The pattern of afferent and efferent connections of the superior colliculus suggests that this area of the brain is involved in sensory-motor integration. In particular, the colliculus is involved in the integration of visual, auditory and somatosensory information, and the subsequent generation of eye and head movements. Despite the accumulated evidence for this role, surprisingly little is known about the intrinsic organization of the superior colliculus. Thus there is a special need for anatomical studies to identify the precise morphology of local circuits in the colliculus. Without this knowledge it is almost impossible to begin to construct a model of how the colliculus is involved in the control of orienting movements. Up to this time it has been almost impossible to analyze the intrinsic organization of the colliculus due to the technical limitations of the techniques available. Recently, a new anterograde anatomical tracer has been discovered: Phaseolus vulgaris-leucoagglutinin (PHA-L). This plant lectin is non-cytotoxic, does not appear to be taken up by axons of passage, and is compatible with light and electron microscopy. Thus, for the first time it is possible to examine the intrinsic connections of neurons in the superior colliculus. Our preliminary studies with PHA-L show axonal arborizations throughout the colliculus following very restricted injections in specific collicular laminae. We have also seen extensive connections between the superficial and deep layers of the colliculus, a finding that has been disputed for several decades. In the proposed experiments specific models of intrinsic circuitry will be tested to establish how functional units in the deep layers of the colliculus are organized. We will determine whether there is direct visual input to the deep layers from the overlying superficial layers, and how this visual input is organized. Finally, we will establish whether efferent neurons in the deep layers also receive direct visual input from the lateral suprasylvian visual cortex. These studies will enable us to outline a model of how sensory information is disseminated throughout the colliculus, how visual and motor maps in the colliculus are woven into functional units, and how they contribute to the generation of eye and head movements.
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