CORTICAL PLANNING AND CONTROL OF SMOOTH PURSUIT

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To maintain clear vision of an object at the fovea, the eyes must be moved to that object. Rapid eye movements, or saccades are used to acquire stationary objects, while smooth pursuit is used to stabilize moving objects. When an object remains stationary, high-resolution information is lost for only about 180 msec, the time required to generate a saccade. However, if an object is moving, the smooth pursuit system could never acquire that object because of this delay if it merely responded to retinal image motion. To solve this problem, the pursuit system uses past experience with tracking a familiar object trajectory and makes computations on ongoing motion to predict the future position of a moving target. Based on this neural computation, the eyes pursue the target at the appropriate time and with the appropriate speed. The substrate for predictive eye movements is not known. The dorsomedial frontal cortex (DMFC) participates in planning and other higher-order motor behavior. The principal investigator has preliminary evidence that this area may participate in planning and execution of smooth pursuit eye movements. The goal of this project is to study this area with electrophysiological techniques to understand how predictive neuronal coding of upcoming target motion is interfaced with the command to move the eyes. Specific aims are to answer the following questions: 1. Is the effect of internal planning on visually-guided pursuit multiplicative, or implemented by a switch? Neuronal activity recorded while the animal pursues a target (motor) will be compared with activity recorded while the target moves, but the animal continues to fixate (planning). 2. Does neuronal activity in the DMFC time fixation duration? We will pit an artificial timing signal against the signal in the DMFC by injecting small currents before an eye movement with durations that are different from the actual fixation period. 3. Does activity in the DMFC increase pursuit gain as a result of a release from inhibition? We propose to employ the GABAergic agents muscimol, which has an inhibitory effect on cortical function, and bicuculline, which produces a disinhibition of cortical cells to explore the balance between inhibitory and excitatory connections in the DMFC necessary to time pursuit initiation. 4. Does the smooth pursuit system use ongoing motion information to compute the trajectory of a target and thereby optimize initiation and perseveration of pursuit? We will monitor eye movements and single-neuron activity in the presence of targets that change curvature or speed in a fashion that would make a simple response to the motion inefficient.