Speech processing in the auditory system I: The representation of speech sounds in the responses of the auditory nerve

In a previous paper the speech evoked spatio-temporal response patterns recorded in large populations of auditory-nerve fibers in the cat were examined M.I. Miller and M.B. Sachs, J. Acoust. Soc. Am. 74, 502-517 (1983). The distribution of the relative phases of synchronized activity emerges as an important response feature reflecting the stimulus spectral parameters. Specifically, each strong low-order harmonic of the stimulus (less than or equal to 1.5-2 kHz) dominates the synchrony of a relatively broad segment of fibers near its corresponding characteristic frequency (CF) location in a pattern which mirrors the underlying traveling wave component. Each such fiber segment can be roughly subdivided into two regions: (1) a region basal to the point of resonance of the harmonic where the fiber PST histograms accumulate only small delays (or phase shifts) relative to each other reflecting the fast speed of propagation of the traveling wave, and (2) a region at or very near the point of resonance where the responses exhibit drastic relative phase shifts owing to the sudden slow down of the traveling wave and the consequent rapid accumulation of phase shifts. These rapid phase shifts thus manifest themselves as steep and localized spatial discontinuities in an otherwise relatively uniform instantaneous pattern of activity across the fiber array, all occurring at the CF locations corresponding to the low-order harmonics of the stimulus.