INTERLAMINAR CONNECTIONS OF THE SUPERIOR COLLICULUS IN THE TREE SHREW.1. THE SUPERFICIAL GRAY LAYER

One of the most persistent problems in the study of the superior colli culus is the relationship between its superficial and deep layers. The superficial tier of layers is considered to be visuosensory in functi on, whereas the deep tier is multisensory and has premotor functions. This fundamental distinction is the primary basis for the view that a visually triggered shift in the direction of gaze depends on the trans fer of information from sensory cells in the superficial tier to premo tor cells in the deep tier. The goal of the present experiments was to examine the interlaminar projections of the superficial gray layer in the tree shrew Tupaia belangeri. We used biocytin as the marker for t racing the pathways. The tree shrew was chosen because its large and d istinctly laminated superior colliculus facilitates the task of examin ing connections between the layers. Biocytin was used because of its s ensitivity and because it allowed us to place very small injections re stricted entirely to the superficial gray layer. The results demonstra ted that a prominent pathway originates in the superficial gray layer and terminates in stratum opticum. In comparison, the projection from the superficial gray layer to the layers beneath stratum opticum is ex tremely sparse. The pathway from the superficial gray layer to stratum opticum has a columnar distribution, extending about 100 mum rostrall y and caudally from the center of the injection site. There were no si gns of more remote intracollicular connections, nor of patches or band s of terminals. The biocytin injection sites also labeled pathways to nuclei as distant from the superior colliculus as the diencephalon, in cluding the dorsal and ventral lateral geniculate bodies, and the pulv inar. The results suggest that stratum opticum may serve as a link bet ween the superficial gray layer and the deeper layers.