Cochlear nerve fiber activity after short‐term noise exposure

Abstract

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.]