Ocular dominance

Ocular dominance, sometimes called eye dominance or eyedness, is the tendency to prefer visual input from one eye to the other. It is somewhat analogous to the laterality of right or left handedness; however, the side of the dominant eye and the dominant hand do not always match. This is due to the fact that both hemispheres control both eyes, but each one takes charge of a different half of the field of vision, and therefore a different half of both retinas. There is thus no direct analogy between "handedness" and "eyedness" as lateral phenomena.

Approximately two-thirds of the population is right-eye dominant ; however, neither eye is dominant in a small portion of the population. Dominance does appear to change depending upon direction of gaze due to image size changes on the retinas. There also appears to be a higher prevalence of left-eye dominance in those with Williams-Beuren syndrome, and possibly in migraine sufferers as well. Eye dominance has been categorized as "weak" or "strong" ; highly profound cases are sometimes caused by amblyopia.

In those with anisometropic myopia (i.e. different amounts of nearsightedness between the two eyes), the dominant eye has been found to be the one with more myopia.

Importance of ocular dominance
In normal binocular vision there is an effect of parallax, and therefore the dominant eye is the one that is primarily relied on for precise positional information. This may be especially important in sports which require aim, such as archery, darts or shooting sports.

It has been asserted that cross-dominance (in which the dominant eye is on one side and the dominant hand is on the other) is advantageous in sports requiring side-on stances (e.g. baseball, cricket, golf) ; however, recent studies have shown this not to be the case. In a study of professional baseball players, hand-ocular dominace patterns did not show an effect on batting average or ERA. Similarly, a recent South African study found that "cricketers were not more likely to have crossed dominance" than the normal population.

Ocular dominance is an important consideration in predicting patient satisfaction with monovision correction in cataract surgery, refractive surgery, and contact lens wear.

Determination of ocular dominance
A person's dominant eye "is determined by subjective alignment of two objects presented at a stereodisparity far beyond Panum's area". There are a number of ways to do this:
 * 1) The "Miles test". The observer extends both arms, brings both hands together to create a small opening, then with both eyes open views a distant object through the opening. The observer then alternates closing the eyes or slowly draws opening back to the head to determine which eye is viewing the object (i.e. the dominant eye).
 * 2) The "Porta test". The observer extends one arm, then with both eyes open aligns the thumb or index finger with a distant object. The observer then alternates closing the eyes or slowly draws the thumb/finger back to the head to determine which eye is viewing the object (i.e. the dominant eye).
 * 3) The observer extends one arm, forms a small, circular opening with the thumb and index finger, then with both eyes open views a distant object through the opening. The observer then alternates closing the eyes or slowly draws the opening back to the head to determine which eye is viewing the object (i.e. the dominant eye).
 * 4) The "Dolman method" also known as the "hole-in-the-card test". The subject is given a card with a small hole in the middle, instructed to hold it with both hands, then instructed to view a distant object through the hole with both eyes open. The observer then alternates closing the eyes or slowly draws the opening back to the head to determine which eye is viewing the object (i.e. the dominant eye).
 * 5) The "convergence near-point test". The subject fixates an object that is moved toward the nose until divergence of one eye occurs (i.e. the non-dominant eye). It is an objective test of ocular dominance.
 * 6) Certain stereograms.
 * 7) The "Pinhole test".
 * 8) The "Ring test".
 * 9) Lens Fogging Technique. The subject fixates a distant object with both eyes open and appropriate correction in place. A +2.00 or +2.50 lens is alternately introduced in front of each eye, which blurs the distant object. The subject is then asked to state in which eye is the blur more noticeable. This is the dominant eye.
 * 10) The Camera Test. The subject brings a camera up to his/her face. Whichever eye they use to look through the eyehole is their dominant eye.

"Forced choice" tests of dominance, such as the Dolman method, allow only a right or left eye result.