Disconjugate Eye Movement Patterns during Optokinetic Stimulation of the African Chameleon, Chameleo melleri

Abstract

OPTOKINETIC nystagmus can be elicited in humans and in many animals by presenting a moving optic pattern, for example, of vertical stripes moving horizontally at constant velocity. The eyes follow (“pursue”) the pattern at an angular velocity that may approach or equal that of the moving stripes: this ocular pursuit movement (the “slow phase” of the optokinetic nystagmus) tends to stabilize the position of the image on the retina^1–3. Each pursuit movement is terminated by a rapid, “saccadic” ocular return movement (the “fast phase”) that transports the image to a new retinal location^1–3. Alternation of slow-phase movements in one direction with fast-phase movements in the opposite direction results in an oscillatory eye movement pattern called a nystagmus. Mammalian optokinetic nystagmus is normally conjugate^4,5, that is, the two eyes move in unison, as if yoked; however, we do not think the extent to which the pattern of optokinetic nystagmus is conjugate in the infra-mammalian vertebrate has been clearly determined.