The proposed research continues on-going efforts to describe cortical-subcortical interconnections in the rabbit visual system. Based on current findings, retrograde axonal transport techniques will be used to define the thalamic cells which project widely to the outermost layer of the visual cortex. Our current data also suggest that a cortical area, medial to the splenial sulcus, receives a discrete projection from the lateral posterior nucleus. Therefore, anterograde and retrograde axonal transport techniques will be used to more clearly relate this recently discovered medial visual area to other known cortical and subcortical visual structures. In addition, current research on projections of the superior colliculus indicates that the tectal input to the pretectum and thalamus is via a series of collaterals from a limited population of ascending fibers. Thus, the current proposal will use fiber degeneration to identify branching tectothalamic axons and retrograde transport of fluorescent markers will be utilized to identify cells of superior colliculus which give rise to branched axons. Finally, a series of three experiments will use a number of techniques to assess the role which the superior colliculus plays in determining input to, and function of, the visual cortex. Transneuronal transport techniques will define areas of the visual cortex which receive thalamically relayed retinal input and those areas which receive relayed tectal input. Ultrastructural studies will describe the termination of retinal and tectal fibers relative to the thalamocortical relay cells. The third set of experiments will attempt to demonstrate, using the 2-Deoxy-D-glucose technique, overall functional alterations in the visual cortex which accompany removal of the superior colliculus.