Change in the coding of interaural time difference along the tonotopic axis of the chicken nucleus laminaris

Interaural time differences (ITDs) are an important cue for the localization of sounds in azimuthal space. Both birds and mammals have specialized, tonotopically-organized nuclei in the brain stem for the processing of ITD: medial superior olive (MSO) in mammals and nucleus laminaris (NL) in birds. The specific way in which ITDs are derived was long assumed to conform to a delay-line model in which arrays of systematically arranged cells create a representation of auditory space with different cells responding maximally to specific ITDs. This model was supported by data from barn owl nucleus laminaris taken from regions above 3 kHz and from chicken above 1 kHz. However, from mammals often do not fit this model, and an alternative has been proposed in which neurons are not topographically arranged with respect to ITD and coding occurs through the assessment of the overall response of two large neuron populations, one in each hemisphere. Modeling studies have suggested that the presence of different coding systems could be related to the animal's head size and frequency range rather than their phylogenetic group. Testing this hypothesis requires data from across the tonotopic range of both birds and mammals. The aim of this study was to obtain in-vivo recordings from neurons in the low-frequency range (<1000 Hz) of chicken nucleus laminaris. Our data argues for the presence of a modified Jeffress system that uses the slopes of ITD-selective response functions instead of their peaks to topographically represent ITD at mid- to high frequencies. At low frequencies, below several hundred Hertz, the data did not support any current model of ITD coding. This is different to that was previously shown in the barn owl and suggests that constraints in optimal ITD processing may be associated with the particular demands on sound localization determined by the animal’s ecological niche in the same way as other perceptual systems such as field of best vision.

Abstract licence: CC BY

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