RATE REPRESENTATION AND DISCRIMINABILITY OF 2ND FORMANT FREQUENCIES FOR EPSILON/-LIKE STEADY-STATE VOWELS IN CAT AUDITORY-NERVE/

Alternate forms of the steady-state vowel /epsilon/ with second forman t peaks located at 1400, 1500, 1700, and 2000 Hz were used to study th e representation and discrimination of second formant frequencies at t he level of the auditory nerve. Recordings from large populations of a uditory nerve fibers in response to these stimuli were used to create rate-place plots, which show second formant peaks that resembled the s timulus spectra. Measures of the peak amplitude decreased as sound lev el was increased and as second formant frequency was lowered. Represen tation of the spectra was degraded at the higher sound level because o f saturation and two-tone suppressive effects. However, formant peaks were clearly represented in plots of rate differences between two vowe ls. Such plots resemble the ratio of the magnitudes of the two vowel s pectra. The results suggest that information concerning the position o f formant peaks is present in the average discharge rate of the audito ry nerve. A measure of discriminability, d', between vowel pairs was a lso calculated. Second formants differing by 125-240 Hz can be discrim inated using the rate responses of individual fibers that are optimall y placed on the basilar membrane; the estimated second formant jnd for the whole auditory nerve is approximately 1 Hz. (C) 1995 Acoustical S ociety of America.