Spatial response properties of acoustically responsive neurons in the superior colliculus of the ferret: a map of auditory space.

Extracellular single-unit recordings were made from auditory neurons in the superior colliculus of ferrets anesthetized with either a neuroleptic or a combination of barbiturate with paralysis. The response properties of these neurons were studied using white-noise bursts presented under free-field conditions in an anechoic chamber. Auditory neurons were found throughout the intermediate and deep layers of the superior colliculus. All neurons were spontaneously active, the rates of discharge varying from 0.1 to 61.1 spikes X s-1. Although the spontaneous discharge interspike-interval histograms for many units approximated to exponential distributions, the histograms of 44% had clear secondary peaks, indicating more than one preferred interval, and could not be modeled by a simple process. Most neurons (50%) responded only at stimulus onset, whereas 12% exhibited sustained discharges and 38% gave onset responses followed by a period of silence or reduced activity and then a period of elevated discharge, which was not apparently related to stimulus offset. Neurons with multipeaked response patterns were concentrated in the stratum griseum profundum. The latencies from arrival of the stimulus at the ear to the onset of neural activity ranged from 6 to 49 ms and decreased with increasing stimulus intensity. Although responsive to sounds over a large region of space, most neurons had clearly defined best positions at which the strongest response was obtained. The response declined as the speaker was moved away from this position, and nearly all units had peaked response profiles. The spatial tuning varied between different neurons, but most were more sharply tuned in elevation than in azimuth. Increasing the stimulus intensity did not, in general, alter the best positions of these neurons, but usually resulted in a broadening of the receptive fields, although other units became more sharply tuned. The best positions of auditory neurons varied systematically in azimuth from 20 degrees into the ipsilateral hemifield to 130 degrees into the contralateral hemifield as the electrode was moved from the rostrolateral to the caudomedial end of the superior colliculus. The best positions shifted in elevation along a rostromedial to caudolateral axis from 60 degrees above to 50 degrees below the visuoaural plane.(ABSTRACT TRUNCATED AT 400 WORDS)