Cyclovergence

Cyclovergence is the simultaneous occurring cyclorotation (torsional movement) of both eyes which is performed in opposite directions to obtain or maintain single binocular vision.

Conjugate cyclorotations of the eye (that is, cyclorotations in the same direction) are called cycloversion. 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.

Listing's law, however, does not account for all cyclorotations. In particular, in the presence of cyclodisparity (that is, when two images are presented which would need to be rotated in relation to each other in order to allow visual fusion to take place), the eyes perform cyclovergence, rotating around their gaze directions in opposite directions, as a motor response to cyclodisparity.

Such additional, visually evoked cyclovergence appears to superimpose linearly onto the cycloversion due to Listing's law.

Visually induced cyclovergence of up to 8 degrees has been observed in normal subjects. Together with the 8 degrees that can usually be compensated by sensory means, this means that the normal human observer can achieve binocular image fusion in presence of cyclodisparity (also called orientation disparity in the case of a line image) of up to approximately 16 degrees. Larger cyclodisparity normally results in double vision. It has been shown that the tolerance of human stereopsis to cyclodisparity of lines (orientation disparity) is greater for vertical lines than for horizontal lines.

The visually evoked cyclovergence relaxes once the cyclodisparity is reduced to zero. 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.

Cyclovergence can also be evoked by cyclodisparity of the visual field; the cyclodisparity can be introduced by dove prisms. 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. Conversely, the range of cyclovergence-based cyclofusion can be trained using dove prisms that actively rotate the field of view: "The patient fixates a vertical line target, and the dove prism is rotated in the direction to increase the action of the insufficient muscle while fusion is maintained."

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Wikipedia