STUDIES OF TONAL INTERACTIONS IN THE AUDITORY SYSTEM

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This is a competitive renewal of electrophysiological studies of stimulus coding in the auditory nerve and ventral cochlear nucleus of intact adult cats. There are two separate but related lines of research. The first line is a continuation of studies of neural coding of acoustic stimuli. The goals of these are to determine how nonlinear features of auditory response combine to define neural firing patterns and to examine ways that frequency and intensity information is encoded in the auditory nerve and transformed in the cochlear nucleus. Specific projects entail quantitative analyses of rate and synchrony suppression, temporal coding of frequency spectra of harmonic complexes in responses of first- and second-order auditory neurons, investigations of stimulus transforms involved in neural processing of complex tones between the auditory nerve and cochlear nucleus, and studies of excessive adaptation in auditory nerve fiber responses at high stimulus frequencies. Among the hypotheses to be tested are that stimulus waveform fine structure determines the instaneous magnitudes of both excitation and suppression, that responses of phase-sensitive cochlear nucleus neurons are quantitatively but not qualitatively different from responses of auditory nerve fibers, and that the high-frequency "hook" region of the cat cochlea is metabolically fragile. These studies contribute to the elucidation of cochlear mechanisms underlying the neural coding of complex tones and speech, to the understanding of human perception of complex tones, and to the validation of theories of frequency analysis and pitch perception. The second line of research consists of quantitative studies of neural coding of electrical stimuli delivered through a banded multi-channel cochlear implant. The goal of these is to describe thoroughly the basic properties and limitations of auditory nerve fiber responses to electrical stimuli and to infer the neurophysiological basis of behavioral performance in implanted humans. Specific projects include detailed quantitative analyses of auditory nerve fiber responses as electrical stimulus parameters are varied systematically, electrophysiological measurements of intracochlear current spread and electrode channel isolation, nerve fiber population responses to speech stimuli encoded via contemporary speech-processing schemes, and examination of alternative ways to encode electrical stimulus intensity so as to expand dynamic range. The studies are relevant to understanding the types and limits of information that cochlear implants are capable of encoding and they provide data that can guide future speech processor designs.