Pitch Representations in the Auditory Nerve: Two Concurrent Complex Tones

Abstract

Pitch differences between concurrent sounds are important cues used in auditory scene analysis and also play a major role in music perception. To investigate the neural codes underlying these perceptual abilities, we recorded from single fibers in the cat auditory nerve in response to two concurrent harmonic complex tones with missing fundamentals and equal-amplitude harmonics. We investigated the efficacy of rate-place and interspike-interval codes to represent both pitches of the two tones, which had fundamental frequency (F0) ratios of 15/14 or 11/9. We relied on the principle of scaling invariance in cochlear mechanics to infer the spatiotemporal response patterns to a given stimulus from a series of measurements made in a single fiber as a function of F0. Templates created by a peripheral auditory model were used to estimate the F0s of double complex tones from the inferred distribution of firing rate along the tonotopic axis. This rate-place representation was accurate for F0s above about 900 Hz. Surprisingly, rate-based F0 estimates were accurate even when the two-tone mixture contained no resolved harmonics, so long as some harmonics were resolved prior to mixing. We also extended methods used previously for single complex tones to estimate the F0s of concurrent complex tones from interspike-interval distributions pooled over the tonotopic axis. The interval-based representation was accurate for F0s below about 900 Hz, where the two-tone mixture contained no resolved harmonics. Together, the rate-place and interval-based representations allow accurate pitch perception for concurrent sounds over the entire range of human voice and cat vocalizations.

Computational Model Predictions of Cues for Concurrent Vowel Identification

“…A periodic sieve (or template) was then defined by placing narrow bins (with binwidth=50 microseconds) at every integer multiple of the template period in the pooled ACF. Template contrast at a given template period (1/ template F0) was the ratio of the mean value in the periodic sieve to the overall mean of the pooled ACF (Larsen et al 2008). Template contrast was then computed for various template periods ranging from 80 to 1,000 Hz.…”

Auditory-nerve responses predict pitch attributes related to musical consonance-dissonance for normal and impaired hearing

“…To estimate the neural pitch salience of each musical interval, each ACF pop was analyzed using "periodic template" analysis. A series of periodic sieves were applied to each ACF pop in order to quantify the neural activity at a given pitch period and its integer related multiples (Cedolin and Delgutte, 2005;Larsen et al, 2008;Bidelman and Krishnan, 2009). This periodic sieve analysis is essentially a time-domain equivalent to the classic pattern recognition models of pitch in which a "central pitch processor" matches harmonic information contained in the stimulus to an internal template in order to compute the heard pitch (Goldstein, 1973;Terhardt et al, 1982).…”

Selective Subcortical Enhancement of Musical Intervals in Musicians

“…DPs have been extensively studied, but most of the focus has been at the level of the cochlea (Kim et al, 1980). Less is known about human subcortical processing of harmonically complex musical intervals (Fishman et al, 2000(Fishman et al, , 2001Tramo et al, 2001;Larsen et al, 2008). Our current research explores this topic by measuring the brainstem responses to consonant and dissonant intervals in musically trained and control subjects.…”