A neural code for auditory space in the cat's superior colliculus.

We have examined the intermediate and deep layers of the cat's superior colliculus for evidence of a neural representation of auditory space. We measured the responses of single units to sounds presented in a free field. The results support the following generalizations. Most auditory units in the superior colliculus have sharply delimited receptive fields which form two discrete classes distinguished by their locations and sizes. The remaining units respond to sounds presented at any location. Each auditory unit responds maximally to sounds at a particular horizontal and vertical location within its receptive field, the unit's "best area." The best areas and receptive field borders of a unit are resistant to changes in the intensity of stimulus. The locations of best areas shift systematically as a function of unit position to form a continuous map of auditory space. The horizontal dimension of space is mapped rostrocaudally, and the vertical dimension is mapped mediolaterally. This map corresponds in orientation with the map of visual space. These data permit us to infer the distribution of unit activity elicited by a sound at any given location. Regardless of its location, a sound activates a substantial portion of the superior colliculus. Indeed, sounds at some locations activate nearly all of the auditory units. However the activated portion of the colliculus contains a restricted region of units which are excited to near their maximum firing rates. The position of this focus of greatest activity varies systematically according to the location of the sound source, thus mapping the location of the sound in space.