Three experiments which aimed to examine the effects of familiarity and task demand on interhemispheric cooperation were conducted to seek a suitable task for the assessment of callosal function. In Experiment 1, subjects were asked to add the two numerals (written in Arabic or Kanji) which were presented in unilateral, bilateral and central visual fields. The results showed weak tendency towards a bilateral advantage which do not surpass central vision. In Experiment 2, the effect of task familiarity on bilateral advantage was examined with the same stimuli in Experiment 1. The results showed no significant variation of bilateral advantage in both the case of Arabic and Kanji numerals. In Experiment 3, the effect of task demand on bilateral advantage using new type of stimuli was examined. The results showed that strong stable bilateral advantage was emerged in more demanded task (add two numerals, each written in different scripts, Arabic and Kanji). Based upon these findings, the relation between task demand and bihemispheric cooperation was discussed from the view points of the development of a measure for the assessment of callosal function.

Key words: interhemispheric cooperation, mental addition, bilateral advantage, task demand