Effects of handedness and script types on visual-field differences in the mental addition task.

The effects of handedness and script types on the difference in performance in a mental addition task by visual field were examined. Right-handers, nonfamilial left-handers, and familial left-handers who were all native speakers of Japanese were asked to add two numbers presented in the visual half-fields tachistoscopically. The two numbers were displayed either to one visual field (left or right visual field) or to the center. The numbers were displayed in Arabic, in Kanji, or in Arabic and Kanji numerals (one in Arabic and the other in Kanji: Mixed stimuli). The subjects were asked to add the two numbers and to state the sum orally. In the right-handers group, a right visual-field advantage was found for the Arabic condition but not for the Kanji or Mixed stimuli. On the other hand, in the nonfamilial and familial left-handers group, no visual-field difference in any of the conditions was found. These findings suggest that pattern of cerebral lateralization for familial and nonfamilial left-handers is the same but it is different from that of right-handers.