Walsh & Hoyt: Saccades

Summary. Saccades function to move the fovea rapidly to a target located on the peripheral retina. Phasic high-frequency discharges of ocular motoneurons provide a pulse of innervation, an eye velocity command, to extraocular muscles that drive the eyes against the viscous resistance of orbital tissue. Tonic discharge of motoneurons provides a step of innervation, an eye position command, to the muscles that maintains eye position against elastic restoring forces of the muscles and their tendons and orbital soft tissue. Other eye movement systems also use phasic velocity commands to overcome orbital viscous forces and tonic step commands to prevent the eyes from gliding back to the midposition of the orbit. For each eye position, there is a level of tonic contraction in agonist muscles and a reciprocal level of lesser tonic contraction in their antagonist muscles.