Cycloversion and cyclovergence: the effects of the area and position of the visual display.

Rotation of a display in the frontal plane evokes a conjugate nystagmic rotation of the eyes (cycloversion) about the visual axes, with slow phases in the direction of stimulus motion--a response known as torsional optokinetic nystagmus (TOKN). Antiphase rotation of large dichoptic displays evokes a disconjugate rotation of the eyes about the visual axes, a response known as cyclovergence. Using the scleral-coil technique for monitoring eye movements we recorded TOKN evoked by black-and-white sectored displays rotating about the visual axis at an angular velocity of 30 degrees/s. The display was confined to central areas with diameters ranging from 5 degrees to full field or with the central 5 degrees to 75 degrees occluded. A 5 degrees central display evoked TOKN with 40% of the gain for the full-field display and gain increased as a function of the size of the display. The gain of TOKN decreased with increasing size of a central occluder. These characteristics of TOKN are similar to those of horizontal OKN. Cyclovergence was virtually absent with a 5 degrees display but was immune to occlusion of the central 40 degrees. Cyclovergence therefore differs from cycloversion in showing no preference for centrally placed stimuli. These effects are free from the influence of stationary edges, since these were concentric with the stimulus motion. The effects are also free from the influence of voluntary pursuit, since humans do not normally have voluntary control over torsional eye movements.