Pitch of Complex Tones: Rate-Place and Interspike Interval Representations in the Auditory Nerve

Abstract

Harmonic complex tones elicit a pitch sensation at their fundamental frequency (F0), even when their spectrum contains no energy at F0, a phenomenon known as "pitch of the missing fundamental." The strength of this pitch percept depends upon the degree to which individual harmonics are spaced sufficiently apart to be "resolved" by the mechanical frequency analysis in the cochlea. We investigated the resolvability of harmonics of missing-fundamental complex tones in the auditory nerve (AN) of anesthetized cats at low and moderate stimulus levels and compared the effectiveness of two representations of pitch over a much wider range of F0s (110-3,520 Hz) than in previous studies. We found that individual harmonics are increasingly well resolved in rate responses of AN fibers as the characteristic frequency (CF) increases. We obtained rate-based estimates of pitch dependent upon harmonic resolvability by matching harmonic templates to profiles of average discharge rate against CF. These estimates were most accurate for F0s above 400-500 Hz, where harmonics were sufficiently resolved. We also derived pitch estimates from all-order interspike-interval distributions, pooled over our entire sample of fibers. Such interval-based pitch estimates, which are dependent on phase-locking to the harmonics, were accurate for F0s below 1,300 Hz, consistent with the upper limit of the pitch of the missing fundamental in humans. The two pitch representations are complementary with respect to the F0 range over which they are effective; however, neither is entirely satisfactory in accounting for human psychophysical data.

Human Cortical Activity during Streaming without Spectral Cues Suggests a General Neural Substrate for Auditory Stream Segregation

“…Thus, the limits of tonotopic resolution illustrated in the model likely reflect peripheral limitations that are maintained throughout the tonotopic areas of the auditory pathways. This conclusion is supported by a recent study in the cat auditory nerve, which showed that the highest resolved harmonic varied from around the fifth at an f 0 of 200 Hz to around the 10th at an f 0 of 1000 Hz (Cedolin and Delgutte, 2005). Thus, even if human tuning is sharper than that found in cat by a factor of 2 (Shera et al, 2002), within our range of f 0 s (Ͻ 200 Hz) we would not expect harmonics above the 10th to be resolved in the auditory periphery or beyond.…”

Neural Representation of Concurrent Harmonic Sounds in Monkey Primary Auditory Cortex: Implications for Models of Auditory Scene Analysis

“…Surprisingly, neural resolvability of double HCT harmonics in A1 is comparable with or greater than that observed in the auditory nerve of the anesthetized cat, where responses to HCTs tend to saturate at moderate to high stimulus levels (Cedolin and Delgutte, 2010). Despite their relatively high level in the present study (60 dB SPL per component), many of the lower harmonics of the double HCTs were well resolved in rate-place profiles.…”

“…Using a stimulus design employed in auditory nerve studies of pitch encoding (Cedolin and Delgutte, 2005;Larsen et al, 2008), the present study examined whether spectral and temporal information sufficient for extracting the F0s of two simultaneously presented HCTs, a prerequisite for their perceptual segregation, is available at the level of A1. F0s of the concurrent HCTs differed by 4 semitones, an amount sufficient for them to be reliably heard as two separate auditory objects with distinct pitches in human listeners (Assmann and Paschall, 1998;Micheyl and Oxenham, 2010).…”

“…Rate-place and temporal representations of double HCTs were evaluated using a stimulus design used in auditory-nerve studies of pitch encoding (Cedolin and Delgutte, 2005;Larsen et al, 2008) and in studies of neural responses to single HCTs in macaque auditory cortex . At each recording site, 89 double HCTs with variable F0s were presented in random order.…”

Physiology, Psychoacoustics and Cognition in Normal and Impaired Hearing

“…For example, neither the sigmoidal shape of rate-level functions (firing rate as a function of stimulus SPL), their dynamic range, nor maximum firing rate, are dependent on stimulus frequency per se but on stimulus frequency relative to characteristic frequency (CF). This behavior, combined with cochlear band-pass filtering, underlies rate-place coding: spectral components are translated into a firing rate profile of the population of AN fibers (Cedolin and Delgutte 2005;). This is a tonotopic or "vertical" view of the audi-tory system in which strength of response along the tonotopic axis is proportional to spectral energy.…”