Mechanisms involved in the phase sensitivity of auditory nerve fibers in the bullfrog

Abstract

Auditory nerve fibers in the bullfrog (R. catesbeiana) are sensitive to shifts in the phase spectra of complex sound stimuli. Fibers innervating both auditory organs, the amphibian papilla (AP) and basilar papilla (BP), exhibit changes in their synchronization to the fundamental period of a multi-harmonic stimulus with shifts in its phase spectrum. In addition, AP fibers also exhibit changes in their firing rates. To determine whether these changes actually reflect sensitivity to the peak amplitude of a stimulus waveform, the changes in spike rates and synchronization characteristics of AP and BP fibers were compared with the waveform’s peak amplitude. In BP fibers, synchronization to the fundamental period is highly correlated with changes in the stimulus waveform’s peak amplitude. However, changes in the waveform’s peak amplitude cannot account for changes in the spike outputs of most AP fibers. Therefore, additional mechanisms must be involved in the phase sensitivity of AP fibers. Suppression and distortion nonlinearities contribute to the phase sensitivity of some low-frequency AP fibers, but cannot explain the phase sensitivity observed in all AP fibers. Other micromechanical interactions and/or sensitivity to the fine-temporal structure of the stimulus waveform may play a role. [Work supported by NIH.]