Walsh & Hoyt: The Optokinetic System

The optokinetic system is the helpmate of the angular VOR. Optokinetic smooth eye movements are necessary to sustain compensatory eye speed at the same speed as the head during prolonged rotations, after the VOR fades away. It has been emphasized above that the angular VOR responds best to brief, high-frequency changes in head position. During a constant-velocity head rotation in darkness, the cupula returns to its normal position and the compensatory eye movements that are stimulated by the VOR cease. During rotation in the light, therefore, there must be a backup optokinetic mechanism to ensure adequate image stabilization during very-low-frequency head motion. The optokinetic system aids the VOR to keep the eyes still in space during low-frequency head motion by moving them at the same speed as the head. The optokinetic image stabilizing system also uses the vestibular nuclei to generate slow phases of nystagmus. Smooth pursuit is induced with both voluntary assent and sensory stimulation by small objects whose images are guided to the foveal or parafoveal retina, but optokinetic slow phases are induced reflexively by large scenes that stimulate large areas of the retina.