Representation of stop consonants in the discharge patterns of auditory-nerve fibers

Abstract

The representation of the speech syllables /da/ and /ba/ in populations of auditory-nerve fibers was studied. Post-stimulus-time histograms were computed from 20-ms segments of fiber spike trains occurring in response to the stimulus. Discrete Fourier transforms with a resolution of 50 Hz were computed from each histogram. As a measure of the response of the population of fibers to each harmonic of the 50-Hz resolution frequency of the transform, the magnitude of the response to that frequency was averaged across all fibers whose characteristic frequencies were within one-fourth octave of that harmonic. We have previously called this measure the average localized synchronized rate (ALSR). Response profiles for the 20-ms segments of the stimulus were generated by plotting the ALSR versus frequency. Time-varying spectral features of the /da/ and /ba/ stimuli are well preserved by such profiles. For example, the onset spectrum and formant transitions of the consonant-vowel syllable are well represented. Furthermore, the fine structure in the speech spectrum related to the pitch of the excitation source is maintained in these ALSR plots. Average discharge rate profiles were generated in a manner similar to that for the ALSR; in this case average rate replaces Fourier transform components as response measure. Such average rate profiles can represent the transitions of at least formants two and three. However, such average rate profiles do not represent the steady-state formants or the voice pitch.

Discrimination of Voiced Stop Consonants Based on Auditory Nerve Discharges

“…In neurons with BFs near F2, the representation provides little information about F3, whereas in neurons with BFs near F3, both F2 and F3 are represented. This difference is consistent with the inferences from direct studies of the neural representations of both vowels (Young and Sachs, 1979;Miller et al, 1997) and stops (Miller and Sachs, 1983;Sinex and Geisler, 1983), in which a corresponding result has been obtained by studying the phase-locking of AN fibers to the F2 and F3 components of the stimulus.…”

“…AN fibers provide a tonotopic representation of the spectrum of the transition period, in which the movement of the formants is represented by a moving maximum of response among fibers, the BFs of which correspond to the formant frequency (Miller and Sachs, 1983;Sinex and Geisler, 1983). Given those results, it was expected that differences in the neural responses to two stops would be observed as in Figures 3-5.…”

“…Presumably, this difference reflects the generally weaker neural representation of F3 relative to F2 (Miller and Sachs, 1983;Sinex and Geisler, 1983). This point will be more evident below when JNDs for formants are calculated.…”

“…The peripheral neural representation of speech has been studied both for vowels (Sachs and Young, 1980;Delgutte and Kiang, 1984a;Conley and Keilson, 1995;Miller et al, 1997) and stop consonants (Miller and Sachs, 1983;Sinex and Geisler, 1983;Delgutte and Kiang, 1984b;Carney and Geisler, 1986). These analyses focused on the formant transitions and showed that the tonotopic representation of a stimulus reflects its spectra, formant energy peaks being signaled by peaks of response.…”

Neural representation of spectral and temporal information in speech

“…The study of more realistic stimuli began almost as early as work on vowels, with studies that focused on stop-consonantvowel stimuli (Miller & Sachs 1983;Sinex & Geisler 1983;Carney & Geisler 1986). Stops at the beginning of syllables are characterized by a release burst of broadband noisy sound followed after some delay by the onset of voicing; the voicing produces a vowel-like formant structure in which the formants move rapidly (over a few tens of ms) from frequencies determined by the consonant to frequencies appropriate for the following vowel.…”