HEMISPHERIC ASYMMETRIES AND BIMANUAL ASYNCHRONY IN LEFT-HANDERS AND RIGHT-HANDERS

It is known that, when both forearms are rotated rhythmically and symm etrically, the dominant hand leads in time by about 25 ms, irrespectiv e of movement speed. Positron emission tomography was used to test the hypothesis that the asynchrony results from a functional hemispheric asymmetry. We found that in normal, adult right-handers portions of th e motor and premotor motor areas are more active in the left than in t he right hemisphere. The converse pattern was observed in left-handers . The results suggest that at least some components of the neural proc essing involved in bimanual coordination are carried out only in the h emisphere contralateral to the dominant hand. In particular, between-h ands asynchrony may reflect the time for dispatching pace-setting comm ands to the contralateral hemisphere.