ACENTRAL FACTOR IN PURE TONE AUDITORY FATIGUE

Joel S. Wernick® and Jerry V. Tobias

A long accumulation of psychophysical and physiological evidence indicates that audi-
i tory fatigue has its locus of effect in the cochlea; transfer studies with negative or ques-
tionable results, and studies of cochlear chemistry and potentials with positive results all
lead to the same conclusion, However, Galambos’s reports of inhibition through efferent
stimulation taken together with his findings of changes in cochlear potentials corresponding
to differences in “attentiveness” provide a basis for testing the existence of a central factor
operative in auditory fatigue. The present study was designed to evaluate the possibility
jat a listener's level of mental activity constitutes an adequate central factor. Subjects
were presented with a 4000 cps, 40 dB SL or 90 dB SL tone for 3 minutes under conditions
| of a) a mental task or b) reverie. Pre- and post-fatigue thresholds were measured with
! a switched (0.2 sec on, 0.2 sec off) tone, For low level (40 dB) fatigue the frequency of
: the test tone was 4000 cps and for high level (90 dB) fatigue, 5656 cps. Subjects con-
sistently showed greater temporary threshold shift and longer re time when the
fatiguing tone was presented in the mental task condition. Results thus indicate a central
factor in pure tone auditory fatigue. It is suggested that the degree of effect of a

fatiguing tone is a function of the listener’s degree of mental activity during stimulation.

Empirical investigations of the locus of audi-
tory fatigue have often been concerned with the
question of whether the fatigue producing me-
chanism lies within the cochlea or central to it.
In 1929, von Bekesy’ initiated a new approach
to the problem. Up to that time the evidence
provided by physiological techniques! had in-
dicated that fatigue had its origin in the nerve
endings of the basilar membrane, Bekesy felt
he could rule out any possfblity that fatigue
was produced by the central nervous system
by stimulating one ear and observing the effects
of stimulation on the contralateral ear, The
logical assumption on which his study based
was that if the locus of fatigue, either in whole
or in part, is central to the first neural interac-
tion of the two ears, fatiguing one ear should

pecessarily also cause a decrement in response
in a transfer tg the contralateral ear. Bekesy

* Now at Indiana University, Bloomington, Indiana.
ty, Bt

* G, von Bekesy, Experiments in Hearing (McGraw-Hill
Book Company, Inc., New York, 1960), Chap. 9, pp. 359-
368.

found no decrement in response in the contralat-
eral ear, and concluded, “It is clear that there
is no transfer of fatigue from one ear to the’
other, and hence fatigue is a peripheral
phenomenon.”* Rawdon-Smith’ conducted a
similar study in 1936 and reported that
transfer effects did occur, When one ear was
fatigued he found that the opposite ear suf-
fered a decrement in response. Causse and

avasse‘ have since pointed out that Rawdon-
Smith failed to provide sufficient interaural at-
tenuation for the intensities he was using and
therefore wasxactually stimulating the con-
tralateral ear directly rather than via the postu-
lated” central nervous system route. When
Causse and Chavasse conducted a transfer
study within the limits of interaural attenua-

* Reference 1, Chap. 9, p. 364.

* A. F. Rawdon-Smith, “Experimental Deafness. _Fur-
ther Data upon the Phenomenon of So-Called Auditory
Fatigue,” Brit. J. Psychol, 26, 233-234 (1936).

“RB. Causse and P. Chavasse, “Etudes sur la Fatigue
Auditive,” Ann, Psychol. 43-44, 265-298 (1947).

-l-



reverie condition, .45 dB. A t-test indicated no
significant difference between these means.

Two process recovery. All the obtained curves,
both for high and low level fatigue and for
the task and reverie conditions, showed a
change in slope at about 45 seconds. The two
straight line logarithmic functions which result
indicate that recovery comprises two processes,
with the slower process taking over about 45
seconds after the fatiguing stimulus terminates.
A similar observation has been made by Hirsh
and Bilger,” but the phenomenon becomes

even more distinct when recovery is plotted
logarithmically,

DISCUSSION

The theory of efferent gunk. The results sup-
port the hypothesis that central activity can
modify the fatigue process. The indication is
that the loss of sensitivity is a function of the
level of mental activity exhibited by the subject
during the presentation of the fatiguing stim-
ulus, with a decrease in sensitivity resulting
from an increase in mental activity, It seems
reasonable to assume that this modification
results from inhibitory impulses in the efferent
pathways, Since these pathways do have
peripheral terminations, they can meet the
limitations imposed by the transfer studies —
essentially, that any centrally controlled inhibi-
tion must produce its effect below the level
of any decussation; if the locus of a central
inhibitory mechanism were above the first
neural crossover point, transfer of the effects
would result, and transfer has been showh not
to occur. ‘ «

The finding of an increase in fatigue with
an increase in mental activity appears to con-
tradict current concepts of efferent functioning,
in that a reduction of the evoked potentials
occurs when central pathways are electrically
stimulated, It would appear logical fo prélict
that an increase in central activity would serve
to reduce the activity of the fatiguing stimulus
and thus result in a decrease in fatigue. Topo-
graphically, this prediction appears valid. But
one issue has beerg-overlooked — that of the
mechanism which produces the reduction of

I. J. Hirsh and R. C, Bilger, “Auditory-Threshold Re-
cover} after Exposures to Pure Tones,” J. Acoust. Soc.
“am, 27, 1186-1194 (1955).

—6-

the potentials. If the inhibition is a biologi-
cally active process, that is, is an active chem-
ical decomposition with a resulting accumula-
tion of waste products, then one reaches what
we have termed the “theory of efferent gunk,”
from which follows a prediction in accord with
the results of the present study, This theory
states that there is a chemical mediator required
for initiating the neural response” and that
efferent stimulation tends to decompose this
mediator, The resulting efferent gunk adds to
the fatigue products and thus produces de-
creased sensitivity." Since a stimulus is neces-
sary to elicit the chemical mediator, no decrease
in sensitivity will take place unless stimulation
is present. Therefore, one would predict no
change with no fatigue tone despite the level
of mental activity — as in our control conditions
— but a decrease in sensitivity when the subject
is mentally active during the presentation of a
tone — as our results indicate,

Middle ear muscles. The possibility that the
results are attributable to middle ear muscle
action must be considered. An explanation of
the results in terms of a differential activation
of the muscles as a function of the task con-
ditions has to be discounted because of the
failure of the control conditions to show any
shift, much less a differential shift, The find-
ing of no difference between the control con-
ditions does not completely eliminate the
middle ear muscles as possible sites, for the
possibility still exists that there is an interaction
of the fatiguing stimulus with the task con-
dition, There are two answers to this argument.
One is that there is little or no muscle effect
on high frequeycies; certainly not enough to
account for the 9 dB difference found with
high Iével fatigue. The second answer rests on
the finding of the existence of the phenome-
non with low level stimulation; that is, below

the threshold of muscle contraction,
—_— '

® The existence of a chemical mediator is supported by
H, Davis, for instance, in “Advances in the Neurophysiology
and Neuroanatomy of the Cochlea,” J. Acoust. Soc. Am.
34, 1377-1385 (1962).

%\ A related proposal attributing the results of efferent
stimulation to the release of an “inhibitory transmitter” has
been made by J. E. Desmedt and P, Monaco in “Mode of
Action of the Efferent Olivo-Cochlear Bundle on the
Inner Ear,” Nature 192, 1263-1265: (1961) and in “The
Pharmacology of a Centrifugal Inhibitory Pathway in the
Cat's Acoustic System,” Proceedings of, the First Inter-
national. Pharmacology Meeting 8, 183-188 (1962).

# U.S, GOVERNMENT PRINTING OFFICE : 1963  - 716-031

=

t+o~


Recent neurophysiological findings have gen-
erated a renewed interest in possible central
factors operative in auditory fatigue. In partic-
ular, the demonstration of a subcortical inhibit-
ing system exerting its action in the cochlea via
the efferent auditory pathways has provided a
new basis for looking at the fatigue mechanism.
In 1956, Galambos" electrically stimulated the
roof of the medulla of a cat and observed a
simultaneous decrease in the potentials evoked
by clicks at the cochlear nuclei. Hernandez-
Peon, recording off the cochlear nuclei, no-
ticed a decrease in the evoked potentials when
the cat was presented with stimulation to an
alternate modality, such as exposure to the
smell of fish. Thus he indicated that “attentive-
ness” was a factor that called into play a selec-
tive inhibitory mechanism operating at a locus
peripheral to the higher centers and possibly
within the cochlea itself.

METHOD

Problem and. design. The present study was
designed to determine whether central nervous
system activity could modify the action of a
fatiguing tone, through efferent auditory activ-
ity, for instance, Since it is possible that cen-
tral factors may act differently on high and
low level fatigue, two levels of fatiguing tone
were used in the study: 40 and 90 dB SL,

On the assumption that instructing a subject
to work a mental arithmetic problem produces
a state of central nervous system activity which
functionally differs from the state produced
by instructing him to relax and let hiy mind
wander freely (reverie), four conditions were
used: ‘

Experimental Condition I -

Arithmetic task during. presentation of
fatiguing tone. i

Experimenta] Condition II — /

Reverie task during presentation of fatig-
uing tone.

# BR. Galambos, “Suppression of Auditory Nerve Activity
by Stimulation of Efferent Fibers to Cochlea,” J. Neuro-
physiol. 19, 424-437 @956).

1 R. Hernandez-Peon, H. Scherrer, and M. Jouvet,
“Modification of Electrical Activity in Cochlear Nucleus
During ‘Attention’ in Unanesthetized Cats,” Science 123,

w& 331-332 (1956).

Control Condition I—

Arithmetic task without fatiguing tone.
Control Condition II —

Reverie task without fatiguing tone.

Apparatus. The instrumentation comprised
three basic parts: a fatigue channel, a test
channel, and a warming signal for the subject.
The fatigue tone, generated by a Hewlett-
Packard model 201 C oscillator, was led
through a Hewlett-Packard model 350 B vari-
able attenuator. Pre- and post-fatigue thresh-
olds were obtained on a Bekesy audiometer
(Grason-Stadler model E 800) with the rate of
attenuation set at 5 dB per second. The test
tone, fatigue tone, or no tone, selected by the
experimenter, was led into one ear of a set of
Telephonics TDH-39 earphones with MX-41/
AR cushions. In the testing room, a light under
the experimenter’s control was used to signal the
subject either to initiate his instructed activity
or to start taking a threshold. Figure 1 sho

. the instrumentation. :

Subjects. Subjects were 20 air traffic control
instructors from the Federal Aviation Agency
Academy divided randomly into two equal
groups. Group I received low level fatigue un-
der the two experimental conditions and re-
ceived the two control conditions. Group II
received high level fatigue under the two
experimental conditions and received the two
control conditions.

Procedure. After a series of practice threshold
determinations, each subject was presented
with his four conditions; the trials were sep-
arated by at deast 24 hours and the order of
presentation of the conditions was determined

CONTROL ROOM ‘SUBJECT ROOM
s —-

aes Tone |__[Variabte Lamp
Oncllator Attenuat i
i
fatigue |

Source tet §  1pH-39.
chile, ont i
Audiometer i
‘Audiometer

Control

Figure J. Block diagram of the apparatus.

3



tion, they obtained no transfer. Their con-

clusion was equivalent to Bekesy’s.

Other information on the locus of auditory
fatigue can be drawn from studies of electrical
activity within the cochlea. If the amplitude
of the cochlear potentials is related to the stim-
ulus intensity in a linear fashion, and if the
potentials are decreased after presentation of a
fatiguing stimulus, then the action of fatigue
must take place prior to the initiation of neural
activity, most probably in the structures directly
responsible for the production of the potentials.
In this context, note that Wever and Bray’
found the amplitude of the cochlear potentials
to be directly proportional to the stimulus
intensity up to a certain “critical” intensity
and that, in 1934, Hughson and Witting’ dem-
onstrated that stimuli greater than 95dB SPL
served to induce a poststimulatory reduction of
the potentials. But low intensity stimulation
failed to effect a poststimulatory reduction, The
ability of high level stimulation to reduce the
cochlear response has consistently been demon-
strated, and the inability of low level stimula-
tion to reduce the cochlear response, although it
produces an evident threshold shift, has been
repeatedly confirmed.

Since a low level fatiguing tone has no effect
on the amplitude of the cochlear microphonics,
one must consider the VIIIth nerve action po-
tentials, In 1959, Sorenson’ found both a
poststimulatory and perstimulatory decrement
in action potentials, He interpreted these re-
sults to mean that low level fatigue either acts
upon the nerve itself (if the cochlear potential
are thought of as directly initiating’ the neural
response), or upon the chemical mediator be-
tween cochlear and neural potentials, He re-
jected the possibility that the neural decrement
could be attributed to a central inhibitor since

decrement in the action potential.

+E. G. Wever and C. W. Bray, “The Nature of the
Acoustic Response: The Relation between Sound Intensity
and the Magnitude of Responses in the Cochlea,” J. Exptl
Psychol. 1g 129-143 (1936).

* 'W. Hughson and E. G. Witting, “An Objective Study
of Auditory Fatigue,” Acta Oto-Laryngol. 21, 457 (1934).

" H. Sorenson, “Auditory Adaptation in Nerve Action
Potentials Recorded from the Cochlea -in Guinea Pigs,”
Acta,Oto-Laryngol. 50, 438-450 (1959).

Other observations which indicate that the
locus of fatigue is within the cochlea note the
presence of recruitment following the presenta-
tion of a fatiguing stimulus. Davis et al.,* found
a clear indication of recruitment following high
level stimulation and Ruedi’ noted a decrease in
the difference limen after exposure to white
noise at 95 dB overall SPL. Hood* and
de Mare” were unable to find clear evidence
of recruitment following low level stimulation;
however, Harris and Rawnsley,” using the tech-
nique of binaural loudness balance, were able
to show recruitment for a 1000 cps tone at sen-
sation levels of 10, 20, and 20 dB. Epstein and
Schubert, * using a fatiguing tone at 4000 cps
for a duration of three minutes, report the pres-
ence of recruitment with stimulating intensities
of 70 to 100 dB sensation level. They found that
the degree of recruitment, as obtained from
measurements of the length of the swing on a
Bekesy audiometer, increased as the intensity of
the fatiguing tone increased.

So it has been fairly well established that
fatigue with tones at levels beyond the limits
of linearity has its effect in the cochlea and is
represented by a poststimulation decrease in the
cochlear potentials. Low level fatigue, on the
other hand, has no direct representation in the
cochlea. The first physiological representation
of low level fatigue is a decrease in the action
potential of the auditory nerve, Lack of transfer
leads to a conclusion that the action of low
level fatigue takes place somewhere between
the hair cells of the organ of Corti and the
first decussation in the auditory pathways.

* H. Davis, C. T. Morgan, J. E. Hawkins, Jr., R, Galam-
bos, and F. W. Smith, “Temporary Deafness Followin

Exposure to Loud Tones and Noise,” Acta Oto-Laryngol.
“Suppl. 88 (1950).

)  ° L. Ruedi, “Actions of Vitamin A on the Human and

Animal Ear,” Acta Oto-Larfagol. 44, 502-516 (1954).
contralateral stimulation failed to’ produce a / . - oe )
¥

|. D. Hood, “Studies im Auditory Fatigue and Adap-
tation,” Acta Oto-Laryngol. Suppl. 92 (1950).

TM G. de Mare, “Ein Neues Phenomen im Ohr: Nach-
wirkende Verdeckung,” Skand. Arch. Physiol. 77, 57
(1987).

TM J, D. Harris and A. I. Rawnsley,
Short-Duration ‘Fatigue’ or ‘Adaptation,’”
46, 457-461 (1953).

13 A, Epstein and E, D, Schubert, “Reversible Auditory
Fatigue Resulting from Exposure to a Pure Tone,” A.M.A.
Arch. of Otolamgol. 65, 174-182 (1957).

“The Locus of
. Exptl. Psychol.

2



onds, Only two subjects, each during a task
condition, failed to complete recovery within
the three minute limit, All subjects, with no
exceptions, showed a longer RT for the task
condition,

High level fatigue results. The recovery curves
for high level (90 dB SL) fatigue are presented
in Fig. 3. As with low level, subjects showed
more TTS for the task condition than for the
reverie condition at all the selected points. At
the 10 second point, the mean TTS was 43.3 dB
for the task condition and 34.1 dB for reverie;
that is, there was a mean difference of 9.2 dB.

The RT for high level fatigue was not avail-
able directly for any subject. To obtain an

a
| talus wih 4000%p0- 40m J
‘ teeter soocere

er ‘Task -«
= ‘Reverio- q

of J

ar) 4

2

fae 4

Eat 4

é
ib J
q
ee ee

Thme In Minutes
ag soale)

Figure 2. Recovery following low level fatigue. The
first consistently measurable point was at 10 seconds. Note

that the curves indicate recovery consists of two
logarithmic sses with the slower process t: over
after about & seconds. . 8

estimate of the RT, the curve for each subject
was extrapolated, with an arbitrary limit set at
one hour, and the point at which the curve
crossed the zero line (pre-fatigie threshold
value) was called the recovery ti The mean
RT for the task condition was 44 minutes and
the mean RT for the reverie condition was 34
minutes. , /

In both high level recovery curves a break
is evident just after one minute, This break,
marked “speed change” in Fig. 3, occurred
because of an equipment limitation, The rate
of attenuation was changed from 5 dB per
secofid to 2.5 dB. per second in order to record
the subject’s thresh&fd for a sufficient period
of time. This change in attenuation rate
served to elevate both curves. The continua-
fion of the recovery curve after the speed

change, however, is essentially parallel to the
part before. The possibility that this elevation
is a manifestation of the elusive “bounce”
phenomenon” exists; however, observation of
the individual records indicates that it was the
change in attenuation that elevated the center
point of the tracing as a consequence of a re-
ducion of the ascending portion of the audiom-
eter pen excursion.

An analysis of variance was run on the RT and
on the TTS at 10 seconds both for high and
for low level fatigue, The obtained F for the
task-reverie TTS difference was 26.4 with 18
degrees of freedom, yielding significance be-
yond the .001 confidence level. The obtained
F for the difference in RT between the task

T tt

‘sigue wih 4000ep4- 9048 SL.
Heat 0) 6698 ope

Totes
Reverie «

‘Theashold Shift in 8

Pr ee

Time in Minutes
{leg seas)

Figure 3. Recovery following high level fatigue. The first
caetently measurable point was at 10 seconds. A two

rovess recovery is evident as with low level fatigue. The
Break marked “speed change” represents a change in the
rate of attenuation from 5 dB to 2.5 dB per second.

and reverie conditions was 13.6 with 18 de-
grees of freedom, yielding significance beyond
the .01 confidence level,

Control, conditions results. As a control for pos-
sible procedural artifacts that might produce
a difference between the two task conditions,
the subjects were fin under the same condi-
tions but without a fatiguing tone present. The
mean TTS (at 10 seconds) was computed for
comparison with the results from the experi-

‘mental conditions, Mean TTS for the control

task condition was .30 dB and for the control

** “Bounce” refers to a reversal in the direction of the
recovery curve before completion of reco Approx-
imately 50% of the work on fatigue demonstrates its exist.
gnce “and approximately 50% shows no. indication of
“pounce” in the obtained recovery curves.

5



t

A CENTRAL FACTOR
IN .
PURE TONE",
AUDITORY FATIGUE

\.
‘4

me

'

FEDERAL AVIATION AGENCY
AVIATION MEDICAL SERVICE
AEROMEDICAL RESEARCH DIVISION
CIVIL AEROMEDICAL RESEARCH INSTITUTE
OKLAHOMA CITY, OKLAHOMA

SEPTEMBER 1963


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sdo QQO} Sem ou0} 4saq o4y Jo Aouanbaxy ayy ondney (GP OF) [242]
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>



randomly, Each condition was preceded by a
two-minute threshold determination for the
test tone, Following each condition, another
threshold was taken for a period of three min-
utes for Group I and five minutes for Group II.
The test tone for Group I was a 4000 cps
pulsed tone (0.2 sec on, 0.2 sec off) and for
Group II a 5656 cps pulsed tone. The test
tone was chosen in each case to approximate
the maximum temporary threshold shift (TTS).
The fatiguing tone was a 4000 cps continuous
tone with a duration of three minutes. For
Group I it was presented at a sensation level
of 40 dB, and for Group II, at a sensation level
of 90 dB.

The task conditions were preceded. by the
following. instructions:

“In a moment I'll ask you to take your
threshold again and after a few minutes I'll
tell you to stop. Then relax, After a while
the light in your room will go on (and you'll
hear a tone in your earphones)."* When the
light goes on turn this card over, There are
two numbers on it. Divide the larger number
by the smaller number, Take the answer
and divide it by the smaller number from the
card. Keep dividing your answers by the
smaller number from. the card, not writing any-
thing at all down, In a few minutes the light
in your room will go off again. The instant
the light goes off, write down your last answer
on the bottom of the card, Don’t worry if it’s
a fraction or a decimal. Immediately start
taking your threshold. Work as fast and as
accurately as you can because your perform-
ance will be graded. If you get mixed’up and
forget an answer you may: start over again,
but be sure to stop wherever you are when the
light goes off, because the tohe will start very
quickly and I want you to be ready for it.
Remember . . . take your threshold . . . when
the light goes on you turn, over the card’ and
start dividing . . . and when the light out
you immediately write down your last”answer
and start taking your threshold. Do you have
any questions?” ,

The reverie conditions were preceded by

ese instructions:

“In a moment Tl ask you to take your thresh-
old again and after a few minutes I'll tell

TM The phrases in parentheses were omitted for the con-
trol conditions.

you to stop. Then relax. After a little while
the light in your room will go on (and you'll
hear a tone in your earphones). Just continue
to relax and try not to follow any train of
thought. In a few minutes the light will go
off again. The instant the light goes off start
taking your threshold. Remember , . . take
your threshold . . . when the light goes on just
relax and try not to follow any train of thought

. and as soon as the light goes out start
taking your threshold. Do you have any
questions?”

The subject established his threshold cver a
two minute period. A three minute rest period
followed. At the end of the rest period the
light in the subject room was turned on and
as he was previously instructed, the subject,
either worked a mental arithmetic problem or
engaged in reverie, For the experimental con-
ditions, the fatiguing tone was initiated at the
same time the light was turned on. At the
end of three minutes the fatiguing tone was
switched off and the test tone initiated in con-
junction with turning off the light in the subject
room, This light served as a cue for the subject
to start establishing his threshold again.

RESULTS

Low level fatigue results. The recovery curves
for low level (40 dB SL) fatigue are presented
in Fig. 2, Subjects showed consistently more
TTS when the fatigue tone was presented while
they were working a mental arithmetic problem
than when they were in reverie during fatigue.
The earliest time at which a reliable measure
of TTS was consistently obtained for all sub-
jects was 10 seconds after the cessation of the
fatigue tone. The mean TTS for the task
condition at 10 seconds was 6.4 dB and for
reverie, 3.5 dB; that is, the mean difference in
TTS measured at the 10 second point was
2.9 dB. With the exception of one subject
who showed no ‘difference, all subjects showed
a greater shift for the task condition,

The recovery times (RT) for the two con-
ditions were obtained with an arbitrary limit
set at three minutes. The mean RT for the
task condition was 90 seconds and the mean
RT for the reverie condition was only 36 sec-

1 The phrases in parentheses were omitted for the con-
trol conditions.

—4—

rad



A CENTRAL FACTOR IN PURE TONE AUDITORY

FATIGUE

JOEL S. WERNICK, M.S.
Psychologist
JERRY V. TOBIAS, Ph.D.
Chief, Communication Processes Section

Psychology Branch

63-19

FEDERAL AVIATION AGENCY
AVIATION MEDICAL SERVICE
AEROMEDICAL RESEARCH DIVISION
+ CIVIL AEROMEDICAL RESEARCH INSTITUTE
OKLAHOMA CITY, OKLAHOMA

SEPTEMBER 1963

4


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~ —

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4



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