Auditory nerve representation of a complex communication sound in background noise

A population study of auditory nerve responses in the bullfrog, Rana catesbeiana, analyzed the relative contributions of spectral and temporal coding in representing a complex, species-specific communication signal at different stimulus intensities and in the presence of background noise. At stimulus levels of 70 and 80 dB SPL, levels which approximate that received during communication in the natural environment, average rate profiles plotted over fiber characteristic frequency do not reflect the detailed spectral fine structure of the synthetic call. Rate profiles do not change significantly in the presence of background noise. In ambient (no noise) and low noise conditions, both amphibian papilla and basilar papilla fibers phase lock strongly to the waveform periodicity (fundamental frequency) of the synthetic advertisement call. The higher harmonic spectral fine structure of the synthetic call is not accurately reflected in the timing of fiber firing, because firing is "captured" by the fundamental frequency. Only a small number of fibers synchronize preferentially to any harmonic in the call other than the first, and none synchronize to any higher than the third, even when fiber characteristic frequency is close to one of these higher harmonics. Background noise affects fiber temporal responses in two ways: It can reduce synchronization to the fundamental frequency, until fiber responses are masked; or it can shift synchronization from the fundamental to the second or third harmonic of the call. This second effect results in a preservation of temporal coding at high noise levels. These data suggest that bullfrog eighth nerve fibers extract the waveform periodicity of multiple-harmonic stimuli primarily by a temporal code.