Cortical and Subcortical Processing of Color |
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asymmetric processing in color detection, further studies should be done including recording of cortical activity during color detection task, and using methods with high time resolution.
2.4. Experiment 1-3: Hemispheric Asymmetry of Color Detection in LeftHanded Individuals
2.4.1. Purpose
In this experiment, the hemispheric difference of color processing was evaluated by comparing reaction times of the right and left hands to chromatic stimuli presented to the ipsilateral visual field of left-handed individuals.
2.4.2. Methods
2.4.2.1. Participants
Eight left-handed mail subjects (7 undergraduate students and one researcher) with normal or corrected-to-normal vision (mean age 25.1 years, SD 9.3) participated in this experiment. All of these left-handed participants were selected based on an assessment of the handedness score using a modified Edinburgh Laterality Inventory (Oldfield, 1971), which included 7 items: write, eat, throw, tooth brushing, drive, key, and hammer. For each item, right-handed responses were scored as 0, left-handed ones as 1, or both hands as 0.5. The total score of the left-handed participants ranged from 2.0 to 7.0 (4.6±1.6, mean ± SD), while that of the right-handed ones was 0.
2.4.2.2. Apparatus and Procedures
The experimental apparatus and procedures for this experiment were similar for the Experiment 1-1 and 1-2, excepting that color stimuli were used to examine the effect of dominant hand on the simple reaction time.
2.4.3. Results
Figure 9 shows that there was no significant difference between the simple reaction times by dominant and non-dominant hands in the left-handed participants.
There was no significant difference between simple reaction times in R-R and L-L conditions among the left-handed participants (Fig. 10 right, R-R 325±5 ms, mean ± SE; L-L 324±9 ms, mean ± SE; t(7) = 0.179, N.S.). However, it might be assumed that right hemisphere is more specialized in color detection among the left-handed individuals with low left-handedness score, while symmetrical processing occurs among the typical left-handers with high score. In order to ascertain this possibility, we performed the correlation analysis. No significant correlation between the handedness scores and reaction time differences (R-R
– L-L) was observed r = 0.006, p = 0.990, N.S. .
2.4.4. Discussion
A more symmetrical hemispheric processing was observed among left-handed individuals compared to right-handed individuals. It is well known that language cerebral dominance is lateralized in the left hemisphere in 88-96% of right-handed individuals and in
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43-76% of left-handers (Pujol et al., 1999; Springer et al., 1999; Khedr et al., 2002). It has been suggested that the hemispheric specialization of brain functions is less clear among lefthanders than among right-handers (Zangwill, 1962, Bryden et al., 1982). Consistent to this concept, the present study showed that color detection was less clearly lateralized among the left-handed participants than among the right-handed participants
Figure 9. Simple reaction times to target stimuli presented at the center of visual field, responded by the right hand (R-hand, non-dominant) and the left hand (L-hand, dominant) in 8 left-handed participants. There was no significant difference between reaction times by dominant and non-dominant hands. Mean with SE.
Figure 10. Simple reaction times to lateralized stimuli, responded by the right hand to targets presented in the right visual field (R-R), and by the left hand to targets presented in the left visual field (L-L) in 8 left-handed participants. Target stimulus was achromatic (left, 20 cd/m2) or chromatic (right). There was no significant difference between reaction times in R-R and L-L both to achromatic and chromatic stimuli. Mean with SE.