COMPARISON OF THE DISTRIBUTION AND SOMATODENDRITIC MORPHOLOGY OF TECTOTECTAL NEURONS IN THE CAT AND MONKEY

The presence of a commissure connecting the two superior colliculi sug gests they do not act independently, but the function of the tectotect al connection has never been firmly identified. To develop a better un derstanding of this commissural system, the present study determined t he distribution and morphology of tectotectal neurons in the cat and m acaque monkey, two animals with well-studied, but different orienting strategies. First, we compared the distribution of tectotectal cells r etrogradely labeled following WGA-HRP injections into the contralatera l superior colliculus. In monkeys, labeled tectotectal cells were foun d in all layers, but were concentrated in the intermediate gray layer (75%), particularly dorsally, and the adjacent optic layer (12%). Tect otectal cells were distributed throughout nearly the entire rostrocaud al extent of the colliculus. In cats, tectotectal cells were found in all the layers beneath the superficial gray, but the intermediate gray layer contained the greatest concentration (56%). Labeled cells were almost exclusively located in the rostral half of the cat superior col liculus, in contrast to the monkey distribution. In the context of the representation of visuomotor space in the colliculus, the distributio n of monkey and cat tectotectal cells suggests a correspondence with o culomotor range. So these neurons may be involved in directing orienti ng movements performed within the oculomotor range. The somatodendriti c morphology of tectotectal cells in these two species was revealed by homogeneous retrograde labeling from injections of biocytin or biotin ylated dextran amine into the contralateral colliculus. The cell class es contributing to this pathway are fairly consistent across the two s pecies. A variety of neuronal morphologies were observed, so there is no single tectotectal cell type. Instead, cell types similar to those found in each layer, excepting the largest neurons, were present among tectotectal cells. This suggests that a sample of each layer's output is sent to the contralateral colliculus.