Smooth Pursuit Eye Movements: Physiological Considerations

Smooth pursuit eye movements are expressed in their finest form in primates and have been studied extensively in humans and in monkeys. Several features define pursuit and distinguish it from the other kinds of eye movements. First. pursuit requires a moving visual target and cannot generally be elicited by stationary or imagined targets. For example, attempts to "move the eyes from left to right as slowly and smoothly as possible" produce a stai rcase of small saccades. Second, pursuit can be elicited by the motion of a small target across an otherwise stationary background. In the laboratory, the pursuit stimulus is commonly a small circular spot of light less than 1 deg in diameter, and the background can be dark, diffusely illuminated or highly patterned. Although the properties of the background have some subtle effects on the quality of smooth pursuit, effective tracking can be maintained in spite of the powerful optokinetic stimulus provided by a patterned background as the eyes sweep smoothly across.