Time-domain analysis of auditory-nerve fiber firing rates

Abstract

A time-domain analysis of firing-rate data from over 200 fibers from the auditory nerve of cat has been used to estimate the formants of synthetic speech syllables. Distinct groups of fibers are apparent in the neurograms of the firing-rate responses. The intervals between peaks in the firing-rates of the fibers in each group are very similar, and reflect the period of the formant that dominates the group's response. Analysis of these intervals confirms that they correspond directly to the formant periods. It is concluded that the cochlear filters have much shorter impulse responses than the formants to which they respond. The time-domain analysis tracks the changes of lower frequency formants with more precision than previous analyses of the same neural data [M. I. Miller and M. B. Sachs, J. Acoust. Soc. Am. 74, 502–517 (1983); S. A. Shamma, J. Acoust. Soc. Am. 78, 1622–1632 (1985)]. The direct representation of the formant period in the time domain is contrasted with the diffuse spectral representation of the formant, the dependence of spectral peaks on nonformant parameters, and the distortion of the spectrum by physiological nonlinearities.