Pitch perception and harmonic resolvability in normal-hearing and hearing-impaired listeners

Listeners with sensorineural hearing loss are often impaired in their ability to perceive the pitch associated with the fundamental frequency (FO) of complex harmonic sounds. Four studies investigated the relationship between FO discrimination performance and the ability to resolve individual harmonic frequencies of a complex, testing the hypotheses (1) that the accurate FO discrimination performance associated with low-order harmonics is due to their being resolved, and (2) that listeners with sensorineural hearing loss experience a pitch discrimination deficit due to a reduction in frequency selectivity. The first study revealed that resolved harmonics were not sufficient for accurate FO discrimination. Increasing harmonic resolvability by presenting even and odd harmonics to opposite ears did not improve pitch discrimination, raising the possibility that complex-tone pitch discrimination is not governed by harmonic resolvability per se, but is related to harmonic number. Based on this idea, the second study found that an autocorrelation model of pitch perception, modified to include place dependence by limiting the range of periodicities accurately processed by a given frequency channel, could account for the more accurate FO discrimination associated with low-order harmonics without relying on harmonic resolvability. However, further results in the third and fourth studies suggested a role for harmonic resolvability in pitch discrimination, inconsistent with the lack of dependence on resolvability of the modified autocorrelation model. In normal-hearing subjects at high stimulus levels and in hearing-impaired subjects, a wider spacing between adjacent frequency components, related to a reduction in frequency selectivity, was required to yield accurate FO discrimination performance. Thus, resolved harmonics may be necessary for accurate FO encoding, and the pitch discrimination deficit associated with sensorineural hearing loss may be related to a reduction in frequency selectivity. These results support spectral or spectrotemporal pitch models that derive FO from resolved harmonics, or a place-dependent temporal model whereby peripheral filter bandwidths limit the range of detectable periodicities. Because spectral processing plays an important role in pitch discrimination, hearing-impaired and cochlear-implant listeners may benefit from hearing-aid fitting procedures and cochlear-implant processing algorithms that emphasize or enhance spectral place cues. Thesis Supervisor: Andrew J. Oxenham Title: Principal Research Scientist, Research Laboratory of Electronics; Lecturer, Division of Health Sciences and Technology