The study of the cell populations that control gaze has contributed greatly to our understanding of sensorimotor integration in the vertebrate brain. The long term goal of the proposed research is to increase understanding of the neural control of gaze by studying synaptic relationships within the intermediate grey layer of the superior colliculus, an area whose central role in gaze mechanisms is well-established. The intermediate grey layer contains cells which are premotor in the sense that they respond before saccadic eye movements and send axons, by way of a pathway called the predorsal bundle, to the lower brainstem gaze centers. These brainstem centers, in turn, project to the motor neurons that are involved in shifts in gaze. For understanding the mechanisms of sensorimotor integration in the intermediate grey layer, it is essential to characterize the inputs responsible for the responses of the predorsal bundle cells. Previous studies have shown that the intermediate grey layer is heavily innervated by fibers that utilize the neurotransmitters gamma- aminobutryic acid (GABA) and acteylcholine. The proposed experiments examine the relationships between these two fiber systems and the predorsal bundle cells in the cat. The experiments test the hypothesis that zone incerta, a nucleus located in the ventral thalamus, is an important source of the GABA ergic input to these cells. They also test the idea that a population of cells in the reticular formation, the pedunculopontine tegmental nucleus, is a cholinergic relay between the output of the basal ganglia and the predorsal bundle cells. Specifically, these experiments are designed: 1. To locate, using retrograde axonal transport methods, the cells of origin of the incertotectal pathway and to determine, by using a combination of retrograde tracing and immunocytochemical methods, whether zona incerta is a source of GABAergic input to the intermediate grey layer. 2. To characterize the ultrastructural features and postsynaptic targets of the incertotectal terminations in the intermediate grey layer and to examine whether those terminations directly contact predorsal bundle cells, by using a combination of anterograde and retrograde tracing methods in tissue prepared for electron microscopy. 3. To determine, using a combination of anterograde and retrograde methods in tissue prepared for electron microscopy, whether the cholinergic cells in the pedunculopotine tegmental nucleus receive input from substantia nigra pars reticulata, one of the efferent nuclei of the basal ganglia. 4. To characterize the postsynaptic target(s) of the cholinergic projection from the pedunculopotine tegmental nucleus to the intermediate grey layer, and to examine whether the cholinergic terminals contact predorsal bundle cells, by using a combination of anterograde, retrograde and immunocytochemical methods in tissue prepared for electron microscopy.