Mechanisms for Coding Pitch

Abstract

Pitch, the perceptual correlate of a sound’s frequency, is a fundamental attribute in speech and melody perception. We utilized individual differences across listeners with normal and disordered hearing to better understand how pitch is represented in the auditory system. Results from young, normal hearing listeners and listeners varying in age suggested the bulk of variability in sensitivity to modulations in frequency (FM) and amplitude (AM) likely reflects central, rather than peripheral, limitations. For listeners varying in degree of sensorineural hearing loss, however, sensitivity to FM was directly related to the fidelity of tonotopic (place) coding within the cochlea. This was contrary to the widely accepted understanding that FM is represented by precise, phase-locked spike times in the auditory nerve. To test the role of central processes on pitch perception, several experiments were conducted on listeners with congenital amusia, a neurogenetic disorder characterized by poor fine-grained pitch perception, unrelated to peripheral coding. We found that amusic deficits extend beyond poor pitch discrimination, including poor discrimination for high frequencies as well as poor detection for FM and AM tones. Despite the long-held understanding that amusia is a life-long deficit for pitch and music, impervious to training, we found rapid learning for pitch and melody discrimination in amusia. The learning effects were large and maintained for at least one-year. Overall, the findings suggest peripheral place coding is important for the fidelity of pitch, but many processes beyond the periphery can also contribute to variability in pitch perception.