Asymptotic Threshold Shift Research Articles

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.

Effect of asymptotic threshold shift in the neural firing patterns of the rat cochlear nucleus

Rats were exposed to broad-band noise (spectral level = 50 dB ± 5 dB between 1000 and 10 000 Hz) for three to five days. Single neurons (N = 60) were sampled in the dorsal and ventral cochlear nucleus. Tuning curves were elevated 30–40 dB re normal [A.R. Møller, Acta Physiol. Scand. 76, 503–512 (1969)]. The slopes of the low-frequency sides of the tuning curves were considerably shallower than normal, thereby making the bandwidth of the tuning curve wider. The inhibitory areas of the tuning curves were either not measurable, or were much more elevated than the excitatory area. Light microscopic analysis of the cochleas revealed no changes in the sensory epithelium.

Asymptotic Threshold Shift (ATS) in Man from 24 Hour Exposure to Continuous Noise

Ten men were exposed to a 300–600 Hz band of noise at an octave-band-level of 90 dB in a sound field for a period of 24 hours. Hearing thresholds were measured in one ear at 11 test frequencies ranging from 125 to 8000 Hz prior to exposure and at selected time intervals during and after exposure. Temporary threshold shift (TTS) appeared to reach asymptotic levels by 12 hours of exposure. Maximum TTS was approximately 11 dB and occurred at 500, 750 and 1000 Hz. TTS was appreciable at 1500 Hz amounting to 7 dB and was less than 5 dB at other frequencies. The growth pattern of TTS was triphasic; slow development during the first two hours of exposure, a rapid increase from 2 to 8 hours, and then apparently reaching an asymptote by the twelfth hour. Recovery was prolonged for the relatively small magnitude of TTS, requiring 24 hours before most of the subjects returned to preexposure threshold levels. Asymptotic TTS (ATS) showed dependence on preexposure threshold hearing levels.

Adaptation of quantitative biochemical approaches to the normal and experimental inner ear

The anatomical complexity of the cochlea poses numerous obstacles to the quantitative biochemical study of its tissues and fluids. For instance, the preparatory stages of the quantitative histochemical method of Lowry cannot be directly applied to the inner ear, primarily because the tissues are encapsulated by bone, which makes frozen sectioning virtually impossible. However, temporal bones can be freeze-dried in toto with preservation of three-dimensional tissue structure. In the organ of Corti three-dimensional preservation offers the advantage that reasonably homogeneous and comparatively large populations of cell types can be dissected out and analyzed by microanalytical techniques. Analysis of individual cell types is of potential importance for the elucidation of the biochemical counterpart of moderate noise trauma (e.g., asymptotic threshold shift) in which gross cell damage is absent but electron-microscopic changes are detectable in the outer hair cells. A topic of great current interest is the profound effect of salidiuretic drugs upon the endolymphatic potential and energy metabolism of the stria vascularis. Pertinent electrophysiological experiments combined with specially adapted biochemical approaches are presented. Difficulties arising from interactions with cochlear fluids and other potential sources of artifact are discussed.

Asymptotic temporary threshold shift in chinchilla measured by the auditory evoked response

Monaural chinchilla were exposed to octave-band noise (4 kHz center frequency) at 80 dB SPL for 96 hours. Quiet thresholds at 1, 2, 4, and 8 kHz were estimated before, during, and after the noise exposure using the auditory evoked response technique. The greatest threshold shift that was measured occurred at 24 hours after noise onset; then hearing level recovered by approximately 5 dB to an asymptotic level. After cessation of the noise, hearing returned to normal by 72 hours. The results agree with previous behavioral studies of asymptotic TTS [Mills, “Threshold Shifts Produced by Exposure to Noise in Chinchillas with Noise-Induced Hearing Losses,” (in press)] in showing comparable levels of TTS, and also that the hearing level during the noise exposure is determined by the level of the noise rather than the animal's preexposure threshold. [Study supported by Grant No. 1RO1 OH 00364-01, National Institute for Occupational Safety and Health, Public Health Service, Department of Health, Education and Welfare.]

Morphological changes in the ears of noise-exposed parakeets

Parakeets exposed to 13-octave bands of noise for 12 or more hours sustain asymptotic threshold shifts (Dooling and Saunders). The ears of three parakeets from the Dooling and Saunders study were prepared for phase-contrast microscopic examination as embedded whole mounts after final audiograms had been obtained. Findings in these ears were then compared to ears of other parakeets exposed for 12 hours to a 13-octave band of noise centered at 2 kHz with a sound pressure level of 76, 86, or 106 dB. The ears of half the parakeets were prepared for microscopic examination 30 minutes following exposure, while the rest were prepared after the birds had recovered from their ATS. Preliminary findings in parakeet ears with ATS include: no sensory cell loss, no tectorial membrane displacement, no reticular lamina breakage, and no abnormalities in the stereocilia. In addition, the cytoplasm of some tall hair cells bulged from the surface of the basilar papilla, thus separating nerve endings from the hair cell bases. This latter feature was not present in parakeet ears which had recovered from ATS.

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.