Saccadic anomalies: vergence induces large departures from ball-and-socket behavior.

The configuration of muscular forces, which maintains a given orientation of the eye, varies with vergence state. As a consequence, changes in vergence produce both static and dynamic violations of simple ball-and-socket behavior: during strong convergence, the entire eye is displaced temporally within its orbit at steady state by as much as 200 microns; and the axis of ocular rotation for small horizontal saccades is consistently displaced forward within the globe by an average of about 1 mm. These phenomena occur regardless of whether vergence is maintained by accommodation or by binocular disparity. Hence, systematic errors of as much as a full degree can arise in measurement of vergence movement, unless monitoring methods are used which are insensitive to translational motion. The observed displacement on the axis of rotation for saccades may be involved in subjective shrinkage of visual targets during convergence ("experimental micropsia ").