Auditory-nerve fiber encoding of two-tone approximations to steady-state vowels.

Responses to two harmonically related tones, approximating the lowest formants of nine American English vowels, were recorded from single auditory-nerve fibers. Data were compiled as period histograms for tones presented singly and in combination using the fundamental frequency of the two-tone complex as the time base. The amplitudes of the primary frequency components present in a histogram were estimated by least-squares fitting a half-wave rectified sum of the stimulating sinusoids plus a constant. Nonlinear interactions resulted for most two-tone stimuli: one tone dominated the response. When one tone was equal to best frequency, that tone always controlled discharge timing, usually suppressing the response to the second tone. Complicated interactions took place when the stimulating frequencies bracketed best frequency. The tone nearest best frequency was most effective near threshold, while higher stimulus levels usually favored the low-frequency tone. Nevertheless, the suppression mechanisms appear to provide an effective spatial separation in the cochlea for the response components to each vowel approximation. Fourier analysis of the period histograms yielded qualitatively similar results.