The case of the missing pitch templates: How harmonic templates emerge in the early auditory system

Periodicity pitch is the most salient and important of all pitch percepts. Psychoacoustical models of this percept have long postulated the existence of internalized harmonic templates against which incoming resolved spectra can be compared, and pitch determined according to the best matching templa tes [J. Goldstein, J. Acoust. Soc. Am. 54, 1496-1516 (1973)]. However, it h as been a mystery where and how such harmonic templates can come about. We present here a biologically plausible model for how such templates can form in the early stages of the auditory system. The model demonstrates that an y broadband stimulus, including. noise and random click trains, suffices fo r generating the templates, and that there is no need for any delay lines, oscillators, or other neural temporal structures. The model consists of two key stages: cochlear filtering followed by coincidence detection. The coch lear stage provides responses analogous to those recorded in the auditory n erve and cochlear nucleus. Specifically, it performs moderately sharp frequ ency analysis via a filterbank with tonotopically ordered center frequencie s (CFs); the rectified and phase-locked filter responses are further enhanc ed temporally to resemble the synchronized responses of cells in the cochle ar nucleus. The second stage is a matrix of coincidence detectors that comp ute the average pairwise instantaneous correlation (or product) between res ponses from all CFs across the channels. Model simulations show that for an y broadband stimulus, a degree of high coincidence occurs among cochlear ch annels that are spaced precisely at harmonic intervals. Accumulating coinci dences over time results in the formation of harmonic templates for all fun damental frequencies in the phase-locking frequency range. The model accoun ts for the critical role played by three subtle but important factors in co chlear function: the nonlinear transformations following the filtering stag e, the rapid phase shifts of the traveling wave near its resonance, and the spectral resolution of the cochlear filters. Finally, we discuss the physi ological correlates and location of such a process and its resulting templa tes. (C) 2000 Acoustical Society of America. [S0001-4966(00)04804-9].