AbstractThe origin, course, and termination of the commissural projection of the superior colliculus were studied using the orthograde autoradiographic tracing method and the retrograde method utilizing horseradish peroxidase.The complementary and mutually confirming sets of data showed that the commissural fibers interconnect a restricted region of the colliculi. This region includes the strata grisea intermedium and profundum and to a lesser degree the stratum opticum. It extends throughout only the rostral part of the colliculus where it ends abruptly at a level slightly less than half the distance from the anterior border of the deep gray layers. By using the needle used for isotope injection to record multiunit reponses to somatic and visual stimuli, direct evidence was obtained that this region falls within that functional area of the colliculus devoted to face representation and central vision. The results also suggested that more commissural fibers arise from lateral than medial parts of this region and that many fibers interconnect corresponding points in the colliculi. In addition to intertectal connections, the commissural projection contains decussating axons which terminate in tegmental structures and within a restricted zone of the central gray matter directly overlying the oculomotor complex.The results are discussed in relation to the possible role the commissural projection plays in the regulation of eye and head movements.