Entracking generates horizontally flattened rate profiles that encode pitch as firing rate

Evidence

To some extent (increasingly so with increasing SPL), the representation hypothesized here is “orthogonal” to the tonotopic representation. The rate-place profile in the AN to a low-frequency tone increasingly broadens with increasing SPL, and flattens due to rate saturation (Kim and Molnar 1979 ). If a population of central neurons shows entracking, this property causes a stratification in the rate-place profile. Instead of a vertical or “hilly” profile where resolved components cause local increases in firing rate and unresolved components lead to a broad mound of activity, entracking generates horizontally flattened profiles or buttes, for which firing rate is dictated by the dominant interval between spikes fed to neurons in these frequency channels. For multiple, truly resolved components, a staircase of buttes would result with increasing firing rates corresponding to the frequency of successive harmonics. For unresolved components, many factors come into play (filter shape, limits to fine-structure and envelope, component phase), but buttes could be formed by neurons entracking to the dominant stimulus period. An obvious limitation in this scalar code is the upper limit of firing (usually near ~ 500 Hz). We surmise that at low SPLs and fundamental frequencies > 500 Hz, a spectral or place mechanism codes for pitch (Cedolin and Delgutte 2005 ; Larsen et al. 2008 ). With increasing SPL the place code degrades but entracking improves and first occurs in the neurons with the lowest thresholds for the spectral components present. For a given stimulus and time window, a neuron can only have one firing rate. One may wonder what distinguishes the low firing rate of an entracking neuron to a low frequency component vs. the low firing rate to a component above the entracking limit. The key would again be in the uniform firing rate, with low variability, across neurons in the case of a low frequency component. Above the entracking limit, firing rate is no longer clamped to the value dictated by the dominant ISI of a neuron’s inputs and will vary across neurons. — Dijk, Pim van, Physiology, Psychoacoustics and…, 4519

Categorization

• DEFINE: Defines the ‘butte’ hypothesis — entracking producing horizontally flattened rate-place profiles whose firing rate is clamped by dominant interspike interval.
• XREF: Plausibly connects to rate saturation flattening AN rate-place profiles (Kim and Molnar 1979) and to pitch-coding models contrasting place vs. temporal mechanisms.