Patterns of organization of the corticotectal projection of cats studied by means of the anterograde degeneration method.

By using the Fink and Heimer method, patterns of organization of the cat corticotectal projection have been studied in more detail than previously known. Collicular projections from various cortical areas show specific laminar distributions in tectal layers, while there is a considerable overlap in areas of terminations. Thus the superficial layers of the superior colliculus receive fibers almost exclusively from the visual cortex, while the deep layers from the auditory and the somatic sensory areas. In the intermediate gray layer, fibers from the visual, auditory and somatic sensory areas terminate in extensive areas throughout the colliculus. These findings are in good accord with physiological observations. Fibers from the "association", orbital, proreate and cingulate areas end in the intermediate and deep layers as well. Patch-like terminations of projection zones from the somatic sensory cortex are observed in the superior colliculus, longitudinally arranged with approximately 200-250 micrometer width and spaced at 200-500 micrometer. Similar patterns of terminations appear to exist in projections from the visual cortex. The lateral portion of the mesencephalic periaqueductal gray matter has been found to be a recipient of cortical fibers from the somatic sensory, orbital and cingulate areas. Possible neuronal interconnections within the superior colliculus are discussed in connection with the recent physiological data.