a rate of 35/s. t

>

Averaging was carried out over 2000 trials using a sweep duration of 25.6 mj/ . The subjects

X

average are

wave

plotted

V in

latencies and Figure

8.

amplitudes

There

were

for

eight

significant

increases in V latency with decreasing stimulus freuency and with increasing

rise-time.

There

was also an

interaction

between

\

these

two

increasing

effects êuch rise-time

was

that

the

greater

increase in' latency at

the

lower

with

frequencies.

i R e p ro d u c e d with perm ission of the copyright owner. Further reproduction prohibited without permission.

S ta p e lls and P ic to n B ra in s te m re s p o n s e s t o

Regression each

frequency.

latency lines Hz.

lines

of

to n e s

were calculated for the rise-time

effects

The equation for these regression

lines

wavfe V= a (rise-time) + b.

The slopes (a) of

at

was: these

were 0.44, 0.40, 0.30 and 0.26 at 500, 1000, 2000 and 4000 The respective latency intercepts (b) were 6.88, 6.48, 6.33-

and 6.11 ms. ............... Insert Figure 8 about here.................... v.. These results show a definitely increasing effect of rise-time

on

explanation acoustic

V latency at the lower frequencies.

for

this

could

be the

increased

One spread

of . the

energy in the spectra of low-frequency tones with short

rise-times.

These rapid-onset tones could thus evoke

i through more

basal regions o£ the cochlea than the

longer rise-times. dB

possible

tones

tones

with

The regression line calculated for 500 Hz 100

presented in notched noise.(data from

however,

responses

Experiment

6),

showed a slope of 0.49 as well as a generally increased

latency with a latency intercept of 8.65 ms.

The regression line

for the 2000 Hz 100 dB tones in notched noise had a slope of 0.23 and

an

intercept

of

7.12 ms.

It

therefore

seems

that

the

alteration in slope with stimulus frequency is a true function of

J

stimulus frequency and not an artifact of acoustic distortion. Several latency

researchers

with

Brinkmann and V.' trigger time" rise-time connecting

of to

increasing Scherg which the

have

reported an increase in

stimulus

rise-time (3,

(3) introduced the appears

6,

concept

to represent

the

qîmulus when the majority

of

the brainstem response generator

wave 13,

of

17).

"virtual

point nerve are

V

on

the

fibers

activated.


I Stapells and Picton Brainstem responses to tonAs 0

58 .*>

This virtual trigger time is a function of both the rise-time and the

intensity of the stimulus.

also

Our results indicate that it

a funotic^n of stimulus frequency.

where

there

phase, major This

is

nerve

the

fiber

delay

At the lower frequenies,

locking of nerve fiber activation

increasing

would

rise-time

could

delay

activation by one or more be

greater

is

to

stimulus

the

time

of

stimulus cycles.

the lower thefrequencyof

the

stimulus. The

V-V'

significant These was

amplitude

of

the

brainstem

response' showed

changes with both stimulus frequency and rise-times.

effects are shown on the right of Figure 8. greater

increasing

at

the

lower

rise-time.

frequencies

The major change)^Ln

and

The amplitude decreased

amplitude

with

occurred f

between the rise-times of 5 and 8 ms. The

effect of stimulus rise-time on the brainstem response

amplitudi^ appears "" stimuli. does

not

cause

a

tones.

than

2.5 ms.

amplitude

impulses

whitenoise

any definite change in the

(3, 13).

with

in

differ between

and

tonal

) Varyingythe rise-time of white noise bursts up to 10 ms

amplitude indicate

to

to

brainstem

response

Our results and those of Kodera et al

definite decrease in amplitude at longer

(17)

rise-times

This is particularly true at rise-times of

greater

At shorter rise-times there maynot be any

change

(6).

nerve

It is possible that the

locking

the phase of low-frequency stimuli may

of cause

more

jitter at longer rise-times in tonal as opposed to noise stimuli. This jitter would result in broader responses of lower amplitude. Our results indicate that for tonal stimuli rise-times of 5 ms or


S ta p e 1 1 s an d P ic to n B ra in s te m re s p o n s e s t o

shorter

would

be

to n e s

preferable

since at

longer

rise-times

the

brainstem response is quite attenuated.

Experiment 6 - The Effects of Notched Noise ' / This experiment was designed to investigate * frequency-specificity of the brainstem responses to tones different rise-times. intensities mixed

with

Tones of 500 and 2000 Hz were presented at

120, 100, 80 and 70 dB peak SPL either alone

or

with notched noise (with the rejected band centered on the

frequency was

of

the

25

of the tone). dB

The noise intensity measured in RMS SPL

less than the tone intensity measured in

peak

SPL.

Stimuli were presented at 35/s and averaging was carried out over 2000 80

trials with a sweep duration of 25.6 ms. and 70 dB were replicated.

was

The responses

at

The rationale for this experiment

that if the notched noise caused the response to change, the

response to the tone alone was in part mediated by frequencies in f* the tone away from its nominal frequency. ) The quite in

effect

complex.

Figure

9.

significantly

of the notched noise on the wave V latency

was

The average data from eight subjects are plotted The latency of wave V in the 500 Hz response

was

increased by- notched noise at all intensities

for

the 1 ms rise-times, at 100 dB or more for 2 ms, at 80 dB or more > for 5 ms, and at 100 dB or more for 8 ms. The latency of the 2000 HS response was significantly increased by the notched noise at for

80

dB or more for the 1 ms rise-time, and at 100 dB or

more

2 ms. There were no significant effects of the notched ntfise v_


S ta p e lls and P ic to n B ra in s te m re s p o n s e s to

to n e s

on the 2000 Hz response latency for rise-times of 5 or 8 ms. .....

Insert Figure 9 about here...... .........

The notched noise significantlyXreduced the amplitude of the brainstem the

response.

higher

This reduction ih amplitude was greater

at

intensities and at the losûf-jise-times. At 500

Hz

there

were

noise

at 80 dB or more for the 1 and 2 ms rise-times, at 100

or V-V*

more

significant amplitude differences with

for the 5 and 8 ms rise-times.

amplitude

for

the

At 120 dB

the

5 ms rise-time tones was 1.20 pV

tones were alone and 0.87 pV when in notched noise. amplitudes

were

significant for

all

0.42

rise-times

At

2000

average

when

the

At 70 dB the were

differences in the V-V' amplitude at 100 dB or

more

was

0.32 pV.

dB

there

rise-times.

and

notched

Hz

At 120 dB the average amplitude for. 5

1.16 pV when the tones were presented alone

0.56 pV when in notched noise.

ms and

At 70 dB the amplitudes were 0.37

and 0.36 pV. Two

possible explanations for the effects of notched

noise

t

can be considered. the

effective

The first is that the masking noise decreases

intensity

of the tone',

thereby

decreasing

the

amplitude and increasing the latency of the response.

The second

is

particular

that

the notched noise limits the response £o

a

area of the cochlea, masking out those parts of the response that are

mediated

the

spread

dynamics

of

through other regions of the cochlea activated

of

acoustic

the

energy in the brief

travelling

wave.

The

tone

first

or

by

explanation

probably not a major cause of the experimental findings.

by the is

It does

9

not

explain

the different effects of the masking noise


at

the

s

S ta p e lls and P ie to n B ra in s te m r e s p o n s e s to

different

61 to n e s

frequencies

Furthermore,

increasing

9

or ' at

the

different

intensities.

the intensity of the masking noise

has

little

effect on the amplitude or latency of the 500 Hz response

(23).

The second line of explanation can account reasonably well

for

all

evident

of

the

observations.

The latency

changes

are

at 500 Hz because the 500 Hz response latency is

determined activated

by

thp

sharp-peaked

frequency

mainly

of the cochlea * by the spread of frequencies in the brief tone. These

high-frequency

higher

most

regions

regions are more readily synchronized and give component

latency.

The

frequency

region

in

the

response

that

determines

notched noise masks the response from the

the

higher

leaving a braod, longer latency response

from

the

500 Hz region of the cochlea.

has

less

effect because of the higher frequency-specificity

the

2000

Hz

cannot

the .cochlea. A because the of

noise of

thev response ^ shift far to an earlier or more synchronizable region of

*

stimulus (cf

At 2000 Hz the notched

a

Figure 1) and because

The amplitude changes occur at

both

frequencies

of the masking of the response to frequencies outside of

rejection band of the notched noise.

The decreasing effects

notched noise at lower intensities occur because at the lower

intensities

the

skirts

of the tone frequency

spectrum

become

subthreshold. If

we

correct,

therefore accept the second line of

explanation

as

then the results of this sixth experiment indicate that

the brainstem response to brief tones at intensities of 100 dB or more is not frequency-specific regardless of the rise-time of the tone.

At rise-times of 1 or 2 ms the response to 500 Hz tones is


62


to n e s

S' not even frequency-specific at lower intensities. :

j

CONCLUSIONS

The recorded low

J

largest brainstem responses to low frequency tones

are

using

and

rolloff

low high-pass filter settings (10 or 20 Hz)

slopes.

vertex-positive

Under

wave

these

is recorded.

conditions At higher

a

large

high-pass

clear filter

settings, particularly if higher rolloff slopes (24-48 dB/oct&ve) are

used, the response is smaller and tends to show a

prominent

vertex-negative wave. There response rolloff

are

no significant changes in the amplitude

recorded

using low high-pass filter settings

of

the

and

low

slopes when stimulus presentation rate of up to 35/s are

used.

There is, however, a significant increase in the

latency

of wave V at the higher presentation rates. Postauricular tones

and

mid-mastoid

muscle reflexes are evoked by high

intensity

can distort the brainstem response recorded reference electrode.

using

a

A reference located lower down

on

the mastoid is therefore preferable.

If the wave I recording

is

not essential, the reference electrode can be located on

the

lower part of the neck. Tones specificity. smaller occur

with

longer

They

amplitudes. with

elicit

rise-times

have

responses with

greater longer

acoustic

latency

and

Xn general, latency increases of 0.2-0.5 ms

increases

of

1

ms

in

rise-time.

The' exact



63 to n e s

^ Stillman, R.D., G. Moushegian, and A.L. Rupert. 1976. Early tone-evoked subjects.

27.

67

responses

in normal and

hearing-impaired

Audiology 15, 10-22.

Suzuki, T., Y. Hirdi, and K.'Horiuchi. 1977. Auditory brain stejn responses to pure tone stimuli. Scand. Audiol. 6, .51-56.

28.

Suzuki,

t

T.,

filter v

and K. Horiuchi. 1979. Effect ah

on

-

Paper presented

Auditory

high-pass

the auditory brain stem responses to •

~pips.

of

Responses

at the

from, the

US-Japan Brain

tone

Seminar

on

Stem, 'Honolulu,

Hawaii. • ‘29.-

Terkildsen,

K., P. Osterhammel. 1981. The

reference /Wl

30.

eletcbrode ° position

recordings

Terkildsen,

K.,

P. Osterhammel, and F.

Far-field

positions. Scand.

Huis

the

Veld.

electrode

Audiol. 3, 123-129.

Terkildsen, K., P. Osterhammel, an^F.-Huis in't Veld. 1975. '

32.

in't

electrocochleogrpphy,

it

*

Far-field electrocochleography. Frequency *

of

of

audito auditory brainstem responses. Ear and Hearing 2, 9-14.

1974.

31.

on

influence

specificity

of the response. Sând. Audiol.-4, 167-172. Weber, B.A. and R.C. FolsdSu^ Brainstem wave V latencies to tone' pip stimuli. .. J. Am. AiA. Aud. Soc., 2, 182-184.

33.

Wood,

* ' ? i

•

M./M.R. Seitz, and J.T. Jacobson.

• ■

1979.

Brainstem

’ •


J

. S ta p e lls and P ic to n . B ra in s te m re s p o n s e s t o

to n e s

.

^

electrical responses from selected tone pip stimuli* J* Am. 34.

Yamada,

Aud. Soc. 5, 156-162. 0.,T. Yagi, H. Yamane,

Clinical v\

and J.-I.

Suzuki.

1975.

evaluation of the auditory evoked brain

stem

response. Aurix Nasus Larynx 2, 97t -105. 35.Yoshie, N ., and T. Okudaira. responses 252,

1969. Myogenic evoked potential

to clicks in man.

Acta Otolaryngol.

89-103. '

%


Suppl.

2e ?


69 to n e s

f i g u r e legends

Figure 1 acoustic by

the

The effect of different rise and fall times

on

the

spectra of 590 and 2000 Hz tones. Tones were generated TN-3000

equivalent

system

fall

with rise-times of 1, 2, 5

and

times, and plateau durations of 0.01

8

ms.

ras, The

tones were presented through a TDH-49 earphone at an intensity of .115 and

I The acoustic signal was recorded using a Bruel

dB peak SPL.

Kjaer microphone and analyzed using a

TN-1500 sfgnal analyzer. each

laboratory-programmed

The power spectral density function for

tone isj^lotted between 100 and 10,1)00 Hz using logarithmic

' intensity

arid-frequency axes.

The intensity axes are

arbitrary

(0 dB is approximately 40 dB SPL). „

Figure 2 the at

- The effects of different high-pass filter settings on

brainstem response to 110 dB peak SPL 500 Hz tones presented a

rate

of 10/s.

Recordings were taken between

vertex

and

mid-mastoid electrodes and each tracing represents the average of 2000 responses.

Relative negativity at the vertex is represented *

f

by

an upward deflection. ^The vertex-positive wave V,

by

the open triangles, is most prominent at the lowest

of^ the

high-pass

indicated

by

filter.

The

vertex-negative

V'

indicated settings component,

the filled triangle, is particularly prominent

at

the 40 Hz 24 dB/octave filter setting. p Subject D.S. c % Figurp 3 rolloff

J*

- The effects of changing high-pass filter settings and slopes

on the brainstem response to 500 Hz tones.

\


The

I 70


to n e s

average

data

wave

is shown on the left and of wave V' on the

V

from eight subjects are plotted.

The

latency

of

right.

The

V-V' amplitude is plotted in the center.

Figure 4

-

high-pass tones*

The

effects

filter

of

settings

stimulus

presentation

rate

on the brinstem response to

The duration of the averaging sweep was 50 ms.

and

500 At

Hz 35/s

the

interstimulus time (28.6 ms)‘is less than

and

therefore a second response is initiated prior to the end of

the

sweep.

lines.

As

This

is illustrated in the figure

the

the

dotted

well, this second response has been superimposed

the first in the initial portion of the tracing. (S.S.)

by

on

In this subject

increase in stimulus presentation’ rate from 20/s

to

35/s causes a decrease in the amplitude of wave V (open triangle) ,and

an increase in the amplitude of V' (filled triangle).

This

is probably because of the superimposition of these components on the negative *wave occurring at 40-45 ms after the preceding tone. When

the

high-pass filter setting is changed, the V'

component

has a shorter latency and small amplitude. V

*

Figure 5

- JJostauricular

recorded

to yo, 90 and 70 dB peak SPL 500 Hz tones presented at

a

rate

vertex

of 10/s. to

reflexes.

low-mastoid electrodes, and the recordings

High-pass

filtering slope.

Responses

The dotted tracings represent recordings

represent

rolloff

muscle

from

vertex

to

continuous

mid-mastoid

was performed at 10 Hz with a

Each

tracing represents the

were

from

tracings

electrodes. 6

dB/octave

average


of

2000

%

V

Stapells and Picton Brainstem responses to tones

responses upward large

and

negativity

deflection. muscle

disappeared

by

an

AT 110 dB this subject (P.F.) exhibited

a

reflex at

71

at the vertex is

represented

that decreased in amplitude at 90

70\dB.

dB

and

The muscle reflex was very focal in

its

scalp

distribution Wind did not show up in the

using

the

brainstem location began

low-mAstoj.d

electrode.

The

V

recordings component

made

of

response

the

of the reference electrode.

before

the

the

peak

of

the k

However, the muscle reflex

V*

wave

and

distorted

any

measurement of this component.

Figure 6

- The effects of high-pass filter settings and stimulus

presentation rate on the response to 500 Hz tones. in

this

figure

responses

from

each represent the average of subject

S.S.,

obtained

The waveforms

4000

using

a

individual vertex

to

low-mastoid derivation and a high-pass filter rolloff slope of 24 •% dB/octave. With decreasing intensity wave V (open triangles) showed

increasing latency and decreasing amplitude. The f vertex-negative V' component (filled triangles) is best seen at

J

the 40 and 70 Hz filter settings.

With decreasing intensity this

V' wave also increased in latency andjdecreased in amplitude.

Figure 7 - The effects of high-pass filtering on the amplitude of “ '“ « the

brainstem response to 500 Hz tones of different intensities.

The

average

tone figure

V-V* amplitude d^ta from 8 subjects is

intensities of 110, 90 and 70 dB peak SPL. only

the

35/s

data

from

Experiment

plotted

at

To simplify the 4

are

30


plotttjed.

Stapells and Picton Brainstem responses to tones

Increasing decreased smaller

the

filter

amplitude

72

setting or the rolloff

qf the brainstem response*

slope

causes

This effect

a is

but still significant at the lower intensities. Ât each

intensity the largest wave V amplitudes are recorded using the 10 Hz

filter settings with either the 6 or 24 dB/octave rolloff

or

using the 20 Hz setting with the 6 dB rolloff.

Figure 8 - The latency and amplitude of the brainstem response to tones

of

different frequencies and rise-times.

The

tones

had

rise-times of 1, 2, 5 and 8 ms, plateau durations of 0.01 ms, and fall-times- equal intensity data

to the.rise-times.

They were presented at

of 100 dB peak SPL and at a rate of 35/s.

from eight subjects are plotted in this figure.

latency

is

plotted

an

Thtf- average The. wave V

on the left and the V-V' amplitude

on

the

effect of notched noise on the latency

of

the

right.

Figure 9

-

brainstem

The

response

to tones. ’ Tones of 500 Hz and 2000 JJz were

presented

at 35/s either alone (continuous lines) or in notched I ** (dotted lines) at 25 dB'below the tone intensity. The

noise average

latencies

of wave V for eight subjects are plotted

tone rise-times of 1, 2, 5 and 8 ms. /

causes

a

for

At 500 Hz the notched noise

prolongation in the latency of the response.

This

is

signifif^tiie averaged evoked potential was measured average of the peak-to-peak amplitudes in the’ waveform, number

and

location

of

these

determined

using


a

A u d ito ry s te a d y S ta p e lls e t a l .

sinusoidal

s ta te p o te n tia ls

94 >

template

of

the

same

frequency

repetition. in

as

the

rate

of

H

*

The latency to the first major vertex-positive peak

the averaged 40-Hz waveform was recorded. At higher rates

of

stimulus presentation, howevervône cannot actually associate any individual "derived was

response

cycle with a particular stimulus cycle.

latency" of^the averaged steady state evoked potentials

therefore 'obtained

(5,6).

A

using the method

proposed

by

Diamond

each subject,, linear regressions were performed on ¥ latency/ISI plots for the prominent positive and negative

the peaks

For

of

latency

the

response between 35/s and 55/s, and

intercept

taken as the derived latency.

the

average

T h y amplitude

X

and phase of the responses evaluated on the Fourier analyzer were calculated

by

the

MINC-11 computer. Replicate

responses

were

combined using vector-averaging. This allowed us' to calcluate the average amplitude and phase of the response over a period of time or

over

several replications.

The amplitude measurements

from

the Fourier analyzer were calibrated on the basis of peak-to-peak ^roltages.

Regan

(18)

"apparent

latency"

repetition

rates.

seconds) portion

was of

has

from For

calculated

described the

the

phase data

each subject the

calculation obtained

apparent

by obtaining the slope of

the phase/repetition-rate function (35 -

at

of

several

latency the

an

(in

linear

55/s)

and

regressions

and

•dividing it by 360. The •

\

data

were

analyzed

using

repeated measures Analyses of Variance. -" *significant at p When

steady

stimulus-rates, latency phase

to

responses

are

obtained

at

different

a latency can be calculated. This latency is the

that portion of the waveform that stays at

regardless of the stimulus-rate.

latency The

state

constant

It can be considered the

to the dominant component of the steady state

response.

method of deriving latency from the averaged data using

the

technique

of Diamond (5,6) is shown for one subject in Figure 6.

The

mean

"derived" latenc” for the 8 subjects calculated

the

Diamond

technique

was 33.3 ms (SD = 8.6).

The

using

"apparent

latency" of the.Fourier-analyzed results was calculated using the method

described by Regan (18). The phase data from the

analysis phase

are

Fourier

plotted against repetition rate in Figure 7. 60/s.

of

The The

mean apparent latency for the 8 subjects was 34.0 ms (SD = 10.8). The

two methods - one a "time-difference analysis", the other

"phase-difference results.

analysis" - thus provide essentially the

The resultant latency of 33 - 34 ms lies in the

of the Pa component of the 10/s transient response.


a

same range

A u d ito ry s te a d y s t a t e p o t e n t i a l s S ta p e lls e t a l.

\

I

99 r'

-

Insert'Figures 6 and 7 about here

Experiment 3; Stimulus Intensity This

experiment

investigated

intensity on the 40-Hz potential Fourier 10—dB

analysis. steps

condition

from

in

the

effects

of

stimulus

using both signal-averaging and

The 500 Hz tonebnrst was presented at 40/s in 90 to -10 dB nHL. There was

also

a

control

which the earphone was disconnected. The order

of

the intensities was randomized. Behavioral threshold (SL) for the stimulus limits

presented

at

40/s was obtained using

the

method

with

responses

5-dB steps. Replicate waveforms of 2000 S each were recorded using the 51.2 ms sweep

intensity

from 10 subjects. At the same time, the amplitude

phase

the 40-Hz fundamental in the EEG were

of

averaged at

each

*c

of

obtained

and using

Fourier analysis. Figjjr-e amplitude

8

shows the effect of stimulus intensity

of the 40-Hz potential. The results from both

of analysis again parallel each other.

the

methods

The decrease in 40-Hz ERP

amplitude

is

fairly

amplitude

at

90 dB nHL being 1.63 (SD=0.65) and 1.46

linear down to threshold,

with

the

mean

(SD=0.60)

pV,

decreasing to 0.26 (SD=0.10) and 0.16 (SD=0.11) pV at 20

nHL

using

These

!

on

signal averaging and Fourier analysis,

represent amplitude decreases

of 20 nV

dB

respectively.

per decibel. The

•


I A u d ito ry s te a d y S ta p e lls e t a l .

change

in

s ta te p o te n tia ls

amplitude

was

*

100

quite linear between 90

and

dB

(r = 0.99

for

averaging

are slightly greater than those obtained using Fourier

analysis

mean data). The amplitudes obtained using

20

for

the

same

reasons

as

noted

for

the

signal

preceding

experiment.

Insert Figure 8 about here

The noise level of the recording technique was determined in the

control condition wherein the earphones were unplugged.

noise

level

was 0.18 (SD=0.04) îV for signal-averaging and 0.09

(SD=0.06) pV for Fourier analysis. decreases, to

these

noise

The 40-Hz potential amplitude

levels

at

(range=-10 - 40) dB for signal averaging - .40) dB for Fourier analysis. below was

The

intensities

below

13

and below 15 (range=—10

These levels are 4.5 and 2.5

the average behavioral thresholds (SL).

dB

This SL threshold

some 15 dB higher than the nHL threshold because there was a

higher level of ambient noise in the laboratory where the potentials

were

recorded

evoked

than in the sound-attenuated

chamber

first

of

where the nHL was obtained. The averaged with

latency

to

the

vertex-positive

peak

the

response and the phase of the 40-Hz fundamental changes

stimulus

intensity.

The

mean

latency

of

the

averaged

response for the 10 subjects was 7.61 ms at 90 dB nHL, increasing linearly

to

15.46

ms at 20 dB nHL, as shown in

Figure


9.

A

L

'/

A u d ito ry s te a d y S ta p e lls e t a l.

linear the

B ta te p o t e n tia ls

regression

continuous

101

analysis performed on these data (plotted

line

a slope of

increase

in

obtained

using Fourier analysis were converted to

and

latency

in Figure S) shows

[r = -0.79; p