Interlaminar connections of the superior colliculus in the tree shrew. III: The optic layer.

These experiments were designed to test the idea that the optic layer in the tree shrew, Tupaia belangeri, is functionally distinct and provides a link between the visuosensory superficial and the premotor intermediate layers of the superior colliculus. First, cells in the optic layer were intracellularly labeled with biocytin in living brain slices. Compared to cells in the adjacent lower part of the superficial gray layer, which have apical dendrites that ascend toward the tectal surface, optic layer cells have dendritic fields that are restricted for the most part to the optic layer itself. The differences in dendritic-field location imply that superficial gray and optic layer cells have different patterns of input. The axons of optic layer cells terminate densely within the optic layer and, in addition, project in a horizontally restricted fashion to the overlying superficial gray and subjacent intermediate gray layers. This pattern also is different from the predominantly descending interlaminar projections of lower superficial gray layer cells. Next, cells in the intermediate gray layer were labeled in order to examine the relationships between optic layer cells and these subjacent neurons that project from the superior colliculus to oculomotor centers of the brain stem. Neurons in the upper part of the intermediate gray layer send apical dendrites into the optic layer and therefore can receive signals from the superficial gray layer either directly, from descending axons of lower superficial gray layer cells, or indirectly, through intervening optic layer cells. In contrast, lower intermediate gray layer cells have more radiate dendritic fields that are restricted to the intermediate gray layer. Thus, these lower cells must depend on descending projections from optic or upper intermediate gray layer cells for signals from the superficial gray layer. Together, these results support the idea that the optic layer is a distinct lamina that provides a link between the superficial and intermediate gray layers. They also are consistent with the traditional view that descending intracollicular projections play a role in the selection of visual targets for saccades.