Coding of phase structure in complex sounds by bullfrog auditory‐nerve fibers

Abstract

This experiment explored how the peripheral coding of the spectral and temporal structure of a complex communication signal is affected by the phase relations between individual frequency components in the sound. Complex sounds with 21 components, all of which were harmonics of a common, low‐frequency fundamental (100 Hz), were digitally synthesized. The frequencies and amplitudes of the individual components matched those of the bullfrog's species‐specific advertisement call. For one stimulus, the 21 components were generated in cosine phase, and for the other stimulus, the 21 components were in random phase. Most amphibian papilla (AP) and basilar papilla (BP) fibers synchronized to the envelope only of the phase‐coherent stimulus, and the complex spectral structure of the stimulus was not accurately encoded by population firing patterns. Envelope synchronization to the random‐phase stimulus was not seen in the responses of some mid‐frequency‐sensitive AP and BP fibers, although low‐frequency‐sensitive AP fibers responded similarly to both stimuli. Behavioral experiments suggest that bullfrogs can discriminate between in‐phase and random‐phase versions of their advertisement calls.