I ndian Journal
Vol. XIV, No. 3
OF
SEPT. 1962
OTOLARYNGOLOGY OFFICIAL SCIENTIFIC JOURNAL OF THE ASSOCIATION OF OTOLARYNGOLOGISTS OF INDIA
*A Study of The Reversible Auditory Fatigue in the Normal Human Subject BY
Dr. B. P. R. BHATIA, Profs R. N. MISRA, Dr. M. L. BHATIA & Dr. DAYAL. Department of Oto-laryngology, K. G. Medical College, Lucknow. INTRODUCTION Listening is an extremely complex phenomenon, when we consider it in the general term of the common person listening in normal surroundings. As against experimental conditions of extreme quiet, we usually listen to speech or music against a background of noise or voices. Nevertheless the human ear has the remarkable capacity to single out a given signal, which it wishes to hear, from a whole melee of sounds. The effect of noise is thus suppressed i.e., the extraneous sounds are excluded and the signal heard. Ours is a noisy culture and the noise is apt to increase further, as time passes and the country is further industrialised, It is therefore very necessary that we have this capacity for exclusion, in order to get along with our daily work. Absolute thresholds for speech and puretones are measured in the quiet, or as quiet as can be obtained in the presence of sounds created by our respiration, the heart and the blood vessels. We must therefore consider what will be the effect of other sounds on these thresholds. Such sounds can have two *Lucknow University Research Grant Scheme 1962.
90 B. P. R. Bhalia, R. N. Misra, M. L. Bhatia & D. Dayal effects, for example one cannot always hear the other person speak when a train passes nearby i.e., the sound of the train causes the speech to be masked, since it is much higher in intensity than the speech itself. Thus masking is one way in which one's ability to analyse sounds is affected. Auditory fatigue is the other effect of noise, it differs from masking in, that upon cessation of the masking sound, auditory threshold returns to normal immediateiy, whereas fatigue affects the audibility of a sound, when another sound precedes it in time. And it is only after a certain period of time, depending on various factors, that the absolute hearing threshold returns to normal. Thus as masking is an exception to the normal function of the ear i.e., singling out a meaningful sound from background noise ; similarly auditory fatigue is an exception to the normal function of the ear, to resist fatigue by acoustic stimulation. Auditory fatigue therefore creeps in when the normal function of the ear to hear is overcome by sound stimuli of sufficient duration and intensity. Auditory fatigue will therefore be defined as an elevation of the absolute threshold, or the decrease in loudness of a sound, that comes as a result of stimulation by a preceding acoustic stimulus. Auditory fatigue has been referred to by various authors by different names. Thus Luscher and Zwislocki ; Hallpike and Hood call it 'adaptation ' Causse and Chavasse termed it 'stimulation deafness ', de Mare, Munson and Gardener called it 'residual masking', and Reudi placed it among the acoustic trauma, lastly Glorig terms it as temporary threshold shift (TTS). Auditory fatigue assumes its importance as a slightly academic study because of certain practical applicabilities in in daily audiological work. (1) The fact that it is present in normal hearing individuals warns us not to turn on the audiometer at a high intensity lest it might affect the accuracy of the absolute hearing thresholds.
A Study of the Reversible Auditory Fatigue
91
(2) There seems to be a relationship between the temporary threshold shifts and the permanent threshold shifts of industrial deafness and the future may lead to a measurement of the susceptibility of a particular ear to industrial deafness and effective measures may be evolved to check the occurrence of such deafness, as is wont to occur after prolonged stimulation by high intensity noise. (3) It seems that the phenomenon of auditory fatigue varies with the conductive or perceptive type of deafness present in a particular individual. It may then be possible to differentiate between the two by means of fatigue measurements of short term duration (Gardner 1947). The interest in this study arose from the common observation every otologist experiences when performing the tuning fork tests on a patient. While measuring the absolute period of decay of tuning fork by air conduction, one finds that the period increases slightly when the vibrating tuning fork is removed, from the ear for a short period every few seconds, over the period of decay one gets when it is constantly held near the ear (Lederer 1947). The observation has been confirmed by us on ten test subjects and it has been found that the period of decay can vary anywhere from 0.6 to 2-3 seconds when determined by the above two methods. This phenomenon has been attributed to auditory fatigue, herein lies the day to day application of this academic work. REVIEW OF LITERATURE
At present the literature on auditory fatigue is huge, disjointed, and full of uncertainity and controversy. Some have even denied its very existence, and others have been able to observe it as extremely transient, and a phenomenon of a very small degree. The disjointed nature of the literature and the consequent confusion that it creates, is due, mostly to two reasons ; firstly due to the many variables that have to be dealt with even during experimental work, let alone the field studies
92 B. P. R. Bhatia, R. JV. Misra, M. L. Bhatia & D. Dayai on the subject. Secondly, the unopened realms on this subject are much greater than the vistas already explored. The following will give some of the important findings on the subject. von Bekesy (1922), using a tone of 800 cps. found that the magnitude of temporary threshold shift and the affected frequency range was a function of the sound intensity. He used high intensity and obtained a curve which rises steeply for the first 15 seconds of exposure and then smothens out. Ewing and Littler (1925), and Perlmann (1947) emphasised that the magnitude and frequency localisation of auditory fatigue is dependant upon the frequency and the sound pressure level of the noise exposure. They also said that, longer the exposure time the greater the auditory fatigue. Thcy used puretones arnd.an exposure time of 15-30 seconds. Rawdon Smith (1936), using a 2000 cps. tone and in conditions similar to Bekesy of intensity and duration, obtained effects well beyond the fatiguing itself. Davis Morgan et al (1946), using pure tones found fatigue effects for high tones, but they sometimes showed low tone fatigue also. For producing low tone fatigue very high intensities (110-130 db.) were however used. In this, probably both trauma and overtones seem to bring about the effect, the latter may be present in the stimulus or may be produced in the ear. They also found that after exposure to an octave band noise the resulting fatigue is corresponding in magnitude and distribution to that obtained after exposure to a pure tone of the same intensity and with a frequency corresponding to the mid point on the octave band. They also showed that high frequency noise causes fatigue to a greater extent than low frequency noise. Causse and Chavasse (1947), using a 30 db. above threshold tone found that the lowest tones are quite ineffective, and all frequencies upto 500 cps. give doubtful auditory fatigue.
A Study of the Reversible Auditory Fatigue
93
Beyond 640 cps. the curve rises, it rises swiftly from 1000 to 3000 cps. but beyond this smoothens out. They also obtained changes in the contralateral ear during fatigue. Hood (1950), Theilgrad (1951), and Harris (1950), used relatively brief exposure and carried out threshold measurements within the first few minutes after exposure. Hoods results, that auditory fatigue was proportional to the logarithm of the exposure time, were confirmed later by Ward et al. Harris (1953), studied the recovery curves in persons who had been subjected to experimentally produced sound of sufficient intensity to cause reversible auditory fatigue. He employed an oscillator amplifier circuit supplying the noise to an ear phone. The same ear phone was used to test the hearing, after the tone was turned off. If the subject had not recovered within a minute, his threshold was measured once per minute. Harris found considerable variation in fatigue from individual to individual, and in the same individual from time to time. The shape of the recovery curve was found to depend upon both the duration and the intensity of the stimulus. The hearing loss bore a linear relationship to the length of stimulation. van Dischoeck (1954), experimenting with recorded total noise, found it to shift the thresholds of hearing for 2000 — 8000 cps. Similar results were obtained with octave bend filtered noise. Rol (1956), showed that the temporary hearing loss after exposure to both puretones and third- octave band noise had a maximum not only in the first octave above the stimulus frequency but also in higher octaves. These, he said, were due to the traumatising effects of overtones. Large discrepancy however existed between his experimentally demonstrated damage risk curve and the theoretically obtained damage risk criteria. He, however said, that there is a rapid recovery phase during the first few minutes after exposure. 2
91 B. P. R. Bhatia, R. N. Misra, M. L. Bhatia & D. Dayal Hinchcliffe (1957) tested the hearing after very short exposures to one or few frequencies. Ward et al (1959) in a report of thirteen subjects, have given an exceedingly comprehensive study of the dependence of auditory fatigue on exposure time and sound pressure level as well as study of recovery curves. Each of their subjects underwent 12 different exposures. And in audiometry two test frequencies were used. It was found that the growth and recovery of auditory fatigue are linear in proportion to the logarithm of the exposure time. Rate of growth varies with the frequency range of the exposure band and the frequency of the test tone. The rate was highest for the 400 cps. test frequency and lower for lower test frequencies and octave bands. Gloring (1961) describes the parameters of temporary threshold shift as follows : 1. Relation with industrial audiometry. That an audiogram must not be made at the end of a days work. It should be made preferably in the morning, or thirty minutes after exposure. 2. As permanent loss increases, temporary loss decreases there is thus a linear relationship between the two. 3. Its relation to susceptibility. Many tests of susceptibility to noise induced hearing loss have been based upon the amount of temporary threshold shift produced by short exposures of various types of sound. There is no test for detecting highly susceptible ears, and predicting susceptibility to noise induced hearing Ioss of any degree. Susceptibility follows a normal statistical distribution; that few persons, are highly susceptible to noise induced hearing loss; noise never produces sudden severe hearing loss, there are undoubtedly few such persons, but they have not yet been discovered.
A Study of the Reversible Auditory Fatigue
95
Indeed we all know that sound in the ear is converted into electrical energy which is then transmitted to the brain in the form of impulses through nerves. It strikes the mind therefore, that, is it not the nerve that is involved in auditory fatigue? Or, is fatigue central or peripheral in origin? Wright (1961) states that changes do occur in the excitability and conductivity of a nerve immediately after the passage of an impulse. Firstly, for the duration of the spike potential i.e. the duration of the passage of the actual nerve impulse, the nerve is absolutely refractory. Meaning thereby that neither can the nerve be excited nor can it conduct during this time, no matter how strong the stimulus. Recovery, starts soon after the spike potential, and the nerve responds to progressively weaker stimuli, but the response is neccessarily weaker than that of a normally recovered fibre. Fibres of larger diameter recover to near normal in about one millisecond, that is to say that each fibre is capable of conducting impulses at the rate of a thousand per second. Motor fibres rarely have to conduct naturally at rates exceeding 100 to 150 impulses per second, or more, normally than these fibres can readily cope with the lower frequencies with which they have to deal. Fibres of smaller diameter recover gradually and therefore the ceiling of the impulse frequency they can transmit is lower, than in the case of larger fibres. Wright further goes on to state, that the reason why a nerve fibre can respond to continuous stimulation for long periods without fatigue is that the fibre is conductive not continuously but intermittently; any stimulus that falls dnring the refractory period is ineffective and the fibre respond only when it has recovered. Another feature in a nerve, according to Wright, is accomodation. When a nerve is submitted to the passage of a constant subthreshold current, the part of the nerve under the anode decreases in excitability and the part under the cathode increases in excitablity. Nevertheless if the current be continued for some length of time both these changes revert to normal, this phenomenon is called accommodation. A
96 B. P. R. Bhatia, R. X. Misra, M. L. Bhatia & 1D. Dayal similar feature of nerve endings is called adaptation. The fibres of sensory nerves have a far less power of adaptation than those of the motor nerves; pain fibres show almost no accommodation at all. Pattie (1929) stimulated both ears equally but adjusted the phase of the stimulating tone so that, to the subject the sound seemed to be present in only one ear; under these conditions he found no difference in the fatigue effect in the two ears. Thus irrefutable evidence that fatigue is peripheral and is not due to such central processes as attention was provided. Wever (1949) envisages a theoretical explanation of the fatigue process on his volley hypothesis. According to this hypothesis, the rate of impulses conveyed by a nerve fibre is determined by three conditions ; the frequency of the excitations, their magnitude, and the excitability of the fibre. In fatigue we are concerned with the last of these factors. When a fibre is called upon to respond early in its relative refractory period, its refractory period is prolonged and, its excitability impaired. These effects are cummulative over a period of time. Therefore as the fibre is over-crowded, its impulses diminish in rate. The whole volley of discharge, in which fibres are undergoing this same decline, accordingly suffers a shrinkage in magnitude. Wever explains the fact, that low tones are not subject to fatigue, or only when presented at extra ordinary levels of intensity, as follows. The excitations of the nerve fibres come so well separated in time that they recover fully after every impulse and the train of activity is. undiminished. Fatigue enters only beyond that critical frequency where the fibres begin to be pushed into their relative refractory periods. This frequeney will vary some-what, with the intensity level prevailing. The higher the intensity level the lower the critical frequency, and vice versa. In the upper frequency range where volley action
A Study of the Reversible Auditory Fatigue
97
prevails, the fatigue increases with intensity because the fibres are brought into action more often, and are thus depressed by the effects of forcing. MATERIAL AND METHOD
Sampling.—Test subjects were medical students and doctors of both sexes. All subjects were selected by random sampling off from those doctors and students who attended the Ear, Nose & Throat out patient departmeut. Examination of subjects.—A complete Ear, Nose & Throat check up was carried out. When no defect was noted, hearing examination was done by means of : (1) 512 dv. Tuning fork - The air conduction decay was compared with that of the experimenter (B.P.R.B.) (2) Forced whisper and conversation test was next done. Whisper was produced with the chest fixed in full expiration. (3) A pure tone, Air conduction audiometry was next done. A subject was declared normal when he or she had
(1) A comparably equal decay time with the tuning fork. (2) A complete comprehension of whisper and conversation at a distance of 20 feet. (3) Not more than 20 db. loss of hearing on the pure tone Audiogram. All subjects which did not come to this standard were rejected.
Procedure.—The subjects were exposed to the experimental tones in a sound proof room, at least ten minutes after the pure tone air conduction audiogram had been done on them. 7
98 B. P. R. Bhatia, R. N. Misra, M. L. Bhatia Fr D. Dayal Audiometry and exposure was done by means of the matching ear phones supplied alongwith the Amplivox Audiometer Model 61. Exposure was done in one ear at a time. Absolute thresholds of hearing were measured half a minute after the cessation of the test tones. If fatigue occured, threshold was determined every half minute till it fell to the pretest level. A period of at least ten minutes' rest was allowed after each exposure, and threshold measurement. The following exposure sounds were used : TABLE 1 Nature of sound
Duration of sound
I
1000 cps.
30 sec.
75 db.
2
1000 cps.
60 sec.
75 db.
3
1000 cps.
30 scc.
90 db.
4
1000 cps.
60 sec.
90 db.
5
4000 cps.
30 sec.
75 db.
6
4000 cps.
60 sec.
75 db.
7
4000 cps.
30 sec.
90 db.
8
4000 cps.
60 sec.
90 db.
9
White noise
30 sec.
70 db.
10.
White noise
60 sec.
70 db.
S. No.
Intensity of sound
SUBJECTS
Since all subjects were either medical students or doctors it was expected that the level of intelligence of each would be the same. The age span was also narrowed down to a variation of only ten years (20-30). The average age being 21.8 years. Subjects were selected at random, but from people residing in the same environment, thus another of the variants was kept constant. The sample so obtained was therefore representative of the group to be tested. None of the subjects had ear or nasal trouble prior to or during the course of the test.
A Study of the Reversible Auditory Fatigue
99
VARIANTS
In auditory fatigue there are many variants. It was attempted to keep most of these constant, viz., 1. Intelligence level. 2. Age. 3. Previous experience of noisy suroundings. 4. Total experience of noise. Depending on 2 & 3 (above). 5. State of the ear. Keeping the age limited to young adults i.e., from 20 - 30 years. Threshold measurements were made as follows: TABLE 2 S. No.
Exposure sound
Threshold determined at
1.
1000 cps.
1000 cps. and 4000 cps
2.
4000 cps.
4000 cps.
3 •White
noise
4000 cps.
During the test all activity was kept at a minimum to avoid extraneous sounds which may have interfered with the determination of the absolute threshold of hearing. AUDIOMETRY
The work was conducted on a freshly calibrated audiometer. Both head phones had been accurately matched as was found by a check audiogram on the same ear by both ear phones. Thresholds were measured in 5 db. steps. The same audiometer was used throughout the tests and a check audiogram was done before the start of work every day. Before the start of the test, the subject was instructed concerning the test and the subsequent measurement of thresholds. The ear-phones were then placed over the head of the
100 B. P. R. Bhatia, R. N. Misra, M. L. Bhatia &r D. Dayal' subject, special care was taken to see that the earphone was in correct apposition with the external auditory canal of the subject, and that it was in close contact with the pinna. For audiometry, an automatic interrupter was used which gave the signal for 2 sec. and then cut it off for 0.2 second. The subject was aaked to raise his finger when he heard the tone and to keep it raised till he heard the tone. Thresholds were always measured in decreasing steps of intensity because it has been found experimentally that the threshold determined where the subject just heard the test tone is 5-10 db. higher than when the subject just stops listening to the test tones, e.g., if the threshold were expected to be 5 db.. measurement was started well above it, say at 20 db., and then the threshold was lowered till the subject stopped hearing the tone. The audiogram was started at 1000 cps. and thereafter, the frequency was raised and lowered from here, till the complete audiogram had been taken ; only air conduction was used. MEASUREMENT OF TIME
A stopwatch graduated to 0.2 sec. was used for measurement of the time of delivery of the exposure tone, and for the subsequent measurement of thresholds.. RECORDING
All observations were recorded on a proforma and then tabulated. During the test any subjective sensation was carefully noted and enquiry made into the character of the sensation (Table 9).
A Study of the Reversible Auditory Fatigue
101
TABLE 3 Incidence of auditory fatigue according to sex distribution S.Sex No.
Number of subjects getting fatigue Number
I.
Male
2.
Female Total
Percentage
Number Percentage
Number of Ears not getting fatigue
Number of Ears getting fatigue
Number of subjects not getting fatigue
Number Percentage
Number Percentage
19
100
Nil
Nil
35
92.1
3
7.9
7
87.5
1
12.5
14
87.5
2
12.5
26
96.3
1
3.9
49
80.7
5
19.3
TABLE 4 Distribution of subjects according to sex. Number of Ears
Sex
Number
1.
Male
19
38
2.
Female
8
16
S. No.
TABLE 5 Showing the frequency at which maximum auditory fatigue was noted
Total ears fatigued
Number recording maximum fatigue at 4000 cps.
Number recording maximum fatigue at 1000 cps.
Number recording maximum fatigue at both.
38
5
6
49
TABLE 6 Showing type of fatiguing-tone causing maximum fatigue
Total Subjects fatigued
49
4
Number recording threshold shift with 4000 cycle tone. 29
Number recording threshold shift at 1000 cps.
Number recording threshold shift at both.
8
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A Study of the Reversible Auditory Fatigue TABLE 9
Effect of Fatiguing Tone Duration on the I nten.sity and Duration of Fatigue
S. Case No. No. Ear No.
Maximum Fatigue with 30 second tone Threshold Shift db.
Duration of Shift. mts.
Maximum Fatigue with 60 second tone Threshold Shift db.
Duration of Shift mts.
1
1/2
5
14
10
14
2
2/4
10
2
10
21
3
3/6
15
2
10
14
4
5/9
10
14
15
24
5
5/10
10
21
10
24
6
6/11
--
10
14
7
6/12
5
21
10
24
8
7/14
10
1
10
14
9
8/15
10
74
10
8
10
10/20
10
1
5
1
11
11/22
10
2
10
2
12
12/23
5
1
10
14
13
12/24
15
14
5
14
13/26
10
14
10
14
15
15/29
5
1
10
14
16
15/30
10
2
10
2
17
16/31
10
12
10
13
18
17/33
—
—
10
1
19
17/34
10
j
10
1
20
20/39
15
14
15
2
21
20/40
15
5
15
6
22
21/41
10
64
10
7
23
21/42
—
—
10
14
24
24/48
10
21
10
34
25
25/50
10
10
4
26
2652
5
34 14
10
2
5
—
106 B. P. R. Bhatia, R. N. Misra, M. L. Bhatia tr D. Dayal TABLE 10 Effect of Fatiguing tone intensity on the intensity and duration of Fatigue.
Case No. S. No.
75 db. tones Max. of 90 db. tones Max. of 70db. White Noise only. 1000 and 4000 cps. 1000 and 4000 cps.
Ear No. Threshold Duration of Threshold Duration of Threshold Duration of Shift db. Shift db. Shift db. Shift mts. Shift mts. Shift mts.
1.
1/2
5
1
5
1
10
if
2.
2/4
5
1
5
1
10
2j
3.
3/6
5
1
10
11
15
21
4.
5/9
5
1
5
1
15
21
5.
5/10
5
11
10
2
10
2f
6.
6/I1
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—
—
10
if
11
10
2f
7.
6/12
5
8.
7/14
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9.
8/15
10
10.
10/20
—
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11/22
5
1
5
--
10
if
10
if
2
10
8
10
71
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1
5
1
10
2
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15
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—
1
12.
12/23
—
—
13.
12/24
—
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14.
13/26
5
15.
I5/29
—
16.
15/30
10
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16/3I
18.
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5
1
10
11
2
5
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10
2
10
12
10
13
10
12
17/33
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—
—
10
1
19.
18/34
—
—
5
1
10
1
20.
20/39
5
1
15
1
15
2
21.
20/40
15
6
5
3
15
6
22.
21/41
5
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10
3
10
If
23.
21/42
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—
—
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24.
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10
2f
10
3f
25.
25/50
10
21
10
3
10
4
26.
26/52
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10
2
10
2
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108 B. P. R. Bhatia, R. N. Misra, M. L. Bhatia & D. Dayal TABLE 12 Analysis of Subjective Sensation. S. No. 1. 2. 3. 4. 5.
Subjective sensation. Tinnitus Fullness Intolerance Pain Ticling sensation
No. of subjects complaining. 6 2 2 1 1
DISCUSSION
An attempt at discussing a phenomenon such as auditory fatigue, leaves one groping in the dark at very preamble. The reason for the trouble that one meets at the very outset. is quite obvious ; what is known about auditory fatigue for certain, is much less than what is not known. To make these disjointed facts comprehensible one has to supply a lot from imagination, and when such a thing happens one is apt to rely more on the fertility of one's imagination than on facts. The truth therefore suffers and what we have in our hands is a series of disjointed facts with the window dressing of theories from the arm chair. It will be our attempt in the following to stick to facts, as far as possible., and to reduce the window dressing to the minimum.
(1) Selection of Exposure time and sounds: The selection of a suitable exposure time and particular exposure sounds and of subsequent audiometric frequencies has been very difficult. On going through the literature one finds a wide variety of sounds being used. We have selected ours (Table 1 and Table 2) for several reasons. Davis and Morgan et al (1946) using only puretones found that fatigue occured easily at higher frequencies than at lower. For producing low tone fatigue very high intensities upto 130 dbs. were required. They could cause trauma, and fatigue could occur due to overtones whose magnitude would be quite high at this high level of exposure sounds, these overtones could be present in the exposure stimulus itself or if it were kept
A Study of the Reversible Auditory Fatigue
109
sufficiently pure i.e., the overtones were kept extremely low in intensity, they could also be produced in the ear at such high intensities of low tone sound stimulus. We have therefore resorted to the use of moderately high tone exposure for stimulation. Davis et al also found that after exposure to an octave band noise the resulting fatigue is corresponding in magnitude and distribution to that obtained after exposure to a puretone of the same intensity and with a frequency corresponding the midpoint on the octave band. Since one of our purpose in this study was also to evolve a simple test for detecting susceptibility without sacrificing accuracy, and also keeping the equipment to a minimum. We have used puretones and white noise as exposure sounds which are available on all standard equipment necessary for routine audiological work. Davis etal alo emphasised that high frequency noise causes fatigue to a greater extent than low frequency noise ; we have therefore used high frequency sounds for exposure. Since van Dischoeck (1954) found threshold shifts of hearing for 2000 — 8000 cycles, and since Rol (1956) has shown that the temporary hearing loss after exposure is found not only in the first octave above the stimulus frequency but also in higher octaves, we have used audiometric threshold check up only at two frequencies; one at 1000 cycles per second and other at a moderately high frequency of 4000 cycles per second.
(2) Observations : In all twentyseven subjects, nineteen males and eight females were examined. The presence of fatigue in all was the rule rather than an exception. However on an average females were found to be less prone to fatique than the males. (Table No. 3). Exposure sounds used in the test (Table No I and 8) were all of short duration 30 seconds and 60 seconds. The fatigue also therefore was of short duration and the amount of threshold shift was less. 6
110 B. P. R. Bhatia, R. N. Misra, M. L. Bhatia & D. Dayal
The maximum threshold shift was fifteen decibels, and the maximum duration of the shift was thirteen minutes. In all, no fatigue was detected in five ears, this does not necessarily mean that no fatigue was present, but can also signify that the duration was less than half a minute. The frequency most commonly affected by the auditory fatigue was that of 4000 cps. Out of the 49 ears in which fatigue was recorded thirtyeight ears showed a maximum temporary threshold shift at the 4000 cycle level, 6 at both 4000 and 1000 cycle level and five at 1000 cycle level. Thus we do not agree with the findings of Harris (1953) that the maximum shift is in the region of 1000 cps., our findings seem to indicate that the loss of dearing during fatigue is more at 4000 cps., but our findings are in agreement with Rawdon Smith who found that a 2000 cps. tone affected well beyond the fatiguing tone, thus fatigue was found to occur at 4000 cps. by a 1000 cps. exposure tone, and Ward et al, who found the rate of growth of fatigue to be highest for the 4000 cycle tone and lower for lower test frequencies and octave bands. It was found out that the 4000 cycle exposure tone affected the maximum threshold shift. Thus we found that out of the fortynine ears fatigued twentynine had maximum threshold shift with 4000 cycle tone and only 8 with 1000 cycle tone, 12 showed a maximum shift with both frequencies. The rate of lowering of threshold after the exposure sound ceased also gave a significant finding. Whenever the shift in absolute threshold was more than 5 db. the rate of lowering of threshold was significant in that, that the more the fatigue, the faster was the rate of lowering, and vice versa. Thus, for example, if a subject showed fifteen decibel of fatigue the lowering through the first five decibels was the quickest, the lowering through the next five decibels was slower and was slowest for the last five decibels. By corollary therefore the rate of growth of fatigue was similar, the first five decibels coming early and the next decibels following slower in time (Ward et al)
A Study of the Reversible Auditory Fatigue
1/1
Table 8 shows the rate of lowering of fatigue in the 27 cases in which fatigue was of the order of 10 dbs. or more. We are in agreement with the findings of Rol (1956) that there is a rapid recovery phase during the first few minutes after exposure. The effect of the duration of the fatiguing tone on the duration of fatigue is of definite importance. That the duration directly increases the amount and duration of fatigue is evident from Table 7. We have used only two duration viz., 30 sec. and 60 sec., and we find that as a general rule the duration and the magnitude of fatigue is directly proportional to the duration of the fatiguing tone, provided of course that the intensity is kept constant. That the amount of fatigue and its duration is directly proportional to the intensity of the exposure sound as found by us (Table 8) has been demonstrated by several workers, such as, von Bekesy who using low frequency exposure tone found that the fatigue curve rose steeply for the first 15 sec. of exposure, Ewing and Littler, and Perlmann have emphasized that the magnitude of auditory fatigue is dependant upon the sound pressure level and that the longer the exposure time the greater was the auditory fatigue. We agree with the finding that the magnitude of fatigue is directly proportional to the duration and intensity of the exposure sound, but have failed to establish a definite formula for predicting fatigure before it occurs through the intensity and or the duration of the stimulating tone. We attribute this failure to an important factor. Individual susceptibility varies from person to person and from ear to ear by a normal statistical distribution. It is difficult therefore to predict, the magnitude of fatigue by the intensity and/or the duration of the exposure tone, when the susceptibility itself is uncertain. Since susceptibility depends on several factors such as — 1.
Age.
2.
Previous experience of noisy surroundings.
112 B. P. R. Bhatia, R. N. Misra, M. L. Bhatia & D. Dayal 3.
Total experience of noise.
4. State of the ear depending upon age and its physical condition. It is therefore almost impossible to establish a mathematical relationship between intensity and the duration of the exposure tone and the magnitude of fatigue. Lastly we come to the very interesting phenomenon that is associated with exposure to all sorts of intense sounds, the phenomenon of subjective sensations (Table 11 and 12). Nine subjects complained of subjective symptoms ; the most common symptom of tinnitus was complained by six supjects, a sensation of fullness and intolerance was next with 2 each, whereas pain and a tickling sensation was felt by one sudject each. Some times two symptoms were common to the same subject. Tinnitus — The fact that tinnitus was felt immediately after the sound ceased and that it usually had one of two similaritie s in every case, i.e., either it was of a frequency near about the stimulating tone, namely, high pitched, or it was complimentary to the stimulating tone, viz., low pitched, thus in a way the latter may be considered to be neutralising the sound originating it. We have no suitable explanation for the presence of tinnitus beyond the fact mentioned by Causse and Chavasse (1947) who found changes in the contralateral ear during fatigue. The peculiar relationship of the tinnitus with the exposure sound makes it possible for us to venture an explanation for this phenomenon. Suppose we compare the loud exposure tone in the ear with intense light in the eye. The auditory fatigue then becomes similar to the dark adaptation required for the eye, varying greatly accordiug to the stimulus which has been falling upon the sense organ before the observation is made. (Duke-Elder 1959). The explanation of the after tinnitus is then easy, it is like the after image that we see when the source of intense light is removed. Similar to the phenomenon in the eye, this after tinnitus may be similar to the sensation that causes
A Study of the Reversible Auditory Fatigue
113
it or complimentary to it in quality. We thus have an explanation for the after tinnitus which is both plausible and which satisfies all our experimental findings. Fullness, pain, intolerance and tickling are in our opinion only different modes of expressing the same sensation, i.e., the sensation of intolerance. We thus had only five instances of intolerence, this only goes to prove that our tests were quite harmless, in so far as the subjects were concerned.
(8) Explanation of the phenomenon of auditory fatigue: For the appreciation of a sense we have three distinct intermediaries before it can become complete. Thus we have -(a) The end organ. (b) The nerves that conduct the impulses. (c) The higher centre which appreciates the sensation. That auditory fatigue can be located at any one of the above sites is a matter to be debated. We shall start at the end therefore and end at the begining. That auditory fatigue is not central in origin but peripheral was conclusively proved by an interesting experiment of Pattie (1929), he found that when a subject's ear was stimulated with sound without him knowing it, he developed fatigue which was similar in magnitude and duration to fatigue that he developed after exposure to the sound in full knowledge. Thus irrefutable evidence that fatigue is not central in character but peripheral was obtained. In the peripheral parts of the mechanism of hearing we have two definite structures viz., the conducting nerves and the sense organ of hearing, the ear. It is a well known fact that the fatigue never occurs in nerve. The changes that occur in a nerve after the passage of an impulse seem to support this fact rather than to disprove it (Wright 1961). During the passage of the impulse i.e., during the spike
114 B. P. R. Bhatia, R. N. Misra, M. L. Bhatia & D. Dajal
potential, the nerve is absolutely refractory to all stimuli, howso-ever strong they are. Immediately after, the nerve-enters a recovery phase, during this period the nerve is able to respond to less stronger stimuli till returns to normal. The period of 90% recovery being only 1 m.sec., a single nerve fibre can therefore transmit impulses at the rate of 1000 per second. Beyond this, impulses which increase in number are just not taken up by the fibre, which is refractory. Normally when one has to appreciate impulses at rate higher than 1000 per second, the Volley mechanism (Wever 1949) comes into play. several fibres dividing the impulses amongst themselves, which are then transmitted split and are analysed in the higher centres. Wever says, that the rate of impulses conveyed by a nerve fibre is determined by three conditions ; the frequency of the excitations, their magnitude, and the excitability of the fibre According to Wever, in fatigue, the last of these factors is disturbed. According to his hypothesis when a fibre is stimulated early during its recovery from the refractory phase, its excitability is impaired, that is, the response obtained is not as much as to be expected from a completely recovered fibre, moreover the subsequent refractory period is prolonged. Therefore as impulses overcrowd a fibre its impulses are diminished in rate, and volume. Thus the whole volley of discharge undergoes a decline, and fatigue occurs. Fatigue according to Wever enters only beyond a certain critical frequency, where-in the fibres begin to be pushed into their relative refractory periods. Below this critical frequency fatigue is not present, because fibres have plenty of time to recover after the passage of the impulse. In low tones therefore high intensities are required to force fibres and to cause fatigue, whereas only moderate intensities cause fatigue in high tones. We can thus arrive at a certain formula. Let us suppose the number of impulses a nerve fibre can transmit is 'a', the frequency of the sound to be transmitted is ' v' and the intensity to be transmitted 'i'. Now the volley mechanism will come into play only when v + i is more than 'a'. As v + i exceeds,
A Study of the Reversible Auditory Fatigue
115
more and more fibres come into play; now let us suppose that the nerve contains 'n' number of fibres, the total number of impulses that it can transmit therefore are a X n. Fatigue will therefore set in when--v-}-i > ax n since factors a and n are constant any increase in v or i to an extent beyond the value of a X n will cause fatigue, which will be proportional to the amount by which v+i will exceed a X n. Theoretically this appears to be a very sound and a reasonable explanation. But this does not stand true to one universal finding, and that is the complete absence of fatigue in certain individuals and the marked fatigue experienced by others. It is no doubt true that between a and n, the value of n will vary, but certainly it can never be so Iarge as to defy fatigue to all intensities, neither can n be so small as to give rise to a marked fatigue. Variation in the number of the auditory nerve fibres can be expected to a certain extent, but never to the extent one would expect by the experimental findings on the subject of auditory fatigue. CONCLUSION
On a critical examination therefore of the three methods by which fatigue could be brought about we find that the role played by the higher ccntres is to be ignored (Pattie 1929). The second theory based on Wever's Volley mechanism of hearing also fails to satisfy the observation that all patients do not show a fatigue of constant variety and at all times. Therefore we are left with the third and only alternative that the seat of fatigue is either at the junction between the membrana tectoria and the hair cell or at the junction between the hair cell and the nerve endings, since according to Wever cochlear microphonics are produced at the site of the membrana tectoria and the hair cells by piezoelectric potentials, fatigue can occur at this site in a manner which might be explained by electrochemical and piezoelectric phenomena. SUMMARY
A study of auditory fatigue of short term duration in twentyseven normal subjects is presented. Puretones and white
116 B. P. R. Bhatia, R. . Misra, M. L. Bhatia & D. Dayal noise were used as fatiguing sounds. It was found that the occurence of auditory fatigue is rather the rule than an exception. It was also found that auditory fatigue was directly proportional to the intensity and duration of the fatiguing tone and that after occurence the rate of lowering was universely proportional to the amount of fatigue. An analysis of subjective sensations and an explanation for the occurence of this phenomenon is offered. ACKNOWLEDGEMENT
The authors wish to acknowledge their thanks to the Superintendent, Gandhi Memorial & Associated Hospitals, incorporating King George's Medical College, Lucknow, tor the help rendered in the completion of this work. BIBLIOGRAPHY 1. 2.
v. Bekesy, G. Physik. Ztscht, 30, 722, (Q. Kylin) (1929). Causse, R. and Chavasse, P, (1941) C. R. Soc. Biol. Paris, 135, 1272, (Q. Hirsh) 1941. 3. Davis, H. Morgan, C. T., Hawkins, J. E., Galambos, R. and Smith, F. W. Acta Otolaryng., Suppl. 88, (Q. Kylin) 1950. 4. Dischoeck, H. A. E., van Acta, otol. belg., 8: 46, (Q. Kylin) 1954. 5. Duke-Elder, S. Parson's Diseases of the, J. & A Churchill, London, 1959. 6. Ewing, A. W. G. and Littler, T. S. Brit. J. Psychol., 25, 284, 1935. 7. Fry, D. B., and Watkyn Thomas, F. W. Diseases of the Throat, Nose & Ear, London H. K. Lewis & Co. Ltd. 1953. 8. Gardner, M. B. J. Acoust. Soc. Amer., 19, 592, (Q. Hirsh) 1947. 9. Glorig, A. (1961) Journal of Laryngology L XXV, 5, 447, 1961. 10, Hallpike, C. S. and Hood J. D. J. Acoust. Soc. Amer., 23, 270, (Q. Hirsh) 1951. 11. Harris, J. D. J. Acoust. Soc. Amer., 22, 674, (Q. Kylin) 1950. 12. Harris, J. D. Laryngoscope, St. Louis, 63, 660, Q. Korkis) 1953. 13. Hinchcliffe, R. Acta, Otol., 47, 497, (Q. Kylin) 1957. 14. Hirsh, I. J. The Measurement of Hearing: Mc Graw-Hill Publications in Psychology, New York, 1952. 15. Hood, J. D. Acta, Otol, Suppl, 92, Q. Kylin) 1950. 16. Korkis, F. B. Recent Advances in Otolaryngology, London, J & A Churchill, 1958. 17. Kylin, B. Acta. Otol. Suppl., 152, 1960. 18. Lederer, F. L. and Hollender, A. R. Text book of the Ear, Nose and Throat, Philadelphia, F. A. Davis, 1947. 19. Luscher, E and Zwislocki, J. Acoust. Sec. Amer., 21, 135, (Q. Hirsh) 1919. 20. de Mare, G. Skand. Arch. Physiol., 77, 57, (Q. Hirsh) 1937,. 21. Munson, W. A.. and Gardner, M. B. J. Acoust. Soc. Amer., 22, 177, (Q. Hirsh) 1950. 22. Pattie, F. A., Jr Brit. J. Psychol, 20, 38-42, (Q. Hirsh) 1929,. 23. Perlmann Arch. Otolaryng., 34, 429, 1941. 24. Rawdon-Smith, A. Brit. J. Psychol., 25, 77, (Q. Kylin) 1934-35. 25. Rol, C. Thesis, Leyden, (Q. Kylin) 1956. 26. Theilgrad Thesis, Munksgaard, Copenhagen, (Q. Kylin) 1951,. 27. Ward, W. D., Glorig, A. and Sklar, D. L., J. Acoust. Soc. Amer., 31, 522, (Q. Kylin) 1959,. 28. Wever, E. G. Theory of Hearing, New York, J. Wiley & Sons, 1949. 29. Wright, S. Applied Physiology, Oxford Medical Publications, London, 1961.