Supplementary motor area activation while tapping bimanually different rhythms in musicians.

In 15 musicians, cortical DC-potentials were recorded from the scalp before and during the execution of bimanual motor sequences. Subjects (Ss) either tapped with their two index fingers in synchrony (quavers against quavers; "2 against 2") or they tapped quavers against triplets ("2 against 3"). Either the right or the left finger started tapping the quavers (onset time t1), after about 4 s the other finger joined in (t2) either with quavers as well (easy rhythm) or with triplets (difficult rhythm). Ss were free to start the sequences, i.e. to determine the onset times t1 and t2. Shifts of cortical DC potentials were averaged twice; (1) time-locked to t1 and (2) time-locked to t2. When moving in synchrony (easy rhythm) DC-potential shifts and maps of radial current densities across the scalp indicated activations of the two primary motor cortices (MI). When bimanually tapping different rhythms, there was not only an activation of MI cortices, but in addition a very large activation of the mesial, central cortex was observed. It is suggested that this cortical area which mainly contains the supplementary motor area (SMA) has the function of controlling the initiations of movements in the difficult sequence which have to fit into a very precise timing plan. Interestingly, activation of the mesial, central cortex preceded the actual performance of the difficult rhythm by about 4 s. This finding indicates that the preparatory set differs between the two tasks.