Linear estimation discriminates midline sources and a motor cortex contribution to the readiness potential

Spatiotemporal dipole modelling of the generators of the readiness potential (RP) prior to voluntary movements has yielded diverging results concerning the contributions of supplementary motor area (SMA) and primary motor cortex. We applied an alternative approach (i.e, linear estimation theory) to measurements of the RP preceding fixed and freely selected finger movements, measured at 28 electrodes of the extended 10-20 system. The volume conductor properties of the head were modelled by 3 concentric spheres. Current densities were reconstructed on a spherical surface, placed at a depth of 5 mm from the inside of the skull. Lead field normalization was applied. The analysis shows activity on the midline as well as near the primary motor area. Although some features of the reconstructions are not readily interpretable, separate contributions of midline sources (including presumably SMA) and motor cortex to the RP are clearly distinguished.