Phase-locking in the cochlear nerve of the guinea-pig and its relation to the receptor potential of inner hair-cells.

The temporal fine structure of discharge patterns of single auditory‐ nerve fibers in adult cats was analyzed in response to signals consisting of a variable number of equal‐intensity, in‐phase harmonics of a common low‐frequency fundamental. Two analytic methods were employed. The first method considered Fourier spectra of period histograms based on the period of the fundamental, and the second method considered Fourier spectra of interspike interval histograms (ISIH’s). Both analyses provide information about fiber tuning properties, but Fourier spectra of ISIH’s also allow estimates to be made of the degree of resolution of individual stimulus components. At low intensities (within 20–40 dB of threshold), indices of synchronization to individual components of complex tones were similar to those obtained for pure tones. This was true even when fibers were capable of responding to several signal components simultaneously. Response spectra obtained at low intensities resembled fibers’ tuning curves, and fibers with low spontaneous discharge rates tended to provide better resolution of stimulus components than fibers with high spontaneous rates. Strongly nonlinear behavior existed at higher stimulus intensities. In this, information was transmitted about progressively fewer signal components and about frequencies not present in the acoustic stimulus, and the component eliciting the largest response shifted away from the fiber’s characteristic frequency and toward the edges of the stimulus spectrum. This high‐intensity ‘‘edge enhancement’’ can result from the combined effects of a compressive input–output nonlinearity, suppression, and the fortuitous addition of internally generated combination tones. The data indicate that sufficient information exists for the auditory system to determine the frequencies of narrowly spaced stimulus components from the temporal fine structure of nerve fiber’s responses.

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Coding of spectral fine structure in the auditory nerve. I. Fourier analysis of period and interspike interval histograms

An analytical approach for quantifying the information in auditory-nerve (AN) fiber responses for the task of level discrimination is described. A simple analytical model for AN responses is extended to include temporal response properties, including the nonlinear-phase effects of the cochlear amplifier. Use of simple analytical models for AN discharge patterns allows quantification of the contributions of level-dependent aspects of the patterns to level discrimination. Specifically, the individual and combined contributions of the information contained in discharge rate, synchrony, and relative phase cues are explicitly examined for level discrimination of tonal stimuli. It is shown that the rate information provided by individual AN fibers is more constrained by increases in variance with increases in rate than by saturation. As noted in previous studies, there is sufficient average-rate information within a narrow-CF region to account for robust behavioral performance over a wide dynamic range; however, there is no model based on a simple limitation or use of AN information consistent with parametric variations in performance. This issue is explored in the current study through analysis of performance based on different aspects of AN patterns. For example, we show that performance predicted from use of all rate information degrades significantly as level increases above low–medium levels, inconsistent with Weber’s Law. At low frequencies, synchrony information extends the range over which behavioral performance can be explained by 10–15 dB, but only at low levels. In contrast to rate and synchrony, nonlinear-phase cues are shown to provide robust information at medium and high levels in near-CF fibers for low-frequency stimuli. The level dependence of the discharge rate and phase properties of AN fibers are influenced by the compressive nonlinearity of the inner ear. Evaluating the role of the compressive nonlinearity in level coding is important for understanding neural encoding mechanisms and because of its association with the cochlear amplifier, which is a fragile aspect of the ear believed to be affected in common forms of hearing impairment.

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Quantifying the information in auditory-nerve responses for level discrimination.

Intensity coding in the auditory periphery of the cat: Responses of cochlear nerve and cochlear nucleus neurons to signals in the presence of bandstop masking noise

The greatest obstacle to a straightforward acceptance of the ‘place’ theory of the coding of frequency in the auditory system is the ‘dynamic range problem’ (Evans, 1977ab, 1978ab, 1980a). It is an everyday experience that our ears can operate over an amazingly large dynamic range approaching 100dB, yet the dynamic range of the great majority of cochlear nerve fibres is remarkably limited. The solution to this problem has implications both for our understanding of the fundamental mechanisms of the neural coding of auditory stimuli (Evans, 1978a) and for the design of multi-channel electrical prostheses intended to impart speech information to the profoundly hearing impaired (Evans, 1978b). This paper briefly reviews the problem and discusses various possible solutions in the light of recent physiological data.

E. F. Evans

Papers

‘Phase-Locking’ of Cochlear Fibres and the Problem of Dynamic Range