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

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Abstract
For a tone that is amplitude modulated by two tones (f mod1 and f mod2), neither the stimulus waveform nor the half‐wave rectified waveform has spectral energy at the envelope beat frequency (f mod2−f mod1). The responses 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 f mod2−f mod1 in addition to the peaks at f mod1 and f mod2. 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 f mod1 and f mod2 is greater than the synchrony at f mod2−f mod1. For a given unit, synchrony at f mod1 and f mod2 remains relatively constant as a function of overall level, whereas synchrony at f mod2−f mod1 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 f mod1 and f mod2 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 f mod2−f mod1 is also largest in chopper units, and smallest in onset‐chopper units and primarylike units with sloping saturation.

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