INTERLAMINAR CONNECTIONS OF THE SUPERIOR COLLICULUS IN THE TREE SHREW.3. 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 prov ides a link between the visuosensory superficial and the premotor inte rmediate layers of the superior colliculus. First, cells in the optic layer were intracellularly labeled with biocytin in living brain slice s. Compared to cells in the adjacent lower part of the superficial gra y layer, which have apical dendrites that ascend toward the tectal sur face, 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 horizont ally restricted fashion to the overlying superficial gray and subjacen t intermediate gray layers. This pattern also is different from the pr edominantly descending interlaminar projections of lower superficial g ray layer cells. Next, cells in the intermediate gray layer were label ed 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 an d 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, l ower 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 interm ediate gray layer cells for signals from the superficial gray layer. T ogether, these results support the idea that the optic layer is a dist inct lamina that provides a link between the superficial and intermedi ate gray layers. They also are consistent with the traditional view th at descending intracollicular projections play a role in the selection of visual targets for saccades.