Intern. J. Neuroscience, 112:517–523, 2002 Copyright © 2002 Taylor & Francis 0020-7454/02 $12.00 + .00 DOI: 10.1080/00207450290025626
RIGHT AND LEFT VISUAL CORTEX AREAS IN HEALTHY SUBJECTS WITH RIGHT- AND LEFT-EYE DOMINANCE
A. RIZA ERDOGAN
Department of Anatomy Atatürk University Erzurum, Turkey
Department of Radiology Atatürk University Erzurum, Turkey
M. DUMLU AYDIN
Department of Neurosurgery Atatürk University Erzurum, Turkey
ÖMER AKTAS ¸ SENOL DANE ¸
Department of Physiology Atatürk University Erzurum, Turkey
The aim of this work was to study the differences between the right- and left-visual cortices in relation to eyedness in healthy subjects. Ocular dominance was determined by means of the near-far alignment test. To assess visual cortical areas, the right and left sagittal scenograms of cranium by
Received 10 January 2002. Address correspondence to Prof. Dr. Senol Dane, Department of Physiology, Medical School, Atatürk University, Erzurum, Turkey. E-mail: email@example.com.
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magnetic resonance imaging were used. To calculate the visual cortex areas by using scenograms, Cavalieri’s method was used. In the subjects with right-eye dominance, the right visual cortex was larger than the left visual cortex, and vice versa in the subjects with left-eye dominance. The right and left cuneal areas were found to be larger in males than in females. In light of these results, it was concluded that the human eyes are predominantly controlled by the ipsilateral visual cortex. Keywords eye dominance, hand preference, men, visual cortex, women
In humans, the right cerebral hemisphere is dominant in visuospatial and nonverbal functions such as art, architecture, geometry, and mathematics, whereas the left cerebral hemisphere is dominant in verbal functions such as rhetoric, literature, and poetry (Geschwind & Behan, 1982; Springer & Deutsch, 1998; Gur et al., 1999). Although two eyes with overlapping visual fields are required for stereovision (Howard & Rogers, 1995), this arrangement complicates the selection of a unique egocentric reference point for vision and action (Flanders, Helms-Tillery, & Soechting, 1992; Milner & Goodale, 1995). Despite methodological quibbles, the classic sighting literature agrees that most subjects show consistent ocular dominance in such tasks (Miles, 1930; Crider, 1944; Walls, 1951; Coren & Kaplan, 1973; Porac & Coren, 1976; Baykal, Dane, Akar, Çolak, & Pençe, 1995; Osburn & Klingsporn, 1998), with 53% to 85% of the population preferring the right eye and 12% to 40% preferring the left eye (0% to 22% showing no preference). Rombouts, Barkhof, Sprenger, Valk, and Scheltens (1996) reported that the dominant eye actually activates a larger area of the primary visual cortex than the nondominant eye. In addition, it has been reported that the cuneus (a part of visual cortex) of the right cerebral hemisphere had more sulci than that of the left and the cuneus had also more sulci in males than in females (da Veiga & Prates, 1993). In the present study, the magnitude of the right and left visual cortex areas in healthy subjects having right- and left-eye dominance was investigated.
MATERIALS AND METHODS Subjects were 11 men and 7 women, 18 to 22 years (M = 19.77 years, SD = 1.52), 11 right-handed and 7 left-handed. Hand prefer-
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ence was assessed using the Edinburg Handedness Inventory (Oldfield, 1971). Subjects having handedness scores smaller than zero were considered to be left handed; those with handedness scores greater than zero were considered to be right-handed (see Tan, 1988a, 1988b). All the right-handers had right-eye dominance and all the left-handers had left-eye dominance. Others were not included in the study. Ocular dominance was determined by means of the near-far alignment test (Rombouts et al., 1996). In this test the subject is asked to hold a pencil in one hand directly in front of him- or herself. Then, the subject is asked to align the tip with a point on a distant wall with both eyes open. The subject is then asked alternatively to close one eye. Only when the dominant eye is open and the other eye closed will the tip of the pencil remain in good alignment with the point on the wall. This test is repeated with the pencil in the other hand. When in doubt, a variant of Miles test (Miles, 1930) was administered, in which the subject is asked to focus on a point on a distant wall through a hole formed by the opposed thumb and index finger of one hand. Then the hand is moved toward the face while focusing on the same point, until one of the eyes is reached, which indicates the dominant eye. To assess visual cortex areas, the right and left sagittal scenograms of cranium by magnetic resonance imaging were used. For the right and left sagittal scenograms, the slices distance of 7.5 mm to right and left from mid-sagittal plan were used. To calculate the visual cortex areas, Cavalieri’s method was used. A clear plastic template imprinted with a uniformly spaced point grid was placed over the images, and the number of points that fell on the object were counted. Grids of three sizes were used with absolute point separations of 4, 2, and 1 mm (the majority of measurements were made with the 4- and 2-mm grids). The appropriate grid was selected so that 100 to 200 points were counted in toto for each scan. The area associated with each point in the grid (equivalent to the area contained between four points) was measured using the centimeter ruler appearing on each scan. This was easily accomplished by counting the number of points along a line in the grid that covered an appropriate distance along the centimeter ruler (such that an integer number of points and a whole number of centimeters were counted) and arriving at the linear distance (in centimeters) between consecutive points. This distance was squared to arrive at
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the area associated with each point. The sum of the points that fell on the object was then multiplied by the area associated with each point to arrive at the area of the object (Clatterbuck & Sipos, 1997; Gundersen et al., 1988). For statistical evaluation, one-sample analyses for the significance of the difference between sets of paired data in the SPSS 9.0 for Windows program were used.
RESULTS Table 1 shows the descriptive analyses (mean and SD) for measurements of the right and left cuneus regions for sex and eye dominance groups. The right-left difference in the cuneus areas (cm2) of visual cortex was not present in the total sample, men, and women (t = 1.75, p = .09; t = 0.73, p = .49; t = 1.58, p = .15, respectively). In the right-handers (or subjects having right-eye dominance), the right cuneus area (cm2) of visual cortex was significantly larger than the left cuneus area (t = 6.95, p = .00) and vice versa in the lefthanders (or ones having left-eye dominance) (t = 2.92, p = .03). Table 2 shows the descriptive analyses (mean and SD) for measurements of the right and left sagittal areas of gyrus occipitotemporalis medialis for sex and eye dominance. The right-left difference in the sagittal area of gyrus occipitotemporalis medialis areas (cm2) of visual cortex was not present in the total sample, men, and women (t = 0.33, p = .75; t = 0.04, p = .97; t = 0.38, p = .71, respectively). In the right-handers (or subjects having right-eye dominance), the right-sagittal area of gyrus occipitotemporalis medialis (cm2) of visual cortex was larger than the left sagittal area
TABLE 1. Descriptive analyses (mean and SD) for measurements of the right and left cuneus regions for sex and eye dominance groups. N Total sample Men Women Right-handers Left-handers 18 7 11 11 7 Right 8.8 10.21 7.91 9.12 8.31 ± ± ± ± ± 1.66 0.82 1.41 1.43 1.97 Left 8.05 9.74 6.97 7.16 9.46 ± ± ± ± ± 2.06 1.1 1.79 1.87 1.55 t 1.75 0.73 1.58 6.95 2.92 p .09 .49 .15 .00 .03
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TABLE 2. Descriptive analyses (mean and SD) for measurements of the right and left sagittal areas of gyrus occipitotemporalis medials for sex and eye dominance groups N Total sample Men Women Right-handers Left-handers 18 7 11 11 7 Right 6.6 7.13 6.27 7.38 5.39 ± ± ± ± ± 1.67 1.81 1.57 1.56 1.01 Left 6.75 7.16 6.49 6.24 7.56 ± ± ± ± ± 1.59 1.73 1.52 1.65 1.16 t 0.33 0.04 0.38 5.33 3.9 p .75 .97 .71 .00 .01
of gyrus occipitotemporalis medialis (t = 5.33, p = .00) and vice versa in the left handers (or ones having left-eye dominance) (t = 3.9, p = .01). There were also sex differences in cuneus. The areas of both right and left cuneus were larger in males than in females (t = 3.89, p = .001; t = 3.62, p = .002, respectively). There was no statistically significant male-to-female difference in both right and left sagittal area of gyrus occipitotemporalis medialis (t = 1.06, p = .3; t = 0.87, p = .4, respectively).
DISCUSSION It was reported that in the right-handed population, the number of persons with right-eye dominance is greater than the number of persons with left-eye dominance, but this was opposite in the lefthanded population (see Annett, 1982). Annett and Turner (1974) reported that the left eye is dominant in 36.1% of pure right-handers, 23.3% of mixed right-handers, 67.6% of pure left-handers, and 55.4% of mixed left-handers. Merrell (1957) reported that 29% of 464 right-handed subjects and 61% of 33 left-handed ones had lefteye dominance. Baykal et al. (1995) reported that 85% of the right-handed subjects had right-eye, 11% had left-eye, and 4% had both-eye dominance, whereas 67% of the left-handed subjects had right-eye, 22% had left-eye, and 11% had both-eye dominance. These studies have shown that there is a weak but definite relation between hand preference and eye dominance. Rombouts et al. (1996) reported that the dominant eye actually activates a larger area of the primary visual cortex than the nondominant
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eye, but the relations of activities or magnitudes of the right and left visual cortex with right-handedness and left-handedness and with the right- and left-eye dominance are unknown. In the present study, in the right-handers (or subjects having righteye dominance), the right cuneus area of visual cortex was larger than the left cuneus area and vice versa in the left-handers (or ones having left-eye dominance). In the right-handers (or subjects having righteye dominance), the right sagittal area of gyrus occipitotemporalis medialis of visual cortex was larger than the left sagittal area of gyrus occipitotemporalis medialis and vice versa in the left-handers (or ones having left-eye dominance). That is to say, the right visual cortex was larger in the right-handers and the left visual cortex was larger in the left-handers. In addition, both right and left cuneus were larger in males than in females in the present study. This may be due to a larger cerebral area in women than men (Tan et al., 1999). However, the results of the present study are consistent with a previous study (da Veiga & Prates, 1993). They reported that the cuneus of the right cerebral hemisphere had more sulci than the left and the cuneus had also more sulci in males than in females. Consequently, it may be concluded that the human eyes are predominantly ipsilaterally controlled by the cerebral visual cortices.
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