Walsh & Hoyt: The Smooth Pursuit System

Smooth pursuit stabilizes the image of an object on or near the fovea during slow movement of the object or of the body. Smooth pursuit is needed to hold the eye on a stationary target during locomotion. When one views a target located off to one side during locomotion, smooth pursuit holds its image at the fovea, despite relative motion of the background. The pursuit system ensures optimal visual acuity for relative motion of a small visual target by reducing the speed of target image movement over the retina; this movement is called retinal slip and it is measured as retinal velocity error, the difference between target velocity and smooth eye movement velocity. Attention and the intent to pursue are required to sustain smooth tracking with a target image on the central retina. In that sense smooth pursuit is voluntary, but in another context it is reflexive since a percept of retinal image motion is required. Very few individuals can make conjugate smooth eye movements in darkness without an externally derived sensory percept of target motion. The speed of retinal images that can be tolerated for clear vision depends on the spatial frequency of the image. Image motion exceeding 5 6 per second reduces visual acuity for the higher spatial frequencies of high-contrast letters. Neurophysiology of pursuit eye movements. Cerebral initiation and maintenance of smooth pursuit. Recovery of smooth pursuit. Pursuit control from cerebrum to cerebellum. Cerebellum and smooth pursuit. Brain stem generation of smooth pursuit.