Autocorrelation model for pitch processing in the frog’s auditory nerve

Abstract

Bullfrogs’ reactions to simulated vocalizations are sensitive to disruption of the 100-Hz 1st harmonic periodicity caused by the presence of inharmonic frequency components or changes in component phase. This behavioral preference for a specific range of periods indicates that representation of periodicity ‘‘pitch’’ may be the basis for evaluation of conspecific calls. Anuran auditory-nerve fibers phase-lock to the 1st and sometimes the 2nd or 3rd harmonic of multiple harmonic signals. Interval histograms of fiber responses reflect the complex pitch of both harmonic and inharmonic stimuli. We developed a pitch model that estimates pitch-period autocorrelation functions from all-order interval histograms across frequency channels by implementing model neurons designed to replicate the frog’s auditory nerve. Histograms are aligned in time across channels before being combined into a single pitch image (‘‘strobing’’ in the Patterson pulse-ribbon model; ‘‘dechirping’’ in bat sonar target-ranging). The resultant output yields a sharp pitchlike image, consistent with both behavioral and physiological results. Robust 1st-harmonic pitch images are produced for multiple harmonic signals, and even two-component sounds require only a few parallel fibers to yield a robust 1st-harmonic pitch.