Abstract: Tachistoscopic visual half-field test was used to examine effects of an early perceptual factor of stimuli on the distributing processing across two hemispheres. An odd-even conceptual decision task on pairs of numerals in capitals of Chinese characters was given to subjects involving three different eccentricities( 3.5°, 5° and 6.5° ) of comparative numerals in order to vary the perceptual processing demands of the stimuli. The results showed that the reaction times were slower and the percentage of errors higher while increasing eccentricity of the comparative stimuli. There was a superiority of correct reaction speed for RVF(LH) presentation, regardless of eccentricities and responding hands. A comparasion of the left and the right hand indicated that response with the left hand was more rapid than with the right hand at 6.5°, and response with the right hand was more rapid than with the left hand at 3.5° and 5°. These results suggest that the stimulus and response processing for this task tend to be distributed across the two hemispheres at 6.5°, and operated at 3.5° and 5° by the left hemisphere independently. The effect of distributing processing appeared at 6.5° could result from more difficult perceptual and attentional demands of the comparative stimuli.

Key words: task difficulty, distributing processing, tachistoscopic visual half-field test, eccentricity