Periodicity extraction in the anuran auditory nerve. I. "Pitch-shift" effects.

1. Activity of individual eight nerve fibers in the bullfrog, Rana catesbeiana, was measured in response to complex, multiple-frequency stimuli differing in both frequency composition and harmonic structure. Stimuli were chosen to parallel types of stimuli producing "pitch-shift" effects in humans. 2. The fundamental frequency of harmonic stimuli can be extracted from the autocorrelation of fiber firing, whether the fundamental is physically present in the stimulus or is a "missing" fundamental. The spectral fine-structure of harmonic stimuli is not robustly represented in fiber temporal response. These effects are seen in both AP and BP fibers. 3. The pseudoperiod of inharmonic stimuli is represented by synchronization to successive high-amplitude peaks in the stimulus envelope. Temporal responses to stimuli with high center frequencies are similar regardless of whether their frequency components are harmonically or inharmonically related. Responses remain dominated by the envelope periodicity, and no "pitch-shift" is signaled. In response to stimuli with low center frequencies, temporal responses signal a "pitch-shift" between harmonic and inharmonic complexes. Both AP and BP fibers show these effects. 4. These data suggest that bullfrog peripheral fibers extract the periodicity of complex stimuli by time-domain rather than frequency-domain coding.