On the Coordination of Saccades with Hand and Smooth Pursuit Eye Movements

Saccades are rapid eye movements that relocate the fovea, the retinal area with highest acuity, to fixate different points in the visual field in turn. Where and when the eyes shift needs to be tightly coordinated with our behavior. The current thesis investigates how this coordination is achieved. Part I examines the coordination of eye and hand movements. Previous studies suggest that the neural processes that coordinate saccades and hand movements do so by adjusting the onset time and movement speed of saccades. I argue against this hypothesis by showing that the need to process task-relevant visual information at the saccade endpoint is sufficient to cause such adjustments. Rather than a mechanism to coordinate the eyes with the hands, changes in saccade onset time and speed may reflect the increased importance of vision at a saccade's target location. Part II examines the coordination of smooth pursuit and saccadic eye movements. Smooth pursuit eye movements are slow eye movements that follow a moving object of interest. The eyes frequently alternate between smooth pursuit and saccadic eye movements, which suggests that their control processes are closely coupled. In support of this idea, smooth pursuit eye movements are shown to systematically influence the onset time of saccadic eye movements. This influence may rest on two different mechanisms: first, a bias in visual attention in the direction of pursuit for saccades that occur during smooth pursuit; second, a mechanism that inhibits the saccadic response in the case of saccades to a moving target. Evidence for the latter hypothesis is provided by the observation that both the probability of occurence and the latency of saccades to a moving target depend on the target's eccentricity and velocity.