Neural code for sound localization at low frequencies

Abstract Several nuclei in the auditory pathway perform sound localization. At lower sound frequencies, they compute the interaural time difference. This difference is transduced by the dedicated neuronal circuit into a labeled line difference. The neurons of the circuit fire only when synaptic inputs from both ears arrive within a short time window. We have shown previously the minimum timing difference detectable in the linear model and in the Hodgkin–Huxley-type equations. To better describe the spike code for the sound localization we define a generalized vector strength for point processes. Based on calculations in single cell models, we show how the neural code is transformed depending on the main sound frequency. We conclude that the performance limits of the low-frequency system are set by the frequency of the sound and therefore by both the vector strength and the probability of spike emission in the single unit. This shows the necessity for the existence of two different mechanisms for sound localization (1) one based on the interaural time delay, operating at low sound frequencies, and (2) another based on the interaural intensity difference, operating at high frequencies. The two systems then converge in higher-order neural relay stations.

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