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If you cover one eye (e.g., with your hand) you remove the sensory information about the eye's position in the orbit. Without this, there is no stimulus to binocular fusion, and the eye will move to a position of "rest". The difference between this position, and where it would be were the eye uncovered, is the heterophoria.
Esophoria is an eye condition involving inward deviation of the eye, usually due to extra-ocular muscle imbalance. ... additional terms may apply.
Exophoria is a form of heterophoria in which there is a tendency of the eyes to deviate outward. [1] During examination, when the eyes are dissociated, the visual axes will appear to diverge away from one another. [2] The axis deviation in exophoria is usually mild compared with that of exotropia.
It is the opposite of esotropia and usually involves more severe axis deviation than exophoria. People with exotropia often experience crossed diplopia. Intermittent exotropia is a fairly common condition. "Sensory exotropia" occurs in the presence of poor vision in one eye.
To ensure that the test is executed accurately and that the maximal amount of information is obtained, it is paramount that the appropriate fixation targets are used for near (approximately 33 cm (13 in)), distance (3 m (10 ft)) and far distance (>3 m (10 ft)) and also that a cover consisting of a black 'paddle' is used.
Fixation disparity is a tendency of the eyes to drift in the direction of the heterophoria.While the heterophoria refers to a fusion-free vergence state, the fixation disparity refers to a small misalignment of the visual axes when both eyes are open in an observer with normal fusion and binocular vision. [1]
Diplopia is the simultaneous perception of two images of a single object that may be displaced horizontally or vertically in relation to each other. [1] Also called double vision, it is a loss of visual focus under regular conditions, and is often voluntary.
[3] 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. [ 4 ]