Abstract

Introduction: Ocular Dominance

Walls (1951: p. 394) has observed: “… dominance is a phenomenon of binocular vision: an eye does not become dominant only when the other eye is out of action”.

So what is ‘ocular dominance’? And what conceivable benefit might such a preferent facility – apparently demonstrable in the majority of binocularly sighted persons (Ehrenstein et al., 2005; Miles, 1930) – bestow upon the individual?

This chapter will describe and review the phenomenon of ocular dominance. Historically, discussion of eye dominance has occurred in the context of theories of binocular visual function (Wade, 1998). But while its existence has been acknowledged for over 400 years, are we any nearer to understanding this putative lateral oculo-visual preference?

The human eyes, although naturally paired, not infrequently manifest functional asymmetries. The apparent behavioural performance superiority of one eye is recognised by a variety of terms: these include ocular dominance, eye preference, sighting dominance or, in terminology analogous to ‘handedness’ or ‘footedness’ (motor preference demonstrated by an upper or lower limb, respectively), eyedness (Porac & Coren, 1981). Ocular dominance (OD) is the term that will be used preferentially throughout this chapter to embrace this concept, although it should be noted that this choice is not intended to imply that any dominance is of ‘ocular’ origin or even a unitary concept (Warren & Clark, 1938).

OD means different things to different people. The lay person might perhaps encounter the phenomenon when aligning objects in space during DIY tasks or when threading a needle; when participating in aiming sports activities (eg, clay pigeon shooting); or if engaged in specific occupations or pastimes that require monocular use of an optical (usually magnification) aid (eg, microscopy, astronomy). Under these circumstances one eye is unconsciously chosen (when viewing in binocular free space) or consciously selected (when using a gun or monocular instrument) to undertake the task (Miles, 1929; Porac & Coren, 1976): unconscious and conscious sighting choices as regards right or left eye use are reportedly in agreement approximately 92% of the time (Coren et al., 1979). All sighting tasks determine OD on the basis of the alignment of two objects presented at a stereo-disparity sufficiently far outside Panum’s area such that fusion is denied (Kommerell et al., 2003): the subject is forced to choose between one or the other image (ie, eye).

The clinical research scientist might consider criteria other than a sighting (motor) preference as providing a more appropriate indication of ocular laterality preference under particular circumstances. These alternatives are likely to be

performance-related measures of sensory origin; two longstanding popular examples are the eye with apparently better visual acuity (van Biervlet, 1901; Miles, 1930), or that eye which shows greater facility to suppress a rivalrous retinal image under conditions of binocular viewing (Washburn et al., 1934).

The first recorded description of what we now call OD is usually attributed to Giovanni Battista della Porta (ca1535-1615) [figure 1] in Book 6 of his treatise De Refractione (Porta, 1593: Wade, 1998). Porta (1593, pp. 142-143) described a pointing test to determine the preferred eye: a rod is held directly in front of the body and, with both eyes open, the viewer aligns the tip of the rod with a defined object in the mid-distance – the eye which retains an aligned view of the rod and the fixation object when the eyes are alternately closed is the preferred (sighting) eye. Translations of Porta’s text, originally published in Latin, have been provided by Durand & Gould (1910) and Wade (1998). It has also been suggested (Wade, 1987, p. 793) that the Flemish artist Peter Paul Rubens (1577-1640) might have made an engraving depicting Porta’s sighting test.

Figure 1. Giovanni Battista (sometimes Giambattista) della Porta (born probably late 1535, deceased 4 February 1615), Neapolitan scholar, polymath and playright. Portrait in profile: frontispiece engraving to expanded 20-volume edition of Magiae Naturalis (Naples, 1589).

It might also be noted that around 330BC Aristotle (ca384-322BC) described (without appreciating the significance of his observation) the ability of most individuals to wink or temporarily close one eye despite both eyes having similarly acute vision (Ross, 1927, p. 959).

For the interested reader Wade (1998) provides a wide historical survey of studies of eye dominances: his narrative places discussion of reported oculo-visual preferences in the context of evolving theories of human binocular visual function.

Over the years numerous techniques have been devised to define OD (Crider, 1944). Walls (1951) compiled a list of 25 tests (and indicated that his inventory could not be regarded as exhaustive). Gronwall & Sampson (1971) compared 18 techniques, and Coren & Kaplan (1973) analysed 13 measures. The issue of test appropriateness and comparative inter-technique agreement will be addressed below. Suffice to say here that on the grounds of the results of Coren & Kaplan (1973) if one were to choose a single technique to predict eye laterality it could reasonably – as four hundred years ago – be that of sighting alignment. Furthermore, subsequent research has suggested that the sighting eye thus determined seems to extract and process visual spatial information more efficiently than its fellow (Porac & Coren, 1977; Shneor & Hochstein, 2006 and 2008).

Demography of Ocular Dominance

The majority of individuals can record a sighting (motor) dominant eye. This laterality is apparently established in early life, possibly around four years of age (Barbeito, 1983; Dengis et al., 1996), and becomes stable by the middle of the human development period (Dengis et al., 1993).

Any contribution of genetic factors to an individual’s OD has been little explored. Porac & Coren (1981) have concluded that familial traits are absent. Reiss (1997) has expressed equivocation, although admitting that in an examination of fresh family data OD failed to align with any direct recessive or dominant Mendelian model of genetic transfer.

A number of studies throughout the twentieth century have explored the laterality distribution of OD in large samples of normally sighted human populations. Porac & Coren ( 1976: Table 1, p. 884) have surveyed much of this work, drawing on studies published between 1929-1974, and undertaken in North America, the UK, Japan, Australia, and Africa. The broad conclusion was that approximately 65% of persons sighted with their right eye, 32% with the left and