Exp Brain Res (1998) 120:531±536  Springer-Verlag 1998 RESEARCH NOTE Paolo Viviani ´ Daniela Perani ´ Franco Grassi Valentino Bettinardi ´ Ferruccio Fazio Hemispheric asymmetries and bimanual asynchrony in left- and right-handers P. Viviani Department of Psychobiology, FAPSE, University of Geneva, Geneva, Switzerland P. Viviani University Vita e Salute Scientific Institute H.S. Raffaele, Milan, Italy D. Perani ( ) ) ´ F. Grassi ´ V. Bettinardi ´ F. Fazio Institute of Neuroscience and Bioimaging CNR, University of Milan, Scientific Institute H.S. Raffaele, via Olgettina 60, I-20132 Milan, Italy e-mail: danielap@mednuc.hsr.it, Tel: +39-2-2643-4886, Fax: +39-2-2643-4892 Abstract It is known that, when both forearms are rotat- ed 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 func- tional 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 compo- nents of the neural processing involved in bimanual coor- dination are carried out only in the hemisphere contralat- eral to the dominant hand. In particular, between-hands asynchrony may reflect the time for dispatching pace-set- ting commands to the contralateral hemisphere. Key words Positron emission tomography ´ Bimanual coordination ´ Asynchrony ´ Hemispheric dominance ´ Motor cortex Introduction Most humans exhibit some degree of handedness, that is, a preference to use one hand for tasks requiring dexterity, precise calibration of forces, and accurate timing. For an- atomical reasons, this preference has implications con- cerning the cortical control of movement. Although prox- imal forelimb muscles receive some activation from the ipsilateral hemisphere, uncrossed fibers in the lateral cor- tical spinal tract do not exceed 15% of the total output (Brinkman and Kuypers 1972, 1973; Brodal 1992). More- over, afferent information from the peripheral receptors is also channelled predominantly to the contralateral hemi- sphere (Ruch 1965). The principle of contralateral control applies even more strictly to the distal segments of the forelimbs. Mapping the human brain with functional mag- netic resonance imaging showed that the activation of the contralateral motor cortex during distal hand movements is 20 times stronger than the activation of the ipsilateral cortex (Kim et al. 1993a). Thus, handedness must reflect a functional hemispheric asymmetry in some of the mod- ules responsible for controlling hand movements. The nature and extent of this asymmetry (McManus 1983; Annet 1985; Geschwind and Galaburda 1987) and its relation to hemispheric dominance (Kim et al. 1993b) have long been debated, the most extreme hypoth- esis being that movement plans are generated only in the dominant hemisphere, and that a copy is then sent trans- cortically to the non-dominant one (Liepmann 1908; Heilman et al. 1973; Geschwind 1975). Recently, this view received support from a behavioral study (Stucchi and Viviani 1993) showing that, when hands are simulta- neously engaged in rhythmic identical movements, the dominant hand leads the non-dominant one by about 25 ms, irrespective of movement speed. The presence of such an asynchrony is in keeping with the hypothesis of an inter-hemispheric transfer of timing information. How- ever, kinematic analysis alone is insufficient to rule out other interpretations. In particular, between-arm asyn- chrony may reflect a difference in the rate at which the two hemispheres process timing signals issued by a single pace-keeping module. We report a study in which posi- tron emission tomography (PET) was used to test whether the asynchrony during bimanual movements is indeed as- sociated with a different pattern of hemispheric activation in right- and left-handers.