Showing papers by "Murray B. Sachs published in 1983"

Auditory nerve representation of vowels in background noise.

Abstract: Responses of auditory nerve fibers to steady-state vowels presented alone and in the presence of background noise were obtained from anesthetized cats. Representation of vowels based on average discharge rate and representation based primarily on phase-locked properties of responses are considered. Profiles of average discharge rate versus characteristic frequency (CF) ("rate-place" representation) can show peaks of discharge rate in the vicinity of formant frequencies when vowels are presented alone. These profiles change drastically in the presence of background noise, however. At moderate vowel and noise levels and signal/noise ratios of +9 dB, there are not peaks of rate near the second and third formant frequencies. In fact, because of two-tone suppression, rate to vowels plus noise is less than rate to noise alone for fibers with CFs above the first formant. Rate profiles measured over 5-ms intervals near stimulus onset show clear formant-related peaks at higher sound levels than do profiles measured over intervals later in the stimulus (i.e., in the steady state). However, in background noise, rate profiles at onset are similar to those in the steady state. Specifically, for fibers with CFs above the first formant, response rates to the noise are suppressed by the addition of the vowel at both vowel onset and steady state. When rate profiles are plotted for low spontaneous rate fibers, formant-related peaks appear at stimulus levels higher than those at which peaks disappear for high spontaneous fibers. In the presence of background noise, however, the low spontaneous fibers do not preserve formant peaks better than do the high spontaneous fibers. In fact, the suppression of noise-evoked rate mentioned above is greater for the low spontaneous fibers than for high. Representations that reflect phase-locked properties as well as discharge rate ("temporal-place" representations) are much less affected by background noise. We have used synchronized discharge rate averaged over fibers with CFs near (+/- 0.25 octave) a stimulus component as a measure of the population temporal response to that component. Plots of this average localized synchronized rate (ALSR) versus frequency show clear first and second formant peaks at all vowel and noise levels used. Except at the highest level (vowel at 85 dB sound pressure level (SPL), signal/noise = +9 dB), there is also a clear third formant peak. At signal-to-noise ratios where there are no second formant peaks in rate profiles, human observers are able to discriminate second formant shifts of less than 112 Hz. ALSR plots show clear second formant peaks at these signal/noise ratios.

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.

Speech Encoding in the Auditory Nerve: Implications for Cochlear Implants

Abstract: Discussions of stimulus encoding for cochlear prostheses have focused on replacement of peripheral auditory processing by at least a crude simulation of the function of the normal system.’** The ultimate goal of the prosthesis is to provide the central nervous system with information adequate for the perception of speech. An understanding of the normal encoding of speech in the auditory nerve is therefore fundamental to the development of a simulation-type prosthesis or even to the evaluation of the potential for success of such devices. In this paper we shall review some recent studies of speech encoding in the auditory nerve. In particular, we shall consider representations of speech spectra in terms of average discharge rates of fiber responses and in terms of temporal (phase-locked) properties of the discharge patterns. As we will emphasize, we cannot at this point rule out either representation as the one actually used by the normal auditory system. We will, however, discuss the relationship of both average rate and temporal encoding to stimulation strategies for cochlear implants.