Cyclovergence is the simultaneous occurring cyclorotation (torsional movement) of both eyes which is performed in opposite directions to obtain or maintain single binocular vision.
[1] They mainly occur due to Listing's law, which, under normal circumstances, constrains the cyclorotation in dependence on the vertical and horizontal movements of the eye.
The effect also relaxes when the eyes are presented with darkness; however, experiments show that in the latter case the cyclovergence does not disappear completely straight away.
[6] Here, use is made of the fact that a pair of dove prisms rotate an image optically if they are arranged one after the other and with an angular displacement relative to each other.
However, for a long time the mechanism of the compensation was unclear: many thought that cyclofusion was due exclusively to high-level processing of the visual images, while others suggested a motor cyclovergence response.