Neural representations of pitch : role of peripheral frequency selectivity

Thesis (Ph. D.)--Harvard-MIT Division of Health Sciences and Technology, 2006.Includes bibliographical references (p. 129-140).Investigating the neural mechanisms underlying the perception of the pitch of harmonic complex tones is of great importance for many reasons. Changes in pitch convey melody in music, and the superposition of different pitches is the basis for harmony. Pitch has an important role in speech, where it carries prosodic features and information about speaker identity. Pitch plays a major role in auditory scene analysis: differences in pitch are a major cue for sound source segregation, while frequency components that share a common fundamental frequency (FO) tend to be grouped into a single auditory object. In psychophysics, a positive correlation is commonly observed between the estimated "resolvability" of individual harmonics of complex tones, assumed to depend primarily on the frequency selectivity of the cochlea, and the strength of the corresponding pitch percepts. In this study, possible neural codes for the pitch of harmonic complex tones were investigated in the auditory nerve of anesthetized cats, with particular focus on their dependence on cochlear frequency selectivity, which was measured directly using both complex tones and band-reject noise. A "rate-place" representation of pitch, based on cues to peripherally-resolved harmonics in profiles of average discharge rate along tonotopically-arranged neurons, was compared to a "temporal" representation, based on periodicity cues in the distributions of interspike intervals of the entire auditory nerve.(cont.) Although both representations were viable in the range of FOs of cat vocalizations, neither was entirely satisfactory in accounting for psychophysical data. The rate-place representation degraded rapidly with increasing stimulus level and could not account for the upper limit of the perception of the pitch of missing-F0 in humans, while the interspike-interval representation could not predict the correlation between psychophysical pitch salience and peripheral harmonic resolvability. Therefore, we tested an alternative, "spatio-temporal" representation of pitch, where cues to the resolved harmonics arise from the spatial pattern in the phase of the basilar membrane motion. The spatio-temporal representation was relatively stable with level and was consistent with an upper limit for the pitch of missing-F0, thus becoming the strongest candidate to explain several major human pitch perception phenomena.by Leonardo Cedolin.Ph.D