New Effects of Cochlear Nonlinearity in Temporal Patterns of Auditory Nerve Fiber Responses to Harmonic Complexes

Discharge patterns of single auditory nerve fibers in adult cats were recorded in response to multi-component harmonic complexes. Stimuli possessed a variable number of equal-intensity harmonics of a common low-frequency fundamental. Data were analyzed by examining Fourier transforms of period histograms locked to the period of the stimulus fundamental. At low intensities transfer functions agreed with those defined by more traditional measures such as tuning curves. However, responses generally showed nonlinear level dependence at higher intensities, particularly above 60 dB SPL. In this, response spectra changed from resembling a bandpass filter to resembling a band-reject filter, i.e. responses were synchronized to components at the edges of the signal spectrum and to combination tones near the edges. The nonlinear effects became more pronounced as the number of components in the signal pass-band increased. Comparison of data from fibers with a wide range of thresholds indicated that the nonlinear behavior is mainly related to absolute stimulus level. This suggestes that the site of the nonlineari-ty is located prior to the spike generator.