Representation of sound source direction in the superior colliculus of the guinea pig in a virtual auditory environment.

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The deep layers of the superior colliculus (SC) receive visual, auditory, and somatosensory input. A major function of the SC is the control of orientation movements of the eye, head, and pinna. While a topographical map for sound source direction remains elusive in primary auditory structures of mammals, such a map for azimuthal sound source directions has been reported in the deep layers of the SC. Moreover, a gradient of elevation tuning has been also seen in the SC of ferrets and cats. Here we demonstrate that a virtual auditory environment can be used to reveal azimuthal and elevational topography for auditory spatial receptive fields in neurons in the SC of guinea pigs. Individual, head-related transfer functions (HRTF) were measured in ten guinea pigs for 122 directions in the upper hemispheric field and convolved with white noise. Many neurons (39%) in the deep layers showed robust responses to these virtual sounds, and the majority of these neurons had small spatial receptive fields that were restricted to the contralateral hemifield. Best directions varied from 0 degree to 135 degree azimuth along the contralateral side and from --10 degree to 60 degree elevation. Like previous studies using free-field stimulation, a gradient of best azimuth direction was found along the rostral-caudal axis, with rear directions represented caudally and front directions rostrally. The topographical organization for best elevations had not been studied previously in the guinea pig. We found that it roughly followed the mediolateral axis, with preference for high elevations represented medially and low elevations laterally. A similar organization using free-field stimulation has been reported in the ferret.