Topographic organization of interaural intensity difference sensitivity in deep layers of cat superior colliculus: implications for auditory spatial representation

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Abstract
Sensitivity to interaural intensity difference (IID) was examined for 103 neurons in the deep layers of superior colliculus (SC) in ketamine barbiturate-anesthetized cats. Noise stimuli were presented dichotically, and IID sensitivity functions were generated while keeping the average binaural intensity (ABI) of stimulation constant. Neurons of three binaural classes were found to be IID sensitive. Neurons receiving excitatory contralateral input and inhibitory ipsilateral input (EO/I cells, 55% of sample) had steplike IID functions, with maximum response at IIDs corresponding to contralateral azimuths (positive IIDs), total suppression at IIDs corresponding to ipsilateral azimuths (negative IIDs), and cutoffs at different positions along the IID axis for different neurons. Neurons responsive only to binaural stimulation (OO/F cells, 15% of sample) had IID functions with a sharp peak in the range of 0 to 10 dB IID. Cells receiving excitatory input contralaterally and a facilitatory ipsilateral input (EO/F cells, 7% of sample) had IID functions of intermediate shape, with a peak in the range of 10 to 20 dB IID and a sharper cutoff near zero IID than at larger positive IIDs. The sharpness of IID cutoff for EO/I cells was quantified by measuring an 80% IID dynamic range. Neurons with 80% IID dynamic ranges of less than 26 dB were judged to have sharp cutoffs. The position along the IID axis of the IID cutoff for these cells was quantified by recording the IID at which the response was at 50% of maximum (half-maximal IID). A topographic organization of EO/I cells with sharp IID cutoffs was found along the rostrocaudal axis of SC, such that rostral EO/I cells had IID functions with half-maximal IIDs near zero, while increasingly caudal EO/I cells had progressively larger (positive) half-maximal IIDs. Although detailed maps could not be obtained in individual animals, the topography was observed in each of nine experiments in which EO/I cells were located in two or more rostrocaudal locations (P = 0.00002). The effect of stimulus level on the stability of IID cutoff was examined for 13 EO/I cells. The majority (85%) showed less than 10 dB variation in half-maximal IID across a range of suprathreshold ABIs, indicating that EO/I cells in SC generally exhibit stability in cutoff with changes in intensity of broadband stimuli.(ABSTRACT TRUNCATED AT 400 WORDS)