Brainstem control of saccadic eye movements.

Saccades are rapid shifts in the direction of gaze. They include the fast (reset) phases of nystagmus generated by vestibular or optokinetic stimuli, the catch­ up movements required in the pursuit of a small moving target, and the scan­ ning movements used to explore a stationary visual scene . All these rapid eye movements have a similar trajectory composed of a rapid acceleration to peak velocity followed by a deceleration that brings the eye to its final position. Primate saccades usually last between 15 and 100 msec and can reach peak velocities in excess of 500 deglsec. Because they have such similar time courses, all rapid eye movements are thought to share a common neural saccade generator. If a saccade is made to a visual target, several steps are presumed to precede the activation of the saccade generator. After the target and its visual sur­ roundings fall on the retina and are detected by the visual system, the target is selected from its surroundings , its location is calculated, and a decision is made as to whether the target is worthy of a saccade. The saccade is triggered and appropriate commands are passed first to brains tern centers , then to ocular motoneurons, and finally to the extraocular muscles. Although we know rel­ atively little about the early stages of the process that leads to a saccade, experiments in the last 15 years have provided an understanding of the brain­ stem circuitry involved in saccade production. The relative simplicity of the oculomotor periphery and the easy access to premotor elements in the brain­ stem have provided a unique opportunity to probe into the more central struc­ tures that generate movement . In this review we summarize what is known about the brainstem saccade generator by starting at the motor periphery and moving centrally. We empha­ size new data, particularly from primates, and structure our discussion in terms