Convergence of afferents from frontal cortex and substantia nigra onto acetylcholinesterase-rich patches of the cat's superior colliculus.

The patterns of distribution of frontotectal and nigrotectal fibers were studied with the anterograde horseradish peroxidase method in the cat. Direct serial-section comparisons were made between the afferent-fiber patterns and the compartmentalized arrangements of acetylcholinesterase staining within the intermediate and deep collicular layers. Many of the patches of high acetylcholinesterase activity in the intermediate gray layer proved to be zones in which labeled frontotectal and nigrotectal fibers converged. These acetylcholinesterase-rich patches may thus represent sites at which functional influences from the basal ganglia and frontal cortex are coordinated. In the deeper tiers of the intermediate gray layer and layers ventral to it, there were also zones of heightened and diminished acetylcholinesterase staining. Much of this histochemical patterning was reflected in the arrangement of fibers labeled by large rostromedial frontal injections, but these deeper tiers were not strongly labeled after more lateral frontal injections or after injections placed in the substantia nigra. The deeper parts of the acetylcholinesterase-positive gridwork in the superior colliculus are thus distinct from its upper tier of acetylcholinesterase-positive patches. We conclude that the compartmentalized patterning of dense acetylcholinesterase staining in the intermediate and deep collicular layers represents a mosaic architecture to which collicular afferent circuitry is tightly related. This gridwork may serve to set up functional domains within which different aspects of collicular processing are accommodated.