Hemispheric asymmetries and bimanual asynchrony in left- and right-handers.

It is known that, when both forearms are rotated rhythmically and symmetrically, the dominant hand leads in time by about 25 ms, irrespective 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 the motor and premotor motor areas are more active in the left than in the right hemisphere. The converse pattern was observed in left-handers. The results suggest that at least some components of the neural processing involved in bimanual coordination are carried out only in the hemisphere contralateral to the dominant hand. In particular, between-hands asynchrony may reflect the time for dispatching pace-setting commands to the contralateral hemisphere.