Intrinsic circuitry in the cat superior colliculus: projections from the superficial layers.

The neuroanatomical tracer Phaseolus vulgaris leucoagglutinin (PHA-L) was used to label the local projections of neurons whose cell bodies are located in the superficial layers of the cat superior colliculus. Small, localized groups of neurons in the superficial layers project to all regions of the ipsilateral colliculus, including the deep layers. Comparable distributions of labeled terminals are seen throughout the colliculus when the PHA-L injection site is located in the rostral, middle, or caudal one-third of the colliculus. In addition, there is some evidence of a topographic projection from superficial to deep layers. These results suggest that a complex anatomical substrate exists for communication between the superficial and deep layers of the colliculus. Connections such as these may underlie the transfer of visual information from neurons in the superficial layers to populations of neurons in the deep layers that respond prior to saccadic eye movements.