Responses of ventral cochlear nucleus units in the chinchilla to amplitude modulation by low-frequency, two-tone complexes

Abstract

For a tone that is amplitude modulated by two tones (fmod1 and fmod2), neither the stimulus waveform nor the half-wave rectified waveform has spectral energy at the envelope beat frequency (fmod2-fmod1). The response of ventral cochlear nucleus units in the chinchilla were recorded for best frequency tones that were amplitude modulated by low-frequency, two-tone complexes. Fourier analysis of poststimulus time histograms shows spectral peaks at fmod2-fmod1 in addition to the peaks at fmod1 and fmod2. The peaks in the neural spectra arise from compressive nonlinearities in the auditory system. The magnitudes of these spectral peaks are measures of synchrony at each frequency component. For all units, synchrony at fmod1 and fmod2 is greater than the synchrony at fmod2-fmod1. For a given unit, synchrony at fmod1 and fmod2 remains relatively constant as a function of overall level, whereas synchrony at fmod2-fmod1 decreases as the level increases. Synchrony was quantified in terms of the Rayleigh statistic (z), which is a measure of the statistical significance of the phase locking. In terms of z, phase locking at fmod1 and fmod2 is largest in chopper units, whereas onset-chopper units and primarylike units having sloping saturation in their rate-level functions show the smallest amount of phase locking. Phase locking at fmod2-fmod1 is also largest in chopper units, and smallest in onset-chopper units and primarylike units with sloping saturation.

Correlation Index: A new metric to quantify temporal coding

“…This is analogous to vector strength, which has also been combined in various ways with spike rate (e.g. Kim et al, 1990;Liang et al, 2002;Rees and Palmer, 1989;Sachs et al, 1983;Shofner et al, 1996).…”

Masking release for consonant features in temporally fluctuating background noise

“…To date, the mechanism underlying these masking effects has not been fully elucidated. Masking effects may result either from: (i) a similarity between the signal and masker temporal envelope patterns (Moore et al, 1999), (ii) an interference between the signal modulation and a modulation distortion component of frequency f 0 m generated by some nonlinear mechanism(s) along the auditory pathway in response to the masker (Shofner et al, 1996;Sheft and Yost, 1997;Moore et al, 1999;Verhey et al, 2003;Sek and Moore, 2004;Fü llgrabe et al, 2005), or (iii) an across-modulation-channel interference process (Ewert et al, 2002;Sek and Moore, 2003).…”

Perception of the envelope-beat frequency of inharmonic complex temporal envelopes

“…Since there is no energy at the envelope-beat frequency in the modulation spectrum of the physical stimulus, it has been argued that some nonlinear mechanism͑s͒ along the auditory pathway must generate an audible modulation-distortion component at the envelope-beat frequency in the internal modulation spectrum of complex temporal envelopes ͑e.g., Shofner et al, 1996;Moore et al, 1999͒. This idea has been recently implemented in a computational model of temporalenvelope processing ͑Ewert et al, 2002͒.…”