William Melnick

Human temporary threshold shift (TTS) and damage risk

Information regarding the relation of human temporary threshold shift (TTS) to properties of steady-state and intermittent noise published since the 1966 appearance of the CHABA damage risk contours is reviewed. The review focuses on results from four investigative areas relevant to potential revision of the CHABA contours including effects of long-duration exposure and asymptotic threshold shifts (ATS); equivalent quiet and/or safe noise levels; effects of intermittency; and use of noise-induced temporary threshold shift (NITTS) to predict susceptibility to noise-induced permanent threshold shift (NIPTS). These data indicate that two of three major postulates on which the original contours were based are not valid. First, recovery from TTS is not independent of the conditions that produced the TTS as was assumed. Second, the assumption that all exposures that produce equal TTS2 are equally hazardous is not substantiated. The third postulate was that NIPTS produced by 10 years of daily exposure is approximately equal to the TTS2 produced by the same noise after an 8-h exposure. Based upon several TTS experiments showing that TTS reaches an asymptote after about 8 h of exposure, the third CHABA postulate can be reworded to state the hypothesis that ATS produced by sound of fixed level and spectrum represents an upper bound on PTS produced by that sound regardless of the exposure duration or the number of times exposed. This hypothesis has a strong, logical foundation if ATS represents a true asymptote for TTS, not a temporary plateau, and if threshold shifts do not increase after the noise exposure ceases.

Effects of prolonged noise exposure in chinchillas with severed middle ear muscles

Using the round window-recorded cochlear microphonic as the index of acoustic reflex activity, we noted a decay in the strength of middle ear muscle contraction in the chinchilla following an eight hour exposure to octave band noise (500 Hz. center frequency, 95 dB. sound pressure level). On the basis of this observation it was concluded that the prolonged exposure reduced the effectiveness of the acoustic reflex in protecting the cochlea. This reduction, however, may have been underestimated if the exposure was also sufficient to reduce cochlear output. The present investigation examined this possibility by comparing the effects of a similar exposure in chinchillas with intact (normal) and severed middle ear muscles. Following exposure, the cochlear microphonic magnitude increased slightly in the normal group. Decreases in the cochlear microphonic were observed in the animals with severed muscles even when the overall exposure level was reduced to simulate the effects of middle ear muscle contraction. These findings indicate that although the muscles did afford some degree of protection to the cochlea during the exposure, the protective effects of the acoustic reflex may have been reduced even beyond our original observations.

Acoustic Reflex Decay in Chinchillas during a Long-term Exposure to Noise

Reflex decay to 0.5 kHz octave-band noise at 95 dB SPL was measured in chinchillas during four 2-hr exposure periods separated by 11 -min quiet intervals. Round window electrodes were implanted in six animals. Measures of acoustic reflex decay were inferred from amplitude changes in the cochlear microphonic generated by the octave-band noise. Reflex decay followed essentially the same time course during each exposure period. Vigorous muscular contraction to the signal onset was followed by gradual decay that a symptoted between 30 and 50% of its initial value. The process of decay seemed to be complete sometime between 8 and 30 min. The results suggest that the middle ear muscles in chinchillas provide some protective function during exposures of fairly long duration.

Morphological Correlates of Asymptotic Temporary Threshold Shift in Chinchillas

Using the scanning and transmission electron microscopes, an attempt was made to correlate morphological and behavioral data from conditioned chinchillas following their exposure to a presumably subtraumatic level of noise that produced asymptotic TTS. Varying degrees of morphological change in the sensory cells, which could be attributed to the noise exposures, were observed in these animals. Changes included giant cilia formation, sensory cell debris, cellular swelling, and scar formation. Changes also appeared in the stria vascularis. These morphological changes were most pronounced in the apical turn of the cochlea. Our preliminary data appear to support the idea that sensory cell degeneration occurs even after exposure to a noise level considered subtraumatic and which apparently does not produce permanent changes in behavioral threshold hearing levels.

Shifts in Masking with Time

The time course of masking of a 500‐Hz signal was determined for remote masking (noise from 2000 to 4000 Hz) and for direct masking (noise from 200 to 4000 Hz) under conditions of simultaneous and delayed onset of the tracking task. For half the conditions, listeners varied level of the tone against a constant background noise, and in the other half, they varied the noise level to mask a constant level of pure tone. The results showed a downward shift occurred during the first two minutes of tracking for both direct masking and remote masking. The shift in direct masking was influenced by the onset of tracking and the method of control, while the shift in remote masking was independent of these factors.

TTS in man from a 24‐hour exposure to an octave band of noise centered at 4 kHz

Ten adult males were exposed to a sound field consisting of an octave band of noise centered at 4 kHz for 24 hours at an octave band level of 80 dB. Seven of these men were also exposed to the same noise at 85 dB for 24 hours. Hearing thresholds were measured in one ear at 11 test frequencies from 250 to 10 000 Hz prior to, during, and after exposure. As in previous experiments with a 300–600‐Hz noise band, TTS appeared to reach asymptote after 8 to 12 hours of exposure. Maximum asymptotic threshold shifts (ATS) occurred at 4 and 6 kHz. The magnitude of ATS for this sample of subjects was less than would be expected from previous studies. For the seven men who were exposed to both noise levels, the asymptote at 4 kHz was 9.7 dB from the 80‐dB noise and 18.4 dB for the 85‐dB exposure. ATS increased 1.74 dB per decibel increase in exposure, a growth rate similar to that reported earlier. Thresholds recovered to within 5 dB of pre‐exposure levels by 24 hours and completely by 48 hours.

Electron Microscopy (Scanning and Transmission) of Acoustic Damage in the Cochlea

Guinea pigs were exposed to 300–600 Hz and 1000–2000 Hz octave bands of noise in a sound field measured at 117 dB SPL. Exposure times were 2, 4, 6, 12, and 24 h. Animals were sacrificed immediately after exposure. At least three observations were made which, to our knowledge, have not been previously reported: (1) bleb formation on the sensory hairs; (2) deformation of cuticular lamina of the hair cells; and (3) changes in Golgis apparatus. We have also observed changes in the Organ of Corti which have been described by others. These include swelling of hair cells, increased lysosome‐like bodies, nuclear chromatin aggregation, and vesicular dilatation of the outer hair‐cell plasma membrane. Hair cells were observed in differing states of damage varying from simple blebs on the hairs to complete destruction. When there apparently was total destruction of sensory cells, the space previously occupied by the cell seemed to be filled by a supporting Deiter cell. These findings will be illustrated by numerous electron micrographs. [Supported in part under an Air Force contract.]Guinea pigs were exposed to 300–600 Hz and 1000–2000 Hz octave bands of noise in a sound field measured at 117 dB SPL. Exposure times were 2, 4, 6, 12, and 24 h. Animals were sacrificed immediately after exposure. At least three observations were made which, to our knowledge, have not been previously reported: (1) bleb formation on the sensory hairs; (2) deformation of cuticular lamina of the hair cells; and (3) changes in Golgis apparatus. We have also observed changes in the Organ of Corti which have been described by others. These include swelling of hair cells, increased lysosome‐like bodies, nuclear chromatin aggregation, and vesicular dilatation of the outer hair‐cell plasma membrane. Hair cells were observed in differing states of damage varying from simple blebs on the hairs to complete destruction. When there apparently was total destruction of sensory cells, the space previously occupied by the cell seemed to be filled by a supporting Deiter cell. These findings will be illustrated by numerous ...

TTS with Auditory Task during Exposure

Less TTS was measured at 1400 Hz following a 2 min exposure to 1000 Hz at 110 dB SPL when, simultaneously with the exposure, the listeners were required to trace threshold for 250 Hz in the nontest ear than when the listeners were passively exposed to the intense signal during this interval.

The effects of noise on the measurement of the 2f1−f2 otoacoustic emission

The effects of noise exposure on the measurement of the cubic distortion product emission (2f1 − f2) were measured. TTS was induced using white noise presented at four intensities, each for three different time periods, giving a total of twelve conditions. The subjects were normal‐hearing adults. The trial conditions were randomized, with an appropriate rest period between each condition. The amount of temporary threshold shift (TTS) in dB was obtained by comparing pre‐ and post‐noise pure‐tone thresholds for the 500‐Hz to 8‐kHz range by octave. The level of 2f1 − f2 was measured prior to and immediately following the threshold testing. Changes in the distortion product emission were determined by comparing pre‐ and post‐noise emission measures. Preliminary results suggest that noise exposure may reduce the level of the distortion product emission. The degree to which this effect was seen appeared to be related to both intensity and duration of the noise exposure.

A comparison of the effects of time and intensity during the auditory brain‐stem response to lateralized clicks

A group of 12 otologically normal subjects were presented with binaural clicks at 70 dB nHL. During one condition, subjects delayed the onset of the click to the left ear until a single image was percieved either midway between the center of the head and the right ear (referred to as the “midway” location), or until the image was perceived just at the right ear. In a second condition, subjects attenuated the click to the left ear until the acoustic image was lateralized either midway towards the right ear, or just at the right ear. These same stimuli conditions were then used to evoke auditory brain‐stem responses (ABRs). When clicks were lateralized midway, ABR latencies from delayed versus attenuated clicks were not significantly different. However, when clicks were lateralized just at the right ear, there were statistically significant differences between latencies from delayed versus attenuated clicks. There was also a consistent ordinal relationship noted: In nearly every case, delayed clicks yielded...