Distribution of retinofugal and corticofugal axon terminals in the superior colliculus of squirrel monkey.

The distribution of terminal fields of retinocollicular fibers was studied in squirrel monkeys with the autoradiographic technique. The terminals were aggregated into patches which were separated by intervening gaps. The ipsilateral patches were particularly distinct. The patches as well as th gaps ranged in size from 50 to 200 microns. In the most posterior aspect of the contralateral superior colliculus, the gaps were absent, and the terminals formed an uninterrupted sheet. The corresponding portion of the ipsilateral colliculus had no retinal input, in agreement with the concept that this region most likely represented the temporal crescent of the visual field. In the most anterior portion of the superior colliculus where the fovea is known to be represented, the ipsilateral and contralateral projections were sparse but, nevertheless, discernible. There was a partial laminar segregation of terminals. The majority of the terminal fields in the contralateral colliculus was located in the most dorsal tier of the stratum griseum superficiale, whereas the majority of the ipsilateral input was slightly deeper in the same stratum. The distribution of corticocollicular fibers was studied by the autoradiographic technique. The fibers from areas 17 and 18 terminated predominantly in the dorsal portion of the stratum griseum superficiale. Area 19, in contrast, projected to the ventral portion of the stratum griseum superficiale. Thus the terminal fields of axons from the retina, area 17 and area 18, overlap in the superior colliculus, whereas axons arising from area 19 terminate in another substratum.