EFFECTS OF ACOUSTIC TRAUMA ON THE REPRESENTATION OF THE VOWEL VERTICAL-BAR-EPSILON-VERTICAL-BAR IN CAT AUDITORY-NERVE FIBERS

A population study of cat auditory-nerve fibers was used to characteri ze the permanent deficits induced by exposure to 110-115 dB SPL, narro w-band noise. Fibers in the region of acoustic trauma (roughly 1-6 kHz ) showed a loss of sensitivity at best frequency (BF) of about 50-60 d B and an increased tuning bandwidth. A correlation between weakened tw o-tone suppression and loss of sensitivity was found for fibers with B Fs above 1 kHz. Single-fiber responses to the vowel /epsilon/ were rec orded at intensities ranging from near threshold to a maximum of about 110 dB SPL. In normal cochleas, the temporal response patterns show a capture phenomenon, in which the first two formant frequencies domina te the responses at high sound levels among fibers with BFs near the f ormant frequencies, After acoustic trauma, fibers in the region of thr eshold shift synchronized to a broad range of the vowel's harmonics an d thus did not show capture by the second formant at ally sound level used. The broadband nature of this response is consistent with the bro adened tuning observed in the damaged fibers, but may also reflect a w eakening of compressive nonlinearities responsible for synchrony captu re in the normal cochlea. (C) 1997 Acoustical Society of America.