Vergence

A vergence is the simultaneous movement of both eyes in opposite directions to obtain or maintain single binocular vision.

When a creature with binocular vision looks at an object, the eyes must rotate around a vertical axis so that the projection of the image is in the centre of the retina in both eyes. To look at an object closer, the eyes rotate towards each other (convergence), while for an object farther away, they rotate away from each other (divergence). Exaggerated convergence is called cross eyed viewing (focusing on the nose, for example). When looking into the distance, the eyes diverge until parallel, effectively fixating on the same point at infinity (or very far away).

Vergence movements are closely connected to accommodation of the eye. Under normal visual conditions, looking at an object at a different distance will automatically cause changes in both vergence and accommodation, sometimes known as the accommodation-convergence reflex. When under non-typical visual conditions, like when looking at a stereogram, the vergence and accommodation of the eyes will not match, resulting in the viewer experiencing the vergence-accommodation conflict.

As opposed to the 500°/s velocity of saccade movements, vergence movements are far slower, around 25°/s. The extraocular muscles may have two types of fiber each with its own nerve supply, hence a dual mechanism.^{[citation needed]}

The following types of vergence are considered to act in superposition:

Accommodative vergence is measured as the ratio between how much convergence takes place for a given accommodation (AC/A ratio, CA/C ratio).

Proximal vergence is sometimes also called voluntary vergence, which however more generally means vergence under voluntary control and is sometimes considered a fifth type of vergence. Voluntary vergence is also required for viewing autostereograms as well as for voluntary crossing of the eyes. Voluntary convergence is normally accompanied by accommodation and miosis (constriction of the pupil); often however, with extended practice, individuals can learn to dissociate accommodation and vergence.

Vergence is also denoted according to its direction: horizontal vergence, vertical vergence, and torsional vergence (cyclovergence). Horizontal vergence is further distinguished into convergence (also: positive vergence) or divergence (also: negative vergence). Vergence eye movements result from the activity of six extraocular muscles. These are innerved from three cranial nerves: the abducens nerve, the trochlear nerve and the oculomotor nerve. Horizontal vergence involves mainly the medial and lateral rectus.