Space-mapped auditory projections from the inferior colliculus to the optic tectum in the barn owl (Tyto alba).

The optic tectum of the owl contains a topographic representation of auditory space. We have investigated the source of this space-mapped auditory activity by using retrograde tracing with horseradish peroxidase. The major source of auditory input to the optic tectum is the ipsilateral external nucleus of the inferior colliculus (ICX), which is known to contain a map of auditory space also. Additional minor projections originate in the superficial nucleus of the inferior colliculus and the nucleus isthmi parvocellularis. There is no apparent projection to the optic tectum from the contralateral ICX or any other brainstem auditory nucleus. The projection from the ICX to the optic tectum is point-to-point: rostral ICX projects to rostral tectum, caudal ICX to caudal tectum, dorsal ICX to dorsomedial tectum, and ventral ICX to ventral tectum. Thus, the space-mapped organization that exists in the ICX is passed on by topographic projections to the optic tectum.