Investigation of the horizontal, vertical, and oblique optokinetic nystagmus and afternystagmus in squirrel monkeys.

A moving random dot pattern was projected onto a tangent screen in front of awake untrained monkeys that were always placed in upright position. Eye movements were recorded in two dimensions to study the oblique optokinetic nystagmus (OKN) and compare it to the horizontal and vertical OKN. Any direction of pattern movement across the screen could be achieved. The angular velocity of pattern movement was varied between 6 and 180 degrees/s. To display off-horizontal and off-vertical eye movements, the instantaneous direction and velocity of the eye movements were computed from the horizontal and vertical search coil voltages. At pattern velocities below 90 degrees/s, stimulus-direction and direction of the OKN slow phase matched very precisely. Above 90 degrees/s the slow-phase eye movement direction was systematically shifted toward the horizontal except for pure vertical stimulation. The slow-phase eye velocity at off-horizontal stimulation was inconstant, however; stable periods occurred repeatedly that were used to define the gain of OKN. Up to stimulus speeds of about 90 degrees/s the OKN gain did not depend on the direction of stimulation and of OKN. At higher velocities the gain decreased with the increasing angle between stimulus direction and horizontal. Practically no vertical optokinetic afternystagmus (OKAN) could be observed, in either the up or down direction. At the onset of afternystagmus after oblique stimulation the direction of the OKAN slow phase immediately shifted over to the horizontal. The data indicate that the slow-phase direction and gain of oblique OKN with the monkey's head upright can be described by the sum of a horizontal and a vertical velocity vector obtained during stimulation in these cardinal directions.