Abstract
A major cue to the location of a sound source is the interaural time difference (ITD)–the difference in sound arrival time at the two ears. The neural representation of this auditory cue is unresolved. The classic model of ITD coding, dominant for a half-century, posits that the distribution of best ITDs (the ITD evoking a neuron’s maximal response) is unimodal and largely within the range of ITDs permitted by head-size. This is often interpreted as a place code for source location. An alternative model, based on neurophysiology in small mammals, posits a bimodal distribution of best ITDs with exquisite sensitivity to ITDs generated by means of relative firing rates between the distributions. Recently, an optimal-coding model was proposed, unifying the disparate features of these two models under the framework of efficient coding by neural populations. The optimal-coding model predicts that distributions of best ITDs depend on head size and sound frequency: for high frequencies and large heads it resembles the classic model, for low frequencies and small head sizes it resembles the bimodal model. The optimal-coding model makes key, yet unobserved, predictions: for many species, including humans, both forms of neural representation are employed, depending on sound frequency. Furthermore, novel representations are predicted for intermediate frequencies. Here, we examine these predictions in neurophysiological data from five mammalian species: macaque, guinea pig, cat, gerbil and kangaroo rat. We present the first evidence supporting these untested predictions, and demonstrate that different representations appear to be employed at different sound frequencies in the same species.
Highlights
For many species, including humans, a major cue for soundsource lateralization is the interaural time difference (ITD), the difference in arrival time of a sound at the two ears [1,2]
Recorded distributions of best ITDs were examined as a function of sound frequency for five mammalian species with different head sizes, and compared to the distributions predicted by the optimal-coding model, the Jeffress model, and the two-channel model
In this and subsequent figures, ITD is plotted as interaural phase difference (IPD) - the ITD as a proportion of the period of the sound frequency
Summary
For many species, including humans, a major cue for soundsource lateralization is the interaural time difference (ITD), the difference in arrival time of a sound at the two ears [1,2]. The classic model of the neural representation of ITD, developed by Jeffress [3], proposes an array of coincidence-detector neurons fed by a series of internal delay lines originating from each ear. Each neuron generates a maximal response at its ‘best ITD’, when the difference in internal delay compensates the external ITD, bringing the neuron’s inputs into coincidence. The Jeffress model posits that best ITDs are distributed within the ‘‘physiological range’’ of ITDs, generated by the size and shape of the head. This range is bounded by the maximum ITD, found for sound sources near the interaural axis (Figures 1B). Most instantiations of the Jeffress model posit a unimodal or uniform distribution of best ITDs centred at zero
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