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The importance and relevance of eye dominance, Part 2: in the second part of this two-part series, the latest research relating to our understanding of eye dominance will be explored with its significance considered in relation to sport and monovision correction.

Introduction

In the first part of this series, different methods for establishing eye dominance in the consulting room were discussed. In this second installment, attention will turn to the importance of eye dominance in sport and for monovision correction. The article concludes with an overview of the latest research in this subject area.

Significance for sport

In sports like archery or shooting, identifying the dominant eye appears to be important and various aids are used to partially or completely occlude the non-dominant eye. Jones and colleagues found that the learning of rifle marksmanship is influenced by eye dominance. (1) Specifically, individuals who shoot right-handed and are left-eye dominant, or who shoot left-handed and are right-eye dominant do not learn marksmanship skills as readily as individuals who have matched eye and hand dominance. A relationship between accuracy in archery and uncrossed patterns of hand preference and eye dominance has also been noted. (2)

Some authors have claimed that better sporting performance in batting /hitting tasks, for example, in baseball or cricket, may be associated with crossed dominance in relation to eye and hand dominance (right-hand, left-eye; left-hand, right-eye) since this allows the dominant eye to be positioned toward the bowler/pitcher and the oncoming ball. (3) To test this idea, Laby and colleagues compared batting averages in over 400 major and minor league members of the Los Angeles Dodgers professional baseball team. (4) They found that hand /eye dominance patterns had no effect on performance in the field. With few exceptions, most other studies of the patterns of hand and eye dominance are in agreement with Laby's findings. One exception is the work by Portal and Romano, who in their 1988 study, (5) and in their subsequent 1998 review, (6) found that collegiate baseball players with 'central ocular dominance' (see OT, July edition, Part 1), whether right-handed or left-handed, were the most successful in both pitching and batting. Pitchers who displayed uncrossed eye/hand dominance patterns were 'distinctly more successful' than those with crossed dominance, whereas batters with crossed dominance were 'slightly more successful' than batters with uncrossed dominance. A high incidence of crossed eye /hand dominance was noted in a large sample (>450) of young, highly skilled basketball players. (7) However, these studies are the exception and a large majority of studies have found no association between eye dominance and hitting, fielding or pitching skill in baseball. (8) Similarly, a study of cricketers found that they were not more likely to have crossed dominance. (9) Part of the reason for such starkly different conclusions as to whether patterns of eye/ hand dominance are associated with greater sporting success may have to do with whether the test used to assess sighting dominance allowed for a diagnosis of central dominance. However, even if it is the case that sighting dominance is somehow linked to sporting skill level, the practical implication of finding that someone displays uncrossed or crossed dominance is probably fairly limited.

This is highlighted by the following statement which Portal and Romano make very prominently at the end of their 1998 article: (6) 'Warning: This information (about success in sport associated with uncrossed and crossed dominance patterns) may be useful in career guidance but does not justify or medically indicate attempts to alter ocular sighting dominance or eye/hand dominance patterns as these are probably genetically determined or at such an early age that they cannot be successfully altered later. Attempts to so alter them are historically fraught with irremediable psychological or physical injury to the subject, including permanent incapacitating double vision.' The warning for eye care practitioners here could not be clearer.

Another baseball study showed that a 0.6 neutral-density (ND) filter placed over the non-dominant eye had little effect on hitting ability but the same filter over the dominant eye had a very significant impact. (10) Interestingly though, in this study, rather than using a test of eye dominance, the eye classed as the dominant eye was retrospectively identified on the basis that the hitting ability was more affected by the filter! There is a certain circularity about the logic here. The results of this ND-filter study were interpreted as evidence that motion-in-depth processing, which is needed for hitting a ball, shares the sensitivity to unequal binocular illumination that is demonstrated by side-ways motion processing (as in the Pulfrich phenomenon). The significance of these findings is again unknown because of course batting takes places under conditions of equal binocular illumination, but the invocation of the Pulfrich phenomenon is interesting because of Woolmer's speculation that one eye 'transmits visual signals to the brain more quickly and effectively than the other' (see Part 1). A number of other studies suggest that dominant-versus non-dominant eye viewing differences include slower fading of retinal imagery (11) and a greater ability to absorb prismatic stress to avoid diplopia. (12)

A recent article by Mann and colleagues examined hand and eye dominance, along with the batting stance in professional and inexperienced cricketers.13 There are a number of findings reported in this study which have direct relevance to the topic of eye dominance, particularly in relation to sport. Firstly, in accordance with results summarised here (see 'Latest research') and in Part 1 of this series, when comparing the agreement between the three tests of eye dominance that they used, the researchers found that it was the hand used during testing which caused most of the variability between the test results. Secondly, the proportion of elite batters who adopted a reverse stance (that is, a stance which was opposite to that expected on the basis of handedness, for example, bat left-handed even though right-handed, was vastly (7.1 times) greater in elite batters compared to the inexperienced batters. On the basis of this finding, the authors speculated that batters may be taught to bat 'back-to-front.' More relevant to the discussion of eye dominance, they also speculated that when a reverse stance is adopted, there is an increased likelihood that the 'front' eye (the eye closer to the incoming ball) will be the dominant eye; since most people are right-handed and right-eye dominant, with a reverse stance, the right eye will be closer to the incoming ball when a left-handed stance is adopted. This would fit with the view of Bob Woolmer about getting the dominant eye closer to the line of sight, as referenced in Part 1. However, whether eye dominance alone, or in part, explains why elite batters much more frequently bat with a reverse stance remains to be determined.

[FIGURE 1 OMITTED]

Eye dominance has been also assessed in putting in golf. One study found that uncrossed dominance, for example, right-eye dominant and right-handed stance, was associated with better performance than crossed eye /stance patterns. (14) However, more recently, Dalton and colleagues compared eye dominance in primary gaze with that in downward gaze when putting. (15)

The results showed that the eye dominance measured in primary and putting gazes were frequently not the same, and that the dominance in putting gaze was not as strong as in primary position; this reported change in eye dominance with gaze angle is consistent with some very recent lab-based studies which will be discussed later. The authors concluded that: 'coaches should assess golfers' ocular [eye] dominance in both primary- and putting-gaze positions to ensure they have the most accurate information upon which to base their vision strategy decisions.'

Significance for monovision and other cases of unilaterally reduced visual acuity

One of the main reasons that eye care practitioners assess eye dominance in practice relates to monovision correction. Typically, part of this assessment involves the introduction of monocular blur. The patient is asked to subjectively compare comfort during right-eye versus left-eye blurring. The implication is that, for example, if the left eye is the dominant eye, the patient will feel less comfortable when the left eye is blurred. The rationale for this assertion is that the patient will find it easier to suppress blur in the non-dominant eye than will be the case in the dominant eye. Another approach is to establish which eye will accept less plus power when a distant target is viewed binocularly. Malott and colleagues speculated that agreement between the dominant eye determined by this test and by sighting dominance enhance the chances of successful monovision adaptation. (16) However, we now know that agreement between such different types of eye dominance tests is often poor (see Part 1), hence this conclusion is questionable and is also at odds with the views of others who find that a strong dominance may be associated with unsuccessful monovision correction (see below).

Ninety-five per cent of monovision patients have their dominant eye corrected for distance, leaving the non-dominant eye in focus for near viewing, (17) and between half and three-quarters of patients can successfully adapt to monovision correction. (18) This high success rate is often attributed to careful determination of which eye is 'dominant.'

The relationship between eye dominance and patient satisfaction with monovision by intraocular lens implantation has also been studied. (19,20) The hole-in-the-card sighting test was used (for a distant target) along with a sensory test that used a binocular rivalry approach (Figure 2). The results were that the strength of eye dominance as assessed by the hole-in-the-card test was an important predictor of patient satisfaction with monovision delivered via an intraocular lens. However, a recent study published in the British Journal of Ophthalmology challenged the importance of determining the dominant eye. (21) The results were not significantly different in monovision patients whose dominant eye was corrected for near vision compared to conventionally corrected monovision patients in whom dominant eye was in focus for distance vision. Quite why such between-study differences exist is far from clear.

Another, recent lab-based study of monovision found that, with both eyes open, contrast sensitivity, but not visual acuity, varied with the optical quality in the dominant eye, where dominance was determined using a sensory test. (22) The conclusion was that when assigning the dominant eye to near or distance viewing during the monovision prescribing process, it may be important to optimise sensitivity to contrast at different object distances. Although this study has only recently emerged, its results suggest something quite different, namely that practitioners should rely more on contrast sensitivity measures than on visual acuity when making decisions about how to deliver the monovision correction.

In multifocal contact lens fitting, establishing which eye is dominant is also advised. This is important in instances when different strengths of near additions are being prescribed (usually it is suggested that the higher near add should be given to the non-dominant eye) or when patients have a particular need to enhance either their distance or near vision.

In clinical settings, the lack of agreement between the results of the various tests of eye dominance (see Part 1) is not a reason to avoid testing for dominance. In clinical practice relating to monovision, rather than using tests of eye dominance to establish which eye is dominant, it has been suggested that tests should be used to find the individuals with strong or clear dominance; this is because they are the patients who appear most at risk of unsuccessful outcome with monovision correction. (17)

A small number of studies have examined whether unilateral pathology affecting the dominant eye is more likely to result in the patient presenting for investigation, compared to when the non-dominant eye is affected. One study found that this was indeed the case, (23) but another did not, (24) and so the evidence on this topic, like many others in this field, is somewhat inconclusive.

Latest research

The results of recent studies that could have major significance for our understanding of eye dominance are now summarised.

Dieter and Blake questioned whether, rather than having a dominant eye, people may instead have regional differences in sensory eye dominance (SED). (25) This is akin to saying that the right eye may be the dominant eye for objects imaged in certain retinal locations, but the left eye is dominant when the object is imaged in a different retinal location. How was this finding established? They used a binocular rivalry technique to map patterns of SED in observers' visual fields. Because a dichoptic means for presentation was used and dissimilar objects were presented to the two eyes, observers stated which of the two stimuli (the right or the left eye's) they perceived from instant to instant and as the monocular stimuli moved into different visual field locations.

From this the researchers were able to tell which eye was dominant. The authors found that the strength of sensory eye dominance could be judged by varying the contrast of the images presented to the two eyes. For example, if participant perceives the right eye's image when dissimilar objects located in a particular visual field location are viewed, this suggests that the right eye is dominant for that region of space. However, if the contrast of the right eye's image is reduced sufficiently, there will come a point when there is equi-dominance of the two eyes (the dissimilar images are equally likely to be perceived) (see Figure 1). The amount of balancing contrast required by the weaker eye to equate dominance of the two eyes indicates the magnitude of sensory eye dominance imbalance. The results from this lab-based psychophysical study have direct clinical relevance because they show how the eye which is dominant depends on the location of the information in the visual field and is not tied directly to a single eye as suggested by simple, clinical tests of eye dominance. The significance of this lab study is heightened by the fact that there have been a small number of reports of the gaze dependency of sighting eye dominance in the clinical literature, (26) including the study of eye dominance during putting mentioned earlier. (15)

[FIGURE 2 OMITTED]

While the study above was of sensory eye dominance, recent studies are also beginning to show that sighting dominance may not be the robust trait that was once thought to be the case. Khan and Crawford conducted a simple but elegant study in which a semi-circular array of eye-level target stimuli were viewed. (27) They were positioned at 10 degree intervals from 50 degrees on the left of midline to 50 degrees on the right, and clearly numbered from 1 to 12. Above each target was a hook upon which a wooden ring was placed in a manner that the target number was visible through the centre of the ring. Participants viewed the array binocularly. Head movements were minimised using a chin rest and participants were asked to keep the head directed straight ahead. When a particular number was called out by the experimenter, the task for the participant was to reach out, grasp and transport the ring so that the target number remained continuously visible to the participant within its aperture as they moved the ring towards the face. As the ring was moved towards their face, sighting was therefore from a single eye. Khan and Crawford's results were that eye preference reverses as a function of horizontal gaze angle, and remarkably, that the reversal of sighting from one eye to the other can depend on which hand is used to reach out and grasp the target. (27) These results have been confirmed and extended by Carey and Hutchinson. (28) As with the results from Dieter and Blake, (25) these findings seriously challenge the notion that there is a constant, dominant eye and suggest that the concept is much more fluid and circumstance-dependent.

Conclusion

There is a popular belief that eye dominance has direct relevance in sport and, while this is clearly established in sports where only one eye is used such as shooting and archery, the relevance in sporting situations when viewing is habitual (that is, with both eyes open, as when playing golf or cricket) is not well established. It is important for clinicians to try to determine which eye should be in focus for distance vision during monovision correction and, if possible, to identify those unsuitable for this form of refractive correction. However, the tests suitable for this purpose often give inconclusive results and appear to offer little more than would be gained by trial and error, and establishing which arrangement the patient finds most comfortable. Individuals with the strongest eye dominance may be less suitable for monovision correction. Our knowledge of the significance of eye dominance is growing and long-held ideas that there is a constant, dominant eye may not now be correct. This is not to say that eye dominance has no relevance. Instead the emerging picture appears to be that the concept of eye dominance is fluid because the eye which is dominant shows regional differences (that is, depends on location in the visual field) and also appears highly dependent on additional factors such as gaze direction and the hand being used.

Exam questions

Under the enhanced CET rules of the GOC, MCQs for this exam appear online at www.optometry.co.uk. Please complete online by midnight on 9 September 2016. You will be unable to submit exams after this date. CET points will be uploaded to the GOC within 10 working days. You will then need to log into your CET portfolio by clicking on 'MyGOC' on the GOC website (www.optical.org) to confirm your points.

References

Visit www.optometry.co.uk and click on the CET tab. Click on the 'Related CET article' title to view the article and accompanying 'references' in full.

Course code: C-51889 Deadline: 9 September 2016

Learning objectives

Optometrists

* Be able to explain to patients about the relevance of eye dominance (Group 1.2.4)

* Understand the clinical importance of eye dominance for monovision contact lens patients (Group 5.3.2)

* Understand the clinical relevance of eye dominance (Group 8.1.1)

Dispensing opticians

* Be able to explain to patients about the relevance of eye dominance (Group 1.2.4)

* Understand the clinical relevance of eye dominance (Group 7.1.6)

Contact lens opticians

* Be able to explain to patients about the relevance of eye dominance (Group 1.2.4)

* Understand the clinical importance of eye dominance for monovision contact lens patients (Group 5.5.2)
Optometrists               COMMUNICATION   CONTACT      BINOCULAR
                                           LENSES       VISION

Dispensing opticians       COMMUNICATION   REFRACTIVE
                                           MANAGEMENT

Contact lens opticians     COMMUNICATION   CONTACT
                                           LENSES


Professor Brendan Barrett PhD

* Professor Brendan Barrett is professor of visual development at the University of Bradford where he is the module leader for refraction and refractive error and also teaches on the binocular vision module. He is currently an external examiner for the undergraduate ophthalmic dispensing programme at Glasgow Caledonian University and the undergraduate optometry programme at Aston University. His research interests include amblyopia, vision and sport, and vision and reading.
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Author:Barrett, Brendan
Publication:Optometry Today
Geographic Code:4EUUK
Date:Aug 1, 2016
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