Smooth pursuit and vergence

The intentional tracking of a moving object so that its image remains on the fovea is completed by the smooth pursuit system. The smooth pursuit movements vary from optokinetic reflexes in being voluntary, and in attending to the movement over a small section of visual space. By contrast, the optokinetic reflexes are involuntary and are responses to movement of the whole visual world.

Signals relating to the velocity (that is, speed & direction) of the target are produced by the medial temporal cortex of the visual “where” system that analyzes motion. The lesions of this cortex impair pursuit movements. Such signals are transmitted to the dorsolateral pontine nucleus (DLPN) that translates the target velocity into the motor commands for the pursuit movement. The DLPN project to the vestibulocerebellum and its output to the medial vestibular nuclei drive the smooth pursuit movements.

Vergence is the only disjunctive eye movement. For illustration, shifting gaze to a closer target needs adduction of both eyes that is achieved by contracting both the medial rectus muscles. Signals to generate vergence involve blurring of the retinal image by large degree of retinal disparity, the degree of accommodation, or monocular cues to distance. These all need the visual cortex. The rapid vergence movements take place during the saccades.