Testing the plausibility of harmonic templates based on temporal fine-structure

Abstract

The existence of harmonic templates in the central nervous system is a basic tenet of place models of pitch. Shamma & Klein (2000) propose that coincidences in temporal fine-structure across auditory nerve fibers in response to broadband stimuli suffice to set up harmonic templates for all fundamental frequencies in the phase-locking range. However, their computational model uses cochlear filters with unphysiologically steep slopes, calling into question the plausibility of the proposed scheme. We obtained responses from chinchilla auditory nerve fibers and “high-sync” neurons to pure tones, low-pass noise, and broadband noise, including “warped” noise (Heinz, 2006; Cedolin & Delgutte, 2010). Coincidences were computed using shuffled auto- or crosscorrelograms. We find that the maximum number of coincidences in response to tones of different frequencies, simulating the output of fibers with infinite sharp filters, indeed shows the template structure modeled by Shamma and Klein. This was not the case for responses to low-pass or broadband noise. However, if coincidences were only counted at zero delay, higher numbers of coincidences were indeed found for harmonically-related characteristic frequencies, even in response to noise. We conclude that steep-sloped cochlear filtering is not a necessary condition to obtain harmonic templates based on fine-structure.