Visually evoked cyclovergence and extended Listing's law

Cyclovergence is a simultaneously occurring cyclorotation of the two eyes in opposite directions. Cyclovergence can be elicited visually by opposite cyclorotation of the two eyes' images. It also can occur in conjunction with horizontal vergence and vertical version in a stereotyped manner as described by the extended Listing's law (or L2). We manipulated L2-related and visually evoked cyclovergence independently, using stereoscopic images of three-dimensional (3D) scenes. During pursuit in the midsagittal plane, cyclovergence followed L2. The amount of L2-related cyclovergence during pursuit varied between subjects. Each pursuit trial was repeated three times. Two of the three trials had additional image rotation to visually evoke cyclovergence. We could separate the L2-related and visual components of cyclovergence by subtraction of the cyclovergence response in matched trials that differed only in the image rotation that was applied during pursuit. This indicates that visual and L2-related contributions to cyclovergence add linearly, suggesting the presence of two independent systems. Visually evoked cyclovergence gains were characteristic for a given subject, little affected by visual stimulus parameters, and usually low (0.1-0.5) when a static target was fixated. Gain and phase lag of the visually evoked cyclovergence during vertical pursuit was comparable with that during fixation of a static target. The binocular orientations are in better agreement to orientations predicted by L2 then would be predicted by nulling of the cyclodisparities. On the basis of our results, we suggest that visually driven and L2-related cyclovergence are independent of each other and superimpose linearly.