Finger movement versus toe movement-related potentials: further evidence for supplementary motor area (SMA) participation prior to voluntary action.

The cerebral potentials associated with voluntary, self-paced rapid flexions of (1) right fingers, (2) left fingers, (3) right toes, and (4) left toes were compared in the same experiment using 32 right- and left-handed subjects. The Bereitschaftspotential (BP) or readiness potential was, in the first half of the foreperiod, bilaterally symmetrical for both finger and toe movements of either side. In the later foreperiod there were differences: Finger movements showed two maxima, an early one at Cz and a late one, which was lateralized toward the contralateral precentral region. With toe movements, the maximum BP amplitude was always at Cz and not lateralized and was twice as large as with finger movements. The data are compatible with the view that two principal sources of different spatial and temporal characteristics are active in the foreperiod of a voluntary movement. The early generator is probably the supplementary motor area (SMA) on the mesial surface of the hemispheres; the later is the primary motor cortex (MI) which is lateralized for finger but not for toe movements. In lateral leads, rather remote from the mesial source, the BP for toe movements showed a small but significant ipsilateral preponderance, which is obviously due to the fact that dipole sources located on the mesial surface of the hemispheres point to the opposite direction as compared to those on the convexity.(ABSTRACT TRUNCATED AT 250 WORDS)