Asymptotic Threshold Shift Research Articles

The influence of moderate-intensity noise on the compound action potential evoked by tone bursts in the guinea pig, Cavia porcellus.

Noise-induced changes in the compound action potential (CAP) evoked by tone bursts in the frequency range 0.5-24 kHz were studied in 15 pigmented guinea pigs by means of chronically implanted electrodes positioned near the round window. The animals were exposed for 120 h to continuous pink noise at the intensities 80, 90 and 100 dB SPL. During the exposure period, all the animals exhibited an exponential rise in CAP threshold, leveling out after 24-72 h (asymptotic threshold shift, ATS). The largest threshold shifts were recorded during exposure to 100 dB SPL, for frequencies in the range 8-12 kHz. In the recovery phase, after the end of noise exposure, the threshold to tones at all frequencies tested fell exponentially, reaching the original level in about 72 h in all cases.

The effects of interruption on squirrel monkey temporary threshold shift to a 96-hour noise exposure.

6 squirrel monkeys (Saimiri scuireus ) were exposed to an octave band of noise with a center frequency of 500 Hz under both continuous and interrupted conditions. Continuous exposures lasted for 4, 8, 16, 24, 48, 72 and 96 h, with complete recovery of hearing before the next exposure. The interrupted exposure was 96 h, long with 5-min interruptions so that temporary threshold shift (TTS) could be measured at 2, 4, 8, 12, 16, 24, 36, 48, 60, 72 and 84 h of exposure. There were no differences in TTS from 8 to 96 h of exposure between the two conditions. However, at 4 h of exposure, the interrupted exposure showed significantly less TTS (p less than 0.05) than the continuous exposure. The only difference between the exposures was a 5-min interruption at 2 h of exposure during the interrupted exposure. Thus, the interrupted-exposure method is valid for making generalizations about continuous exposures of 8 h or longer. The results also confirmed our previous finding that there was no asymptotic threshold shift for the squirrel monkey although we extended the exposure time to 96 h in this experiment. The similarity of human and squirrel monkey TTS growth functions is consistent with our earlier results and supports the suitability of the squirrel monkey as an animal model for noise-induced hearing loss in humans.

Temporal resolution in normal‐hearing and noise‐exposed chinchillas

Auditory temporal resolution has been shown to deteriorate with sensorineural hearing loss (Boothroyd, 1973; Fitzgibbons, 1979); however, there is considerable intersubject variability in human studies. The purpose of the present experiment was to investigate the effect of stimulus presentation level on the gap detection thresholds obtained from chinchillas during various amounts of noise‐induced asymptotic threshold shift (ATS). ATS was induced with an octave‐band of noise centered at 500 Hz and having a SPL of 75, 85, 95, and 100 dB. Gap detection thresholds were unaffected by the 75‐dB SPL noise exposure. However, during the 85‐dB SPL noise exposure, gap detection thresholds were longer than normal if compared at the same sound pressure levels, but within normal limits if compared at similiar sensation levels. At ATS for the 95 or 100‐dB SPL noise exposures, gap detection thresholds were significantly wider than pre‐exposure measures regardless of whether the comparison was in terms of sound pressure level or sensation level. The results show that there is an orderly breakdown in temporal resolution as the degree of noise‐induced ATS increased. [Supported in part by grants from NIH (NS 1676), NIOSH (OH‐00364), and the U.S. Army Medical Research and Development Command (DAMD 17‐80‐0133)].

Gap detection in hearing-impaired chinchillas.

Auditory temporal resolution is known to deteriorate with sensorineural hearing loss; however, there is considerable intersubject variability in human studies. The purpose of the present study was to obtain measures of temporal resolution in the chinchilla as the degree of noise-induced hearing loss was systematically varied. Gap-detection thresholds, a measure of temporal resolution, were evaluated at four levels of noise-induced asymptotic threshold shift (ATS). Gap thresholds were normal when the pure-tone thresholds were elevated approximately 15 dB. With a hearing loss of approximately 30 dB, the gap thresholds were longer than normal if compared at the same sound pressure level, but within normal limits if compared at the same sensation level. When the hearing loss exceeded 40 dB, gap thresholds were longer than normal both in terms of sound pressure level and sensation level. These results show that there is an orderly breakdown in temporal resolution as the degree of noise-induced ATS increases. The results are related to neural data and models of temporal resolution.

Is the equal energy rule applicable to impact noise?

The equal energy rule (EER) is a theoretical framework for evaluating the acoustic trauma associated with a wide variety of noise exposures. Most of the data supporting the EER have been large scale demographic studies. Controlled laboratory studies, especially with impulse noise, have brought into question the generality and validity of the EER. The present study assessed the hearing loss resulting from three impact noise exposures having equal energy, but different peak levels (107, 113, 119 dB SPL) and repetition rates (4/s, 1/s, 1/4s). Hearing loss was assessed at 0.5, 2, and 8 kHz in three groups of chinchillas using the auditory evoked response. The animals were exposed for five days and developed an asymptotic threshold shift (ATS) during the exposure. The animals, however, did not develop the same amounts of ATS for each exposure as predicted by the EER; instead the hearing loss increased with peak level at the rate between 0.6 and 1 dB ATS/dB peak SPL. [Work supported by NIOSH.]

Changes in evoked‐response tuning curves with varying levels of hearing loss

Evoked‐response tuning curves from chinchillas were measured before and after exposure to an octave band of noise (CF = 0.5 kHz) that produced an asymptotic threshold shift (ATS) which was nearly flat from 0.5 to 16 kHz. The hearing loss was systematically increased from roughly 15, 30, and 45 dB by increasing the level of the noise from 75, 85, and 95 dB SPL. Tuning curves were measured in awake animals using a simultaneous masking paradigm. As hearing loss increased, there was a progressive broadening of the tuning curve; little or no tuning was present after the highest noise exposure. The systematic loss of tuning will be related to behavioral and single unit tuning curves obtained with the same noise exposure. [Work supported by NINCDS, USAR.]

Influence of vibration on asymptotic threshold shift produced by impulse noise.

Monaural chinchillas were exposed for 10 days to one of these conditions: (1) whole-body vibration (30 Hz, 1 g rms acceleration); (2) impact noise (113 dB peak SPL 1/s) or (3) a combination of whole-body vibration and impact noise. Thresholds were monitored before, during and after the exposure using the auditory-evoked response or behavioral conditioning techniques. Vibration alone had essentially no effect on threshold. Exposure to impact noise produced a stable asymptotic level of threshold shift 2-8 h after exposure onset. The asymptotic threshold shift was roughly 35 and 43 dB SPL at 0.5 and 8.0 kHz, respectively. Exposure to impact noise plus vibration produced an asymptotic threshold shift at 0.5 and 8.0 kHz that was approximately 10 dB greater than noise alone. The combination group also showed greater permanent threshold shifts and greater hair cell losses than the group exposed only to impact noise. The results imply that impact noise and whole-body vibration, at levels commonly found in industrial settings, can interact to increase the susceptibility of the chinchilla cochlea to noise-induced hearing loss.

Lower intensity limits of noise which produces measureable TTS following 24-h exposure

If the assumption that noise which does not produce measureable TTS will not produce NIHL, and if asymptotic threshold shift (ATS) represents the limit of predictable hearing loss which might be expected from a given noise environment, then determination of the lowest intensity capable of producing measureable ATS should provide a reasonable estimate of the lower intensity bound of hazardous noise. To define this lower noise level, ten normal hearing males were exposed to pink noise at each of the LAM-76, 79, 82, and 85 dB for 24 h, a duration previously found to be sufficient for production of ATS. Thresholds were measured at 0.25, 0.5, 1.0, 1.5, 2.0, 3.0, 4.0, 6.0, and 8.0 kHz before, during, and after the exposure. Significant threshold shifts were measureable for the frequencies from 1000 to 4000 Hz when the LA was 85 dB. Although there was a trend toward a systematic change in threshold with increased exposure duration at 82 dB, the variability of the measures precluded attaching any significance to these threshold shifts. [Work supported by Air Force.)

The influence of whole body vibration on asymptotic threshold shift induced by impulse noise

Whole body vibration can alter the susceptibility of the chinchilla cochlea to noise induced hearing loss. Five monaural chinchillas were exposed to a combination of 1 g rms, 30 Hz wholebody vibration and impulse noise at 113 dB SPL, 1/s for 10 days. Thresholds were monitored using the AER technique before, during and after the exposure. Surface preparation histology was obtained from each animal. Whole body vibration did not appreciably affect the ATS levels (30 dB at 0.5 kHz and 40 dB at 8 kHz) but the PTS levels (20 dB at 0.5 kHz; 24 dB at 8 kHz) were significantly higher than in the noise alone control animals. Histologically, the combination animals had a distribution of hair cell loss similar to that of the noise alone animals, but significantly more outer and inner hair cells were missing. The stria vascularis was also severely damaged in some animals. Correlations between hearing levels, and histology will be presented. [Work supported by NIEHS.]

TTS growth and frequency spread to a 500 Hz octave band of noise

Six squirrel monkeys were exposed to an octave band of noise centered at 500 Hz and presented at 95 dB SPL for up to 96 hours. TTS4.5 was measured either during a 5-min interruption in the exposure or at the end of continuous exposures of various lengths. Growth of TTS4.5 measured under interrupted and continuous exposures showed no differences. After 16 hours, both growth functions showed plateaus which were followed by a continuation of the growth of TTS4.5 through 96 hours of exposure. As with our previous reports of TTS in this primate, no asymptotic threshold shift was observed. Spread of frequency effects were measured to 24 hour exposures to the same noise. TTS4.5 showed a broad shift of about 14 dB in the 125 Hz–1.5 kHz range. Second TTS4.5 maxima of 10 dB were measured at 4.5 and 7.5 kHz. While the squirrel monkey growth functions parallel published human data, the frequency spread data appears to show a broader primary effect in the low frequencies for the squirrel monkey. [This study was supported in part by Henry Ford Hospital Institutional Grant from the Ford Foundation, The Deafness Research Foundation, and an NIH Biomedical Research Grant.]

Identification of the minimum level of noise capable of producing an asymptotic temporary threshold shift

Identification of the minimum level of noise capable of producing an asymptotic temporary threshold shift

Growth and recovery of temporary threshold shifts from 24 hour continuous, 48 hour continuous, and 48 hour intermittent noise exposure

The effects on human hearing of 24 and 48 hour continuous and 48 hour intermittent exposures to pink noise at an Leq of 85 dB were measured by pure tone audiometry to address two questions. First, previous studies have demonstrated an asymptotic temporary threshold shift (ATTS), i.e., growth of TTS ceases, after 8–16 hours of exposure to sound. On the assumption that these effects on hearing are the same once asymptote is reached, there should be no difference between the growth of TTS, level of ATTS and recovery from the two exposure durations. Second, if the equal energy concept is appropriate to exposure durations of 48 hours then the effects found for the interrupted conditions should also demonstrate some similarity with the effects from continuous noise exposures. For instance, results of earlier work has shown similar recovery times between 24 hour interrupted and continuous noise exposures. Present results demonstrate that (1) for the 24–48 hour continuous exposures, TTS growth patterns and ATTS levels are similar. However, recovery from the 48 hour exposure was significantly longer than for the 24 hour condition, and (2) for the 48 hour interrupted exposures, growth and recovery patterns are similar to those of continuous exposures, although the magnitude of the TTS was less and the recovery to pre-exposure hearing more rapid than for continuous exposures.

Discharge patterns in the cochlear nucleus of the chinchilla following noise induced asymptotic threshold shift.

Chinchillas were exposed to an 86 dB SPL octave band of noise centered at 4.0 kHz for 3.5--5 days. The noise elevated the hearing thresholds between 4.0 and 16.0 kHz to between 60 and 75 dB SPL. Measurements from single neurons in the cochlear nucleus revealed abnormalities in the response properties of neurons with characteristic frequencies (CF) above 2.0 kHz. Units above 2.0 kHz had elevated thresholds (between 50 and 90 dB SPL) and broad tuning curves due to a greater loss in sensitivity near CF than at lower frequencies. The tuning curve Q10dB values for high frequency neurons were generally less than 3.0 and approached the Q10dB values for basilar membrane displacement. Spontaneous activity rates in units above 2.0 kHz were also low. In a few units, the threshold for single tone inhibition was significantly lower than that for excitation; the best inhibitory frequencies were always below 2.0 kHz. Two-tone inhibition was present in both low and high threshold neurons, but its strength was not assessed. Cochleagrams obtained 12 hours postexposure revealed discrete hair cell lesions in the basal third of the cochlea. The locations of the lesions were consistent with the frequencies of maximum hearing loss. The behavioral thresholds and the thresholds at CF of the most sensitive units were within 10--15 dB of each other. The results indicate that intense sounds reduce the sensitivity, frequency selectivity and spontaneous activity of units in the cochlear nucleus. The findings are similar to those obtained in auditory nerve fibers with ototoxic drugs and hypoxia.

Cochlear nerve fiber activity after short‐term noise exposure

Two chinchillas were studied to determine the critical exposure duration for temporary asymptotic threshold shift (ATS) to occur in single units after exposure to an octave band of noise centered at 0.5 kHz at 95 dB SPL. The same acoustic system was used to expose the ear and record nerve activity immediately after cessation of the noise. Chin 543 was exposed for 2 h, and the pattern of TTS was similar to that found after a 12‐h free‐field exposure [J. Markuszka, ASA paper (1978)]. TTS of 70 dB or greater occurred for units below 4.5 kHz. Above this point, the amount of TTS steadily decreased to normal levels at 7 kHz. Chin 545 was treated differently. First, 45 normal units were studied to provide a baseline for further comparisons. Then the animal was exposed for 1 h at 85 dB SPL. Units below 1 kHz had TTS's of 25–30 dB. TTS declined to zero at 3 kHz and above. A second exposure at 95 dB SPL for 0.5 h produced startling results. Units near 3.5 kHz were shifted 60–75 dB; the TTS decreasing at lower frequencies. An additional 7‐dB increase in TTS was found at 0.5 kHz. A comparison of the two exposures revealed that the first exposure had a decreasing effect on units above 1.3 kHz, and the second exposure had an increasing effect. [Supported by NS 03856.]

Asymptotic temporary threshold shift

Five men having normal pre‐exposure audiograms were monaurally exposed in a diffuse sound field for 24 hours to octave‐band noise centered at 500 or 2000 Hz at 90 or 80 dB SPL, respectively. Each subject's exposures were two weeks apart. The growth of threshold shift was measured 10 times, and recovery was followed for a course of three days post exposure by a fixed‐frequency‐Bekesy‐tracking procedure. Functional asymptotic threshold shift (ATS) was achieved for either noise exposure within eight hours at all 11 audiometric test frequencies. The maximum ATS for either exposure was the upper cut‐off frequency of the noise with significant spread to the first octave above the noise's center frequency. For each exposure, growth was linear in logarithmic space. For multiple exposures on one subject at each band, the relationship between asymptotic level and noise intensity appeared to be linear with a slope of 1.6. Group data demonstrated that recovery from a 16.7 dB asymptotic threshold shift at 707 Hz required 98 h and was biphasic. Recovery from an 11.3 dB ATS at 2828 Hz required 20 h. Recovery from either noise was linear in logarithmic space.

Auditory nerve fiber activity, behavioral thresholds and cochlear histology following noise induced asymptotic threshold shift

Chinchillas were exposed three to five days to a 95-dB SPL octave band of noise centered at 0.5 kHz. During the exposure, the behaviorally measured thresholds reached an asymptotic level that was between 40–60 dB SPL from 0.5–16.0 kHz. During a 27–15 h period following the exposure, single auditory nerve fibers and the whole nerve action potential (AP) were studied with clicks and tone bursts. Single neuron thresholds were generally 40–60 dB above normal values and were in general agreement with the behavioral audiogram. Fiber tuning curves were broader than normal with reduced slopes. Some units had a region of increased sensitivity below CF. Click thresholds for the units were generally 40–60 dB above normal levels. The visual detection level for AP was also 40–60 dB above its normal value and consistent with the single neuron data. The cochleagrams obtained 15 h after the exposure generally showed a peak outer hair cell loss (<50%) near the apex with smaller losses (<20%) at more basal locations. [Research supported by NSE.]

Asymptotic Noise-Induced Temporary Threshold Shift in Chinchilla Measured by the Auditory Evoked Response

Monaural chinchillas were exposed to octave band (4 kHz center frequency) noise at 80 dB SPL for 96 h. Auditory threshold at frequencies of 1, 2, 4, and 8 kHz was estimated by the auditory evoked response (AER) technique prior to noise exposure, and on a fixed schedule, both during exposure and following removal from the noise. Results were analyzed in terms of both threshold and threshold shift. Results indicate that the AER is adequate measure of asymptotic noise-induced temporary threshold shift. Advantages of the use of AER as opposed to behavioral means of threshold determination are discussed. Threshold during this long-term noise exposure was found to be determined by the spectral and intensity components of the noise independent of pre-exposure threshold. Implications for future research in long-term noise exposure are discussed.

Asymptotic Noise-Induced Temporary Threshold Shift in Chinchilla Measured by the Auditory Evoked Response

Asymptotic Noise-Induced Temporary Threshold Shift in Chinchilla Measured by the Auditory Evoked Response

Long-term, asymptotic threshold shift in chinchillas exposed to repetitive, reverberant, impulse noise

Five monaural chinchillas were exposed to a repetitive, reverberant, impulse noise for ten days. The impulse-noise source was a mechanized hammer pounding on a steel plate at a rate of 1 impulse/sec. The average, peak over-pressure within the holding cage was 113 dB SPL. Auditory thresholds were determined before and after exposure at 0.5, 1, 1.4, 2. 2.8, 4, 8, and 16 kHz by means of a behavioral technique utilizing shock-avoidance conditioning. During exposure, thresholds at 0.5 and 8 kHz were monitored on a daily basis. Within eight hours from the start of the exposure, threshold shifts at both frequencies had reached asymptotic levels of from 30 to 40 dB. Upon removal from the noise, the threshold shift at 8 kHz recovered more rapidly than at 0.5 kHz; however, at 40 days, both had returned to within 10 dB of the pre-exposure thresholds. A maximum PTS of 20–30 dB was found at either 2 or 2.8 kHz. Cochleagrams were obtained for each animal 60 days following the exposure. The histological examination provided support for the audiological findings.

Neural firing patterns in the chinchilla cochlear nucleus after noise-induced asymptotic threshold shift

Chinchillas were exposed to an octave band of noise (CF = 4.0 kHz, 86 dB SPL) for three to five days. At the cessation of the noise, unit activity was sampled in the dorsal cochlear nucleus and ventral cochlear nucleus over a period of 1–12 h post-exposure. In previous experiments [J. Mills, J. Speech Hear. Res. 16, 426–438 (1973)], the same exposure produced 40 dB of asymptotic threshold shift at 2–8 kHz measured behaviorally. Single units having CF's below 1.0 kHz had normal tuning curves while those above 1.0 kHz had thresholds that were elevated, on the average, by 40 dB and, in some instances, by as much as 70 dB. Tuning curves from the high-threshold units were abnormally broad due to the shallow slope on the low-frequency side of the tuning curves. In some units only inhibitory areas were present, while in others, the threshold for the low-frequency inhibitory areas was considerably lower than for the excitatory area. Spontaneous activity from the high-threshold units appeared to be normal or lower than normal. The single unit results will be related to the postexposure behavioral audiogram and cochleagram.