W. P. Shofner
S. Sheft
Parmly Hear. Inst., Loyola Univ., 6525 N. Sheridan Rd., Chicago, IL 60626
Nonlinear processing of stimulus envelope was investigated in the discharge of cochlear nucleus (CN) neurons. A standard method of investigating nonlinearities is to examine the output of a system for distortion products when the input consists of two tones. For a tone that is amplitude modulated by a two-tone complex, there is no spectral energy at the envelope beat frequency for either the stimulus or the half-wave rectified version of the stimulus. Post-stimulus time histograms (PSTHs) were generated for CN units recorded from barbiturate anesthetized chincillas to amplitude modulated best frequency (BF) tones modulated by a one- or two-tone complex. Modulation frequencies were between 4--64 Hz. Fourier analysis of the PSTHs shows spectral peaks corresponding to envelope beat frequencies. The magnitudes of the spectral peaks at the beat frequencies are less than the magnitudes of the spectral peaks at the primary modulator frequencies. Spectral peaks in neural spectra at envelope beat frequencies presumably arise from a compressive nonlinearity. Psychophysically, low-frequency envelope beating can be an effective masker of AM detection at the beat frequency (Sheft and Yost, in review). The preliminary results suggest that a spectral representation of the envelope beat frequency does exist at the level of the CN. [Work supported by a PPG grant from NIDCD.]