PERIODICITY EXTRACTION IN THE ANURAN AUDITORY-NERVE .2. PHASE AND TEMPORAL FINE-STRUCTURE

Discharge patterns of single eighth nerve fibers in the bullfrog, Rana catesbeiana, were analyzed in response to signals consisting of multi ple harmonics of a common, low-amplitude fundamental frequency. The si gnals were chosen to reflect the frequency and amplitude spectrum of t he bullfrog's species-specific advertisement call. The phase spectrum of the signals was manipulated to produce envelopes that varied in the ir shapes from impulselike (sharp) to noiselike (flattened). Periphera l responses to these signals were analyzed by computing the autocorrel ation functions of the spike trains and their power spectra, as well a s by constructing period histograms over the time intervals of the low -frequency harmonics. In response to a phase aligned signal with an im pulsive envelope, most fibers, regardless of their characteristic freq uencies or place of origin within the inner ear, synchronize to the fu ndamental frequency of the signal. The temporal patterns of fiber disc harge to these stimuli are not typically captured by that stimulus har monic closest to the fiber characteristic frequency, as would be expec ted from a spectral coding mechanism for periodicity extraction, but i nstead directly reflect the periodicity of the stimulus envelope. Chan ging the phase relations between the individual harmonics constituting the signal produces changes in temporal discharge patterns of some fi bers by shifting predominant synchronization away from the fundamental frequency to the low-frequency spectral peak in the complex stimuli. The proportion of fibers whose firing is captured by the fundamental f requency decreases as the waveform envelope becomes less impulselike. Fiber characteristic frequency is not highly correlated with the harmo nic number to which synchronization is strongest. The higher-harmonic spectral fine structure of the signals is not reflected in fiber tempo ral response, regardless of the shape of the stimulus envelope, even f or those harmonics within the range of phase locking to simple sinusoi ds. Increasing stimulus intensity also shifts the synchronized respons es of some fibers away from the fundamental frequency to one of the lo w-frequency harmonics in the stimuli. These data suggest that the sync hronized firing of bullfrog eighth nerve fibers operates to extract th e waveform periodicity of complex, multiple-harmonic stimuli, and this periodicity extraction is influenced by the phase spectrum and tempor al fine structure of the stimuli. The similarity in response patterns of amphibian papilla and basilar papilla fibers argues that the frog a uditory system employs primarily a temporal mechanism for extraction o f first harmonic periodicity.