Coding of AM tones in the chinchilla auditory nerve: implications for the pitch of complex tones.

Responses to sinusoidally amplitude modulated (AM) tones were recorded in single auditory nerve fibers of anesthetized chinchillas. The tones chosen were of a class which, in humans, gives rise to a percept of low subjective pitch. It was found that, for high-frequency stimuli, neurons tuned to high frequencies signaled waveform envolope periodicity when the stimulus was of sufficient intensity. Neurons tuned to low frequencies responded to low-frequency AM tones in an orderly manner which ranged, as overall intensity was raised, from signaling all frequency components to signaling only the component to which the neuron was most sensitive. This result was interpreted to indicate the presence of two-tone suppression. Variation of modulation frequency and modulation depth produced response patterns which were generally predictable based on the relative effectiveness of each of the component frequencies in eliciting discharges. Neurons tuned to low frequencies did not respond at low and moderate intensities to higher-frequency AM tones, even though the subjective pitch of the tones corresponded to frequencies to which the neuron was sensitive. At high intensities, distortion products locked in-phase to the frequency of the subjective pitch were observed. Responses to a single AM tone recorded from different neurons in the same animal displayed considerable diversity in their responses. Response diversity was correlated with neuronal tuning characteristics, stimulus intensiry, and intrinsic factors such as suppression. Neurons retained stimulus finestructure information in the temporal patterns of their discharges as stimulus components were varied from a harmonic relationship to an inharmonic relationship. The data indicate that responses of auditory nerve fibers contain sufficient information for extraction of the subjective pitch of AM tones by a central auditory processor which operates on temporal features of the responses.