Commissural connections of human superior colliculus.

The superior colliculus of higher mammals is a laminated structure of the midbrain that receives visual input in superficial layers, and visual, auditory and somatosensory input in deep layers. The superior colliculi on either side are interconnected via the intercollicular commissure, which has been proposed to play a role in visual transfer and gaze orienting. Intercollicular connections have been anatomically demonstrated in various species including macaque monkeys but not in man. Here we describe the organization of commissural connections of the superior colliculus in man. A single injection of the carbocyanine tracer 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate was made into the superior colliculus in five post-mortem brains. Contralateral to the injection, labelled axons formed a dense bundle in the deep collicular layers and isolated axons were present in the superficial layers. Synaptic-like boutons were found in all collicular layers. Injections placed at different rostro-caudal levels revealed a roughly topographical organization; the bulk of the labelled axons were found opposite to the injection, with a progressive decrease in labelling at more rostral and caudal levels. Our results demonstrate that superficial and, to a larger extent, deep layers participate in intercollicular connections, and suggest that visual information crosses at the collicular level.