The neural control of disjunctive saccades

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We naturally reorient our eyes to targets of interest located at different eccentricities and depths. However how the brain ensures the precise control of binocular positioning remains highly controversial.Objective: The main objective of this study is to understand how the brain generates disjunctive saccades. In our recent work, we have shown that the neurons of the brainstem saccadic generator, which are typically considered a source of purely conjugate premotor drive, in fact provide temporally appropriate vergence information to drive eye motion during disjunctive saccades. In the present proposal, we will take this work to the next stage and identify the source of the vergence-related signals of premotor saccadic neurons. One obvious candidate is the superior colliculus, which receives inputs from disparity sensitive cortical regions such as the frontal eye fields. Additional vergence inputs to premotor saccadic neurons could come from the cerebellum and/or its deep nuclei. In particular, the oculomotor vermis and caudal fastigial nucleus have important projections to the premotor saccadic region and contain neurons whose discharges are related to saccades. Finally, neurons in the midbrain such as vergence-velocity neurons, which have only been characterized during smooth vergence, may also play a role in driving disjunctive saccades by means of their direct projections to the extraocular motoneurons. Each of these possibilities will be systematically explored.Scientific Approach: The activity of single neurons in alert behaving monkeys will be quantified during saccade-vergence interactions in alert behaving monkeys. System identification techniques developed in our laboratory will be used to describe the discharge dynamics of neurons during conjugate and disjunctive movements. The resultant data will be interpreted in the context of current models of the oculomotor system.Experimental Significance: Results will be used to generate theoretical models of the networks that control saccade-facilitated vergence.  These models will, in turn, provide a foundation that can be used to understand how the brain coordinates the movement of multiple muscles to generate more complex voluntary movements.