DESCRIPTION (Author's abstract): Vergence eye movements adjust the angle between the eyes during shifts in gaze between far and near targets and are believed to be responsible for the precise alignment of the visual axes required for binocular vision. Shifts in gaze between objects at different distances from the observer usually involve a combination of saccades and vergence movements. A number of studies have shown that when saccades and vergence movements occur together, the amplitude of the saccade may be markedly larger in one eye than the other. This has lead to the hypothesis that the two eyes are controlled by independent saccadic subsystems which can effect rapid changes in eye alignment. According to this view, when vergence is combined with saccades or in pathologies involving the loss of binocular vision, the underlying independence of the two eyes is revealed. This hypothesis has profound implications for our understanding of the basic neurophysiology of vergence and conjugate eye movements, as well as for our understanding of how eye alignment is controlled in normal and in pathological conditions. An alternative hypothesis, which preserves the conjugate nature of saccades, states that unequal saccades are the result of the sharing of a common neural component by the saccadic and vergence subsystems. A major goal of this proposal is to test both hypotheses exhaustively and determine which, if either, can be developed to account for behavioral observations in normal subjects and those with oculomotor pathology. The proposed studies will involve single unit recording, electrical microstimulation, and drug microinjection in brainstem regions of alert monkeys trained to track visual targets. Accurate measurements of eye movements, together with sophisticated behavioral paradigms, will allow precise, quantitative analyses of neuronal signals and behavioral responses. The results of this work will provide important knowledge about the brain circuits which control saccades and vergence eye movements. Furthermore, these experiments will supply answers to fundamental questions about the etiology of oculomotor disorders resulting in disconjugate eye movements, such as strabismus and certain forms of nystagmus, and will provide strategies for compensating for these disorders.