recovery from high values of temporary threshold shift

DIMENSIONAL ANALYSIS OF COCHLEAR MODELS 497

nonlinear distortion within the model. 16 The inference

had been that this form of distortion might be responsible for the phenomenon of audible harmonic distortion, which appears at relatively low sensation levels within the human ear, according to Fletcher 17 and to Lawrence 20

Figure 3 shows the data by Lawrence 10 on the onset of audible harmonic distortion in normal ears re

sensation level for various primary frequencies. Means

•6 j. Tonndorf, J. Acoust. Soc. Am. 30, 929 (1958). •7 H. Fletcher, Speech and Hearing (D. Van Nostrand, Inc.,

Princeton, New Jersey, 1953). •s M. Lawrence, Tran. Am. Acad. Ophthalmol. Otolaryngol. 62,

104 (1958).

and standard deviations are given. Plotted in the same graph are the onsets of eddy motion for various similar frequencies as observed in the present model but ex- pressed in terms of the dimensional analysis of Table I. The fit is very good at higher frequencies, somewhat less so at lower ones, although the trend of both functions is quite similar. This result appears to lend further support to the hypothesis of the hydrodynamic origin of intra- cochlear distortion.

In many respects, the present paper furnishes mere suggestions of trends rather than final results. However, it is hoped that a presentation of this kind will con- tribute toward a more complete analysis of cochlear modeling.

THE JOURNAL OF THE ACOUSTICAL SOCIETY OF AMERICA VOLUME 32, NUMBER 4 APRIL, 1960

Recovery from High Values of Temporary Threshold Shift* W. DrxoN WAaD

Research Center, Subcommittee on Noise, 327 South Alvarado, Los Angeles, California (Received December 3, 1959)

Twelve normal observers were exposed to noise that produced at least 50 db of temporary threshold shift (TTS) measured 2 min after cessation of the noise. The TTS was measured at regular intervals until recovery was complete. Results indicate that while the recovery from these high values of TTS proceeds as a function of the logarithm of time during the first few hours, later recovery is instead linear in time.

T has been demonstrated 1.2 that during the first two hours after exposure to noise, the temporary threshold shift (TTS) induced by the noise recovers linearly in log t, where t is time since cessation of the noise. As long as the initial TTS or TTS2, (the TTS measured 2 rain after cessation of the noise), does not exceed about 40 db, the slope of the recovery curve is proportional to the TTS•. That is, a higher TTS• will recover at a more rapid rate than a lower TTS•, although, of course, the time for complete recovery will be somewhat greater for the higher TTS•.

However, if TTS2 is allowed to reach 50 db, this rule no longer holds. Recovery in this case is much slower: where a 35-db TTS• will recover at a rate of about 11 db/(log t), a 50-db TTS• may show a slope of only 4 or 5 db/(log t). If the recovery continues at this rate indefinitely, a little calculation will show that this TTS would endure for a rather long time. If a 50-db TTS• decreases only at a rate of 5 db/(log t), then recovery will be complete only after 2)< 10 lø min, or about 38 000 years.

Happily, since few people would enjoy waiting this

* This investigation was supported by research grant B-1122 from the National Institutes of Neurological Diseases and Blindness, Public Health Service.

• W. D. Ward, A. Glorig, and D. L. Sklar, J. Acoust. Soc. Am. 30, 944-954 (1958).

•' W. D. Ward, A. Glodg , .and D. L. Sk!ar, J. Acou•st. Soc. Am. 31, 522-528 (!959)..

long for their hearing to return to normal, we have evidence that the rate of recovery increases some time after 2 hr. Students who have been given 50 db of TTS2 in our experiments and who have recovered only to about 40 db in 2 hr invariably show a full recovery of sensitivity by the next testing session a week later.

The present study was designed to measure the recovery from high values of TTS over a longer period of time than 2 hr in order to determine the point at which the rate of recovery changes.

PROCEDURE

The 12 college students used in this experiment were experienced listeners. Each had served for at least five months as a member of the listening panel for our previous tests. The response of each ear to noise exposure was therefore well known, and it was possible to estimate in advance how long a given individual would have to be exposed to a noise of 1200-2400 cps in order to produce an initial TTS2 of 50 db at some frequency.

Following a pre-exposure audiogram (Rudmose audiometer, pulsed tones) at 1, 1.3, 1.6, 2, 2.3, 2.6, 3, 3.5, 4, 4.5, 5, and 6 kc, each listener was exposed to a noise field of 1200-2400 cps at 105 db SPL. At approximately 12- or 27-min intervals, the subject was removed from the noise for 3 rain during which the TTS• at 3 and 4 kc was determined. If TTS• were

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10:17:12

498 W. DIXON WARD

found to be 50 db or greater in either ear, he was kept out of the noise and allowed to begin to recover. At the end of 2 hr of exposure he was removed from the noise even if the TTS2 had not reached the criterion level of 50 db.

After being removed from the noise, the men were tested as often as practicable during the first hour, hourly through the next 4 or 5 hr, and every 2 hr for the next 14 to 16 hr. That is, the course of recovery in a quiet environment was followed at regular intervals for an entire day. Exposure to noise occurred in the morning. The listeners slept at the laboratory and were awakened for the tests during the night every 2 hr or 4 hr. After completing two or three tests the next morning, they were permitted to leave, returning for test late that afternoon and again once a day until recovery was found to be complete.

During the first hour of recovery, the number of frequencies tested was gradually increased, so that by t--120 (that is, 2 hr after cessation of exposure) complete audiograms were being obtained (at the same 12 frequencies tested before exposure). The reason that all frequencies could not be tested at short recovery times was that 5 men were exposed on the same day, so during the early stages of recovery only 3 min was commonly available for testing a given man.

RESULTS AND DISCUSSION

Figure 1 shows the course of recovery of threshold at 3 and 4 kc for 5 of the listeners (average of right and left ears). These 5 include the extremes: RS showed the greatest initial TTS, NP the least. The other 7 listeners showed recoveries similar to the

middle curves in Fig. 1. In spite of random irregularities in these recovery

curves, it is clear that, although during the first few hours recovery proceeds as a function of the logarithm

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RECOVERY TIME (MIN)

Fro. 1. The average course of recovery at 3 and 4 kc, right and left ears combined, for 5 observers, following an exposure to 105 db SPL of 1200-2400-cps noise whose duration was sufficient to produce 50 db of TTS2 at either 3 or 4 kc in one of the two ears of the given observer. Note that time is represented logarithmically.

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RECOVERY TIME !MIN)

Fro. 2. Data of Fig. 1 replotted in terms of time rather than the logarithm of time (abscissa). Only data for recovery times greater than 100 min are shown here.

of time, a change in slope occurs at some time between 200 and 500 min. However, Fig. 2 indicates that we do not have here a change from one constant slope to another on log-time coordinates, but rather a transition from a recovery that is linear in log-time to one linear in time. In this figure, the data of Fig. 1 are replotted in terms of linear time, without data of the first 2 hr. The lines are fitted (by eye) to the data between t--500 and t= 3000.

At the locus of maximum TTS, then, recovery is linear with log time for the first few hours, and then approximately linear with time thereafter. The transition from one phase to the other seems to occur gradually and is not correlated in any obvious way with the activities of the listener. For example, in Fig. 2, the vertical bars on each curve delimit the time during which the listeners were sleeping: clearly there are no changes in rate of recovery during this time.

Individual differences in rate of recovery are found for both phases of the recovery process. A comparison of Figs. 1 and 2 might suggest that these differences are much greater for the first phase, since, for example, RS recovers initially at a rate of only 4 db/(log t), while HS recovers at a rate of 12 db/(log t) (see Fig. 1), although both later recover at about 0.011 db/min (Fig. 2). (The average slope for all 12 observers was 0.012 db/min during the second phase.) However, the differences in the first phase may be due largely to differences in initial TTS. Figure 3 shows the relation between the initial average TTS and the initial recovery slope for all 12 listeners: except for one observer (NP), the scattergram indicates a strong relation, and implies that if all listeners had been given the same TTS2, as intended, the initial recovery slopes would all have been much more similar. Indeed, the observers with slow rates of recovery in Fig. 3 had all previously shown fast recovery in other experiments in which TTS• was kept below 40 db.


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RECOVERY FROM HIGH VALUES OF TTS 499

Figure 4 illustrates the recovery of 4 ears in a different manner. Here the complete frequency pattern of TTS is shown for different recovery tlmes (parameter). Some ears, like BP-R and SK-R, initially show a maximum loss at 3.5 kc; others, like RS-R and HH-R, display a notch that is initially centered at a lower frequency. The median frequency of maximum TTS after one hour of recovery was 3.3 kc, a frequency just one octave above the nominal center frequency of the noise.

Figure 4 also demonstrates how the frequency of maximum TTS may shift as recovery progresses. This effect, which occurs because of the tendency of low frequencies to recover before high frequencies, was previously noted in the wartime Psycho-Acoustic Laboratory studies 3 of TTS's similar in magnitude to thos6 produced here.

Finally, Fig. 4 also illustrates a tendency toward better-than-normal sensitivity that is often observed just after recovery is complete. For example, SK-R shows 4 db better hearing than normal at 1.6 and 2 kc, at recovery times of 18 and 22 hours. Similar "sensi- tization" can be seen in other ears as well. Although one would expect some negative values of TTS simply from random variability, they occur at frequencies just below those still showing loss more consistently than would be expected on the basis of chance.

This effect may be related to a similar phenomenon that I have observed casually in persons with permanent tonal gaps. Often a person with a high-frequency loss will show his greatest sensitivity (relative to the hearing of the "normal" observer) in the frequency region just below the tonal gap. For example, a man with 40 db HL at 4 kc may be only normal (0 db HL) at 1, 1.5, and 2 kc but better than normal (-5 or even --10 db HL) at 3 kc. While such a region of increased

• 0 0

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4 6 8 I0 12

INITIAl RECOVERY SLOPE (DEVLOGio't;)

Fro. 3. Scattergram illustrating the relation between the average TTS at 3 and 4 kc, right and left ears, 2 min after cessation of the noise (ordinate) and the initial recovery slope (abscissa).

3Davis, Morgan, Hawkins, Galambos, and Smith, Acta Oto-Laryngol. Suppl. 1•1o. 88 (1950).

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Fro. 4. Recovery of TTS for 4 typical ears. The parameter is the number of hours since exposure. The solid circles indicate the TTS 2 min after exposure, at 3 and 4 kc.

sensitivity is not an inevitable correlate of a high- frequency loss (and may not even be detected by the usual octave audiometry), it occurs often enough that one is tempted to try to explain it in terms of a loss of inhibitory action. That is, we assume that the pattern of firing of sensory elements at threshold in a normal ear follows the well-known broad resonance curves

observed by B6k6sy at high intensities, 4 but that a neural peaking mechanism of the sort suggested by Huggins and Licklider 5 restricts, by inhibition, the firing in higher centers to a much smaller number of neural units. When an area of the basilar membrane

is rendered unresponsive (either temporarily as in TTS, or permanently), not only is there an elevation of threshold at the corresponding frequency, but the pattern of excitation at lower (and higher) frequencies is also abnormal. To return to the example given of the 40-db tonal gap at 4 kc, it is conceivable that the absence of the firing normally produced in the 4-kc fibers by a tone of 3 kc may enhance the activity of the "3-kc" units higher up the neural system, thus lowering the threshold at 3 kc.

An attempt was made to relate the individual differences in the shifted audiograms to differences in resting threshold, but, as before, • with only indifferent success. For example, consider listener RS. He had the greatest sensitivity of all the listeners in the neighbor- hood of 1500 cps' his absolute threshold at 1600 cps in the RE and at 1300 in the LE was --8 db SPL (about 25 db better than present audiometric zero Hearing ,•eve,). also u,c 5IlOWt•Ll rilost extensive •'•'ø

listeners (Fig. 1). This suggests that the sensation level of the exposure may be the most important determinant of TTS. However, listener HJ (not shown) whose threshold was nearly as sensitive (--5 db SPL at 1300 cps in both ears), showed a much smaller TTS than listener RS in his (HJ's) Re, and practically no TTS

4 G. v. Bdkdsy, J. Acoust. Soc. Am. 21, 233-254 (1949). 5 W. H. Huggins and J. C. R. Licklider, J. Acoust. Soc. Am.

23, 290-299 (1951).


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500 W. DIXON WARD

at all in the LE. So although the sensation level of the stimulus is probably related in a gross sense to the amount of TTS produced, this factor cannot account for all the differences between listeners. Indeed, there is no good reason to expect it to be the prepotent factor' only when the variations in resting threshold can be ascribed to conductive differences (i.e., differences in the sound-transmission properties of the middle ear) would one necessarily expect a correspondence.

Hirsh and Bilger 6 have postulated two different recovery processes associated with two phases of recovery from TTS. The first, which they call the R-1 process, is responsible for the first minute or two of recovery, after which the second process, R-2, deter- mines the course of recovery. It now appears that, like Gaul, TTS must be divided into three parts:in addition to R-! and R-2, we need a third process to account for the course of recovery from high values of TTS after a few hours have elapsed.

Whether or not this third process (R-3) represents the recovery of a part of the auditory chain other than

6 I. J. Hirsh and R. C. Bilger, J. Acoust. Soc. Am. 27, 1186-1193 (1955).

those involved in the R-! and R-2 processes is still open to question. However, its characteristics appear to be quite different. The R-1 and R-2 phases are essentially linear in log time, but the R-3 process reduces the TTS at a constant rate in db per unit of time (that is, it is linear in time). And while the R-! and R-2 recovery rates are proportional to the initial TTS, the present studies suggest that, ignoring individual differences, the R-3 rate is nearly constant (about 0.012 db/min) regardless of the magnitude of the TTS existing at the beginning of the R-3 phase (Fig. 2).

Future research may more clearly elucidate the differences among these phases of recovery from TTS. Because of the expense involved in getting subjects to return at regular intervals during recovery, it may prove more economical to use animals for these st•xdies of long-term recovery processes. Indeed, recent animal experiments by Miller and Watson 7 in which TTS was measured by the shock-avoidance technique, resulted in a course of recovery from severe initial values of TTS that is just like that of the present Fig. 1.

7 j. D. Miller and C. S. Watson, J. Acoust. Soc. Am. 31, 1574 (1959).


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recovery from high values of temporary threshold shift

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