Andrea Megela Simmons, Ph.D.
Professor, Psychology & Neuroscience
Brown University
Abstract:
Male bullfrogs emit complex advertisement calls to attract females for mating and to defend territories against competing males. Behavioral experiments indicate that the waveform periodicity of the advertisement call is crucial for signal detection and discrimination against background noise. Physiological studies of responses of individual eighth nerve fibers indicate that the estimation of the call's fundamental period occurs by a process similar to pitch extraction as defined from mammalian data. The periodicity of a complex harmonic signal is captured by phase-locked discharges and displayed in all-order interval histograms of individual nerve fibers. Differences between stimuli varying in behavioral relevance are displayed by alignment of summed autocorrelation functions across populations of eighth nerve fibers. These patterns of phase-locked discharges are transmitted to the auditory midbrain, where spatially separate groups of neurons respond selectively to the spectral and temporal characteristics of complex signals. Responses of some cell groups are consistent with hypotheses of pitch extraction based on a delay-line autocorrelation mechanism.