Encoding of phase spectra by the peripheral auditory system of the bullfrog

In this study we have examined the sensitivity of auditory nerve fibers in the bullfrog (Rana catesbeiana) to changes in the phase spectrum of an equal-amplitude multi-harmonic stimulus which spanned the bullfrog's range of hearing. To assess peripheral auditory phase sensitivity, changes in the response properties of VIIIth nerve fibers were measured when the relative phase angle of a single harmonic component nearest a unit's best excitatory frequency was systematically varied. The results revealed that shifts in the phase spectrum are encoded in at least 3 different ways by the peripheral auditory system of the bullfrog: 1) by changes in the degree of spike synchronization of fibers from both inner ear organs (the amphibian papilla and the basilar papilla) to the fundamental waveform period; 2) by changes in the shapes of period histograms of fibers from both organs; and 3) by changes in the spike rates of amphibian papilla fibers. The presence of phase sensitivity in the peripheral auditory system of the bullfrog indicates that information regarding the fine-temporal waveshape and the underlying phase spectrum of an acoustic signal is contained within the spike trains of VIIIth nerve fibers. Similar sensitivities to changes in the phase spectra and temporal waveshapes of acoustic signals may also be present in the peripheral auditory system of other vertebrates. Such studies could provide valuable insight into the role that phase spectra and temporal waveshape may play in bioacoustic communication.