CYCLOVERSION AND CYCLOVERGENCE - THE EFFECTS OF THE AREA AND POSITIONOF THE VISUAL-DISPLAY

Rotation of a display in the frontal plane evokes a conjugate nystagmi c rotation of the eyes (cycloversion) about the visual axes, with slow phases in the direction of stimulus motion - a response known as tors ional optokinetic nystagmus (TOKN). Antiphase rotation of large dichop tic displays evokes a disconjugate rotation of the eyes about the visu al axes, a response known as cyclovergence. Using the scleral-coil tec hnique for monitoring eye movements we recorded TOKN evoked by black-a nd-white sectored displays rotating about the visual axis at an angula r velocity of 30 degrees/s. The display was confined to central areas with diameters ranging from 5 degrees to full field or with the centra l 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 increas ed as a function of the size of the display. The gain of TOKN decrease d with increasing size of a central occluder. These characteristics of TOKN are similar to those of horizontal OKN. Cyclovergence was virtua lly absent with a 50 display but was immune to occlusion of the centra l 40 degrees. Cyclovergence therefore differs from cycloversion in sho wing no preference for centrally placed stimuli. These effects are fre e 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 con trol over torsional eye movements.