A way forward might be provided by considering a taxonomy of eye dominance based on whether OD might be regarded as a unitary concept or a multi-factorial phenomenon. There have been those who claim a generalised laterality (examined by Porac & Coren, 1975), with OD matching hand/foot preferences: an early supporter of absolute unilateral superiority of sensori-motor function was the anatomist G. M. Humphrey (1861; as discussed by Woo & Pearson, 1927, pp. 167-169). Others (Berner & Berner, 1953; Cohen, 1952) considered OD to be composed of two factors, namely sighting dominance and rivalry dominance. Walls (1951) regarded OD as a composite of sensory and motor (eye movement) dominance. Jasper & Raney (1937) added acuity dominance and a dominant cerebral hemisphere to motor control. Lederer (1961) contemplated five varieties of OD (a proposal examined further by Gilchrist, 1976): monocular sighting dominance, motor dominance of one eye in binocular circumstances, orientational dominance, sensory dominance of one eye, and dominance of the lateral (right or left) hemi-field.

A weakness of many of these claims is that essentially they are based on individual observation or founded on theoretical considerations associated with the extant literature. More recently Gronwall & Sampson (1971) and Coren & Kaplan (1973) have both brought some enlightenment to a reappraisal of the issue by actually undertaking fresh examinations and applying modern comparative statistical analyses to the results. The outcome of the study by Coren & Kaplan (1973) in particular provides us with an entry into our probing of the form and function of OD.

Is Ocular Dominance Test Specific?

Coren & Kaplan (1973) assessed a group of fifty-seven normally sighted subjects using a battery of thirteen OD tests that intentionally covered the broad range of sensori-motor approaches suggested by other investigators over the years. The test scoring methodology indicated the strength as well as the laterality of dominance. A factor analysis of the results identified three orthogonal determinants of OD: (i) sensory – as revealed by (form) rivalry of stereoscopically presented images; (ii) acuity – a visual functional superiority indicated directly by the comparative inter-eye level of Snellen visual acuity; and (iii) sighting – ocular preference indicated by an aiming, viewing or alignment type of task.

These three alternative bases for a dominant eye will each be considered further.

I. Tests of Rivalry

Suppression of rivalrous stimuli has been a consistently recognised feature of eye dominance since the earliest writing on the phenomenon (Porta, 1593: Wade, 1998).

The viewing of superficially similar but subtly different stimulus pairs, separated using different coloured optical filters or cross-polarised lenses, can be used to quantify the proportion of time that the view of one or the other eye holds sway in the binocular percept (Washburn et al., 1934). Suppression of competing stimuli is demonstrably not limited to one eye, being usually in a state of flux. But this inter-eye rivalry is only possible when the stimuli are small and discrete (Levelt, 1968), immediately questioning the ability of such a rivalry technique to provide a wider ‘global’ indication of OD.

II. Tests of Asymmetry

Historically (as summarised by Wade, 1998) Aristotle in the third century BC contended that, although both eyes possessed equal visual acuity, superior accuracy was achieved using one eye; this opinion prevailed for nearly two millennia. Only in the eighteenth century was the possibility of an inter-ocular acuity difference given wider credence. Nowadays, it is recognised that in normally sighted (non-amblyopic) individuals, the level of visual acuity is demonstrably similar in the two eyes (Lam et al., 1996) but not atypically one eye might perform very slightly better than its fellow. Unfortunately many investigators have been tempted to claim the better-sighted eye as the dominant one (Duke-Elder, 1952; Mallett, 1988a; Miles, 1930).

The problem is that the evidence base for such a supposition is weak (Pointer, 2001 and 2007). In addition, laterality correlations with other recordable ocular asymmetries (eg, vergence and version saccades: Barbeito et al., 1986; Pickwell, 1972) are absent or unexplored, throwing into question reliance upon oculo-visual functional asymmetries as indicators of OD either in the individual or on a universal scale.

III. Sighting Tests

Probably the most direct and intuitive demonstration of OD originates with the pointing/alignment test described by Porta (1593). Test variations are many but include viewing a discrete distant target either through a circular hole cut in a