The presence of a commissure connecting the two
superior colliculi suggests they do not act independently,
but the function of the tectotectal connection has never
been firmly identified. To develop a better understanding
of this commissural system, the present study determined
the distribution and morphology of tectotectal neurons
in the cat and macaque monkey, two animals with well-studied,
but different orienting strategies. First, we compared
the distribution of tectotectal cells retrogradely labeled
following WGA-HRP injections into the contralateral superior
colliculus. In monkeys, labeled tectotectal cells were
found in all layers, but were concentrated in the intermediate
gray layer (75%), particularly dorsally, and the adjacent
optic layer (12%). Tectotectal cells were distributed throughout
nearly the entire rostrocaudal 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 colliculus, in contrast to the monkey
distribution. In the context of the representation of visuomotor
space in the colliculus, the distribution of monkey and
cat tectotectal cells suggests a correspondence with oculomotor
range. So these neurons may be involved in directing orienting
movements performed within the oculomotor range. The somatodendritic
morphology of tectotectal cells in these two species was
revealed by homogeneous retrograde labeling from injections
of biocytin or biotinylated dextran amine into the contralateral
colliculus. The cell classes contributing to this pathway
are fairly consistent across the two species. 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.
Sensorimotor transformation elicits systematic patterns of activity along the dorsoventral extent of the superior colliculus in the macaque monkey
