Supplementary motor area in spatial coordination of bilateral movements: a new aspect to 'the SMA debate'?

To test whether the supplementary motor area's (SMA) role is confined to determining the 'temporal' but not the 'spatial' properties of a movement (H.H. Kornhuber et al., in: W.A. Hershberger (Ed.), Volitional Action, Elsevier, Amsterdam, 1989, pp. 107-168), movement-related scalp-recorded negative DC potential shifts were recorded in bilateral movements requiring complex spatial coordination. In such bilateral continuous rotation movements, the effect of the rotation sense (symmetrical vs. antisymmetrical), i.e. the direction in which an arm or a finger rotated in relation to the other, heavily affected DC shifts over the frontocentral midline. Antisymmetrical rotation of upper limb segments was associated with higher negative DC shifts than symmetrical rotation was. This was true for rotations in the sagittal plane, irrespective of whether the rotation involved predominantly proximal muscles (by a rotation predominantly in the shoulder) or only distal muscles (by a rotation in the metacarpo-phalangeal joint of the index finger). If these negative cortical DC-shifts over the frontocentral midline relate to activity of mesial frontocentral structures including the SMA, then the present results suggest that there is a role for these cerebral areas in spatial coordination of bilateral movements. Surprisingly, this was not the case for similar finger movements performed in the frontal plane. The results of the present study and particularly the considering of some fundamentals of theoretical physics and of Popper's philosophy of science, made us revise our assumption motivating the present study, that time and space would represent two orthogonal factors of a movement and that the contributions of a particular cerebral motor area (such as the SMA) to 'spatial parameters' versus 'temporal parameters' of a movement can thus be teased apart.