Neural activity in cortical areas MST and FEF in relation to smooth pursuit gain control.

Two cortical areas that crucially contribute to the generation and maintenance of smooth pursuit eye movements (SPEM) are the medial superior temporal area (MST) and the pursuit area of the frontal eye fields (FEF). They both project to the brainstem premotor structures via different parallel pathways. A special property of the pursuit system is the increased sensitivity to retinal image motion for increasing pursuit velocities (dynamic gain control), which might be attributed to the FEF. On the other hand, MST is regarded as the main site for estimating the target velocity in space and therefore for maintaining an ongoing SPEM. However, neural activities in these areas during visually driven SPEM have not been fully understood. We present a simple model of smooth pursuit gain control that comprises population activities, as well as single unit sensitivities with respect to eye and retinal image motion in MST and FEF. We show that the nonlinear computation necessary for dynamic gain control may be accomplished by units that exhibit a curvilinear sensitivity to retinal image motion and eye velocity.