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DISCUSSION
The data considering the relationship between handedness and eye dominances were inconsistent. Dominant eye and hand preference were stated to be related significantly in some studies[5,9,16]. On the other hand there are also statements concerning the absence of any significant relation between eye dominance and hand preference in the literature[4].
Methodological diversity concerning these studies which was the main target of the present study seems to be a devastating feature leading to inconsistent results[12,16,1922]. In the present study determined ratios for left handedness was 8.4% by the Oldfield test and it was found 8.9 % by the McManus test and right handedness was 91.6% by the Oldfield test and it was found 91.1% by the McManus test. The detection of handedness in the present study correlates with our previous findings and the literature in terms of right hand preference [1315,21]. The ratio for left eye dominance was found 38.5% by the Gündoan test and it was found 28.5% by McManus test for the same subject. The right eye dominance was found 61.5% by the Gündoan test and it was found 71.5% by the McManus tests which were correlated with our previous findings and the literature in terms of high right eye dominancy[6,8,9,16,22]. These results indicate the reliability of the methods used in the present study for determination of eye dominance and handedness. In the present study consistent left handleft eye dominance ratios of 46.7 %, 56.2 %, 50.0 % 46.7% and right handright eye dominance ratios of 62.2%; 74.2%; 62.6%; 73.2% as shown in Table 6 were found compatible with the previous reports stating 61.0%; 67.6%; 22.0%; 50.0%; 46.6% for the left and 85.0%;85.5%;83.5% for the right, respectively[1,57,22]. Inconsistent handeye preference ratios of 53.3%;43.8%; 50.0%; 53.3% for left handright eye and of 37.8%; 25.8%; 37.4%; 26.8% for right handleft eye were also compatible with the literature considering 67.0%; 50.0%; 53.3%; 40.0% for left handright eye and 11.0%; 10.2%; 10.1%; 53.2 % for right hand left eye [1,57,22]. The discrepancy concerning the incidence of left hand but right eye dominant subjects observed with McManus tests seems to be related with the previous statement considering individuals who were lefthanded for writing but were righthanded for throwing known as inconsistent lefthanders representing 28.8% of lefthanders [2325]. Eyedness was concluded to be more likely related to throwing than to writing hand in the literature. The eye is of fixed mass and in a fixed orbit so that it can be moved remarkably precisely and predictably by such a ballistic mechanism; but that apart, the mechanism is in principle no different from throwing a ball with a hand. If correct, then eyedness should show a clearer and more straight forward relationship with the throwing hand than with the writing hand. In passing, it might be noted that the frontal eye fields, which are important for the control of voluntary saccades, are located only about 20 mm anterior to the sensory motor cortex for the hand [26].
In a previous study of Manus et al[16] 28.8% of lefthanded writers and 1.6% of righthanded writers showed inconsistent handedness, preferring to throw with the hand contralateral to their preferred writing hand. These proportions are compatible with the previously reported figures of Gilbert et al[27] in a massive survey of over one million individuals where, considering the 251881 males and 336483 females who were in the age range 10±40 years in which the incidences of handedness did not vary with age, 32.4% of male and 37.5% of female lefthanded writers threw with their right hand, and 1.55% of male and 1.05% of female righthanded writers threw with their left hand; and with the smaller study of Grimshaw et al[28] where 35.4% of 322 lefthanded writers and 2.25% of 1640 righthanded writers were inconsistent for throwing. We can therefore conclude that crossed writing and throwing hand is a reliable phenomenon in a minority of both lefthanders and righthanders. In the present study, inconsistency considering handedness was 0.5% in left handers and 0.05% in right handers indicating the homogeneity. The Rosenbach[17] and Miles[4] methods were modified by the Gündoan et al[11]which restricted eye, head and hand movements during test period. Many forms of assessment in the measurement of lateralization of eye dominance were contaminated by handedness: thus, for instance, observing the eye used to sight down a rifle may produce a spurious association with handedness due to the rifle typically being held with a finger of the dominant hand on the trigger.
Similarly, if a subject is asked to look through a small object such as kaleidoscope, held in one hand, the holding of the object in dominant hand may well distort measures of association between handedness and eye dominance [9]. Additionally eye dominance has sometimes been assessed using questionnaires concerned with monocular activities restriction [29,30].
In our study, we applied individual performance test according to Gündoan method for dominant eye assessment without using monocular activities. During this test hand movements were restricted so that eye dominance was not contaminated by dominant handedness. Dominant eye which was stated to be altered according to visual field is being left or right side of the target point restriction [31]. The restricting head movements we prevented shifting visual field during dominant eye assessment. Moreover, when we asked the subjects to close one of their eyes, the deviation distances between near (N) and far (F) points was smaller than in the previous study of Baykal[6]. In addition, deviation distance between N and F points results of our study were more homogenous than Baykals[6] findings. The homogenous measurement of our study seems to confirm the reliability of our method. Right handednessright eye dominance was higher in the majority of our subjects indicating left cerebral hemisphere dominance. Dominant eye and hand preference was shown to be related significantly when the McManus method was applied for determinacy of eye dominances and hand preference (P<0.05, Table 4). This finding correlates with the literature in that the throwing hand is independent from training just like the dominant eye [16].
In conclusion, the dominant eye seems to be significantly related to the throwing hand more consistently than the writing hand. Our results support the literature in terms of reflection of life long functional cerebral asymmetry of the brain also via throwing hand besides the dominant eye as it was previously mentioned by other authors[5,15,16]. Future studies investigating the role of inheritance to model properly the interactions between the writing hand the throwing hand, and eye dominance seem to be mandatory.
Acknowledgments
Do Dr. Ersin ü�瘙塂, Yrd. Do. Dr. Canan Yazc for their excellent statistical studies, Bio.Ayten im�瘙塂ek, Fatih zkul for his technical support during design of the eye dominance measurement device, and our university students for their voluntary participation to our reseach programme and to our medical research group students as Alp Tuna Beksac, Mehmet Emre G naydin, Sema Kurban and Isik Ocak for their excellent assistance during data collection.
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