piece of card or through a short tube (Durand & Gould, 1910), or through the circular aperture created at the narrow end of a truncated cone when the wider end is held up to the face (A-B-C Test: Miles, 1929).

Prima facie OD thus determined is usually clearly defined and consistent within (Miles, 1928 and 1929; Porac & Coren, 1976) and – importantly (see below) – between tests (Coren & Kaplan, 1973; Gronwall & Sampson, 1971). Sighting dominance in the study by Coren & Kaplan (1973) accounted for the greatest proportion (67%) of the variance among all of the tests analysed.

Unfortunately, it has come to be realised that even sighting dominance is not entirely robust, being subject to possible corrupting influences which include: observer (subjective) expectation and test knowledge (Miles, 1929); aspects of operational detail for even this simplest of tests (Ono & Barbeito, 1982); the fact that dominance, possibly as a result of relative retinal image size changes (Banks et al., 2004), has been shown to cross between eyes at horizontal viewing angles as small as 15 degrees eccentricity (Khan & Crawford, 2001; also Henriques et al., 2002; Quartley & Firth, 2004); and not least (for sighting tests which have to be held), the potentially modulating influence of hand use (Carey, 2001).

The dilemma that arises when OD appears to be test specific is simply where to place one’s reliance: has dominance switched between eyes or is the outcome an artefact of the testing technique? This rather begs the question: what is the purpose of OD?

Some Misconceptions

It remains a belief in some quarters that OD is a fixed characteristic in a given individual (a claim disputed by Mapp et al., 2003) or even, in an unwarranted leap of reasoning, displays the same laterality as limb (hand/foot) preferences (Delacato, 1959; Humphrey, 1861; Porta, 1593). Apparently one has only to apply one’s choice of test(s) as summarised in the previous section (or identify the writing hand or the ball-kicking foot) and the laterality of the dominant eye is established. But as we have just discussed, these several tests unfortunately indicate that more often than not OD appears to vary with test selection or circumstances. Also, the majority of studies investigating OD in tandem with hand (and rarely, foot) preference have failed to show a congruent relationship: selected examples include Annett (1999), Coren & Kaplan (1973), Gronwall & Sampson (1971), McManus et al. (1999), Merrell (1957), Pointer (2001), Porac & Coren (1975), and Snyder & Snyder (1928).

Resolving the Paradox of Ocular Dominance

From the foregoing discussion of the phenomenon of OD and attempts to define and measure it, we have apparently arrived at a paradoxical position. On the one hand the majority of binocularly sighted persons would quite reasonably claim to have experienced a demonstration of OD; on the other hand we are unable to confirm uniformity of laterality in the individual, or identify a clearly defined oculo-visual role for the phenomenon.

Of the non-human species, only primates have been considered to show characteristics consistent with having a sighting dominant eye (Cole, 1957; Smith, 1970). This has led Porac & Coren (1976) to speculate that perhaps OD is important to animals (including man) where the two eyes are located in the frontal plane of the head: this anatomical arrangement means that the left and right monocular visual fields display a substantial binocular overlap, with the functional consequence of enhanced depth perception through binocular disparity. However, given that there is fusion only for objects stimulating corresponding retinal points, perhaps suppression of the image in the non-dominant eye removes the interference arising from disparate and non-fusible left and right images that would otherwise confuse the visual percept. Thus the result when undertaking a sighting task, for example, is that the image in the dominant eye prevails. The apparent consistency of right and left OD proportions across the human population, and the almost universal demonstration of OD from an early (preschool) age, have inclined Porac & Coren (1976, p. 892) to the opinion that: “… monocular viewing via the dominance mechanism is as natural and adaptive to the organism as binocular fusion”.

But how reasonable is it to suggest that the highly evolved human visual system, which normally seeks to maintain binocular single vision, will ‘naturally’ occasionally resort to monocularity? And in this regard, as we have touched on in the previous section of this chapter and also stated when discussing tests of rivalry, in the individual with normal (ie, non-amblyopic) sight suppression of competing or rivalrous stimuli is not limited to one eye and one eye alone but rather is fluid from one moment to the next.

It is evident that the sighting (syn. aiming or alignment) test format is the only technique that apparently has the potential to identify OD consistently in a binocularly sighted individual (Coren & Kaplan, 1973). This form of task specifically allows the selection of only one eye to accomplish its demands (Miles, 1928); whether by ease or habit most individuals usually perform reliably and repeatedly in their choice of eye under these constrained circumstances (Porac & Coren, 1976).

experience suggests that inattention to this point might fail to resolve the patient’s asthenopia or visual discomfort.

A specialist clinical area that has come to the fore in recent years is that of monovision, a novel method of correcting presbyopia (the physiological agerelated deterioration in near vision as a consequence of impaired accommodative ability). In this approach (Evans, 2007), one eye is optically corrected for distance viewing and its companion focused for near. Prospective candidates for this procedure include middle-aged (usually longstanding) contact lens wearers, and persons electing to undergo laser refractive surgery.

Typically in monovision it is the dominant eye (usually identified by a sighting test) that is provided with the distance refractive correction; near focus tasks are allotted to the companion eye. This allocation recognises (Erickson & Schor, 1990) that performance with the dominant eye is superior for spatiolocomotor tasks (including ambulatory activities and driving a vehicle), such actions relying on an accurate sense of absolute visual direction. In addition, it is claimed that this ‘dominant/distance’ clinical approach produces better binocular summation at middle distance and reasonable stereo-acuity at near (Nitta et al., 2007). Others have disputed the necessity to adhere to such a rigid rule, not only when fitting contact lenses (Erickson & McGill, 1992) but also when undertaking refractive surgery (Jain et al., 2001). However, it might be remarked that a fundamental area of concern remains centred on the identification or the procedural choice of the ‘dominant’ eye, ie, that eye which is likely to take on the distance-seeing role.

While the visual sensori-motor system shows great adaptability and, as we have seen, OD can switch between eyes depending upon circumstances, great care should be taken when practitioners choose to prescribe monovision. Given the wealth of stimuli and changeable viewing circumstances in the ‘real world’, even sighting tests may not reliably indicate which eye should have the distance correction; furthermore, such tests cannot accurately predict or guarantee the success of monovision in an individual.

Medico-legal, occupational and vocational caveats surround such a specific approach to visual correction. Appropriate patient selection and screening are essential features (Jain et al., 1996). Subjects usually require a variable period of spatial adaptation due to compromised binocular function and the evident necessity for visual suppression as viewing circumstances demand. These and related features associated with this slightly controversial clinical modus operandi have been well reviewed as experience with the procedure has evolved (Erickson & Schor, 1990; Evans, 2007; McMonnies, 1974) so will not be considered further here.