Coding of spectral fine structure in the auditory nerve. II: Level-dependent nonlinear responses

Abstract

Phase-locked discharge patterns of single cat auditory-nerve fibers were analyzed in response to complex tones centered at fiber characteristic frequency (CF). Signals were octave-bandwidth harmonic complexes defined by a center frequency F and an intercomponent spacing factor N, such that F/N was the fundamental frequency. Parameters that were manipulated included the phase spectrum, the number of components, and the intensity of the center component. Analyses employed Fourier transforms of period histograms to assess the degree to which responses were synchronized to the frequencies present in the acoustic stimulus. Several nonlinearities were observed in the response as intensity was varied between threshold and 80-90 dB SPL. Response nonlinearities were strong for all signals except those with random phase spectra. The most commonly observed nonlinearity was an emphasis of one or more stimulus components in the response. The degree of nonlinearity usually increased with intensity and signal complexity and decreased with fiber frequency selectivity. Half-wave rectification introduced synchronization to the missing fundamental. The strength of the response at the fundamental was related to stimulus crest factor. Signals with low center frequencies and high crest factors often elicited instantaneous discharge rates at the theoretical maximum of pi CF. This suggests that the probability of spike generation approaches one during high-amplitude waveform segments. Response nonlinearity was interpreted as arising from three sources, namely, cochlear mechanics, compression of instantaneous discharge rate, and saturation of average discharge rate. At near-threshold intensities, fibers with high spontaneous rates exhibited responses that were linear functions of stimulus waveshape, whereas fibers with low spontaneous spike rates produced responses that were best described in terms of an expansive nonlinearity.

Effects of contrast between onsets of speech and other complex spectra

“…Duifhuis (1970) and Wightman (1973) observed that, when listeners are presented with harmonic complexes from which specific harmonics have been removed, missing harmonics could potentially be derived from the temporal waveform. Horst, Javel, and Farley (1990) found that, when a center component of a harmonic complex was left out, this missing component was restored in the pattern of discharges in AN at higher stimulus levels. In recent studies of basilarmembrane mechanics, Rhode and Recio (2001) presented equal-interval seven-tone complexes.…”

Encoding of phase spectra by the peripheral auditory system of the bullfrog

“…Multi-harmonic stimuli are unique in that their waveforms have a distinct stable periodicity which corresponds exactly to the fundamental frequency of their harmonic series. A number of studies in the mammalian peripheral auditory system have shown that VIIIth nerve fibers can synchronize with high precision to the fundamental period of such multi-harmonic stimuli (Delgutte and Kiang 1984;Miller and Sachs 1984;Horst et al 1986;Deng et al 1987;Horst et al 1990;Palmer 1990).…”

“…And so, the obvious question arises as to whether an auditory fiber's synchronization to the fundamental period is sensitive to those changes, and thus, constitutes a mechanism of encoding phase information in a complex, periodic signal. The effects of phase shifts within multi-harmonic stimuli on the synchronization characteristics of auditory nerve fibers have been examined in several previous studies in cats (Horst et al 1986;Deng et al 1987;Horst et al 1990) as well as more recently in frogs (Simmons et al 1993). In cats, it was found that auditory nerve fibers exhibited a strong degree of synchronization to the fundamental period for stimuli which had "inphase" spectra, (i.e., stimuli in which the relative phase angles of all of the components were the same).…”

“…However, when the phase spectrum was "randomized" so that there was no systematic phase relationship within the harmonic structure, then synchronization to the fundamental period was significantly reduced. In addition, Horst et al (1990) used several other phase spectra including an "alternating" phase spectrum in which the relative phase angles of the harmonic components alternated between 0 ~ sine and cosine. These additional phase manipulations also could produce changes in the synchronization characteristics of cat auditory nerve fibers.…”

Multi-tone suppression of distortion-product otoacoustic emissions in humans

“…Horst et al (1986Horst et al ( , 1990 used multicomponent stimuli to analyze the representation of complex stimuli in responses of the auditory nerve in cat. In their study, the response to the center component became increasingly suppressed as the number of stimulus components increased (from 4 to 64).…”