Electrophysiological correlates of movement initiation

By averaging the electroencephalogram (EEG) or the magnetoencephalogram (MEG) of waking man in a manner time-locked to the onset of self-initiated volitional acts, movement-related potentials (or magnetic fields) can be recorded. In the preparation period, a slowly increasing cortical negatively called the Bereitschaftspotential (BP) or readiness potential, can be recorded. The BP precedes all our self-initiated (i.e. endogenous or willed) movements and actions by 1 sec or more prior to the onset of muscular contraction. The BP has 2 principal components, an early one which is bihemispherically symmetric, even prior to unilateral movements, and a late one which is lateralized towards the contralateral hemisphere. We have provided evidence (which was confirmed by regional cerebral blood flow studies) that the principal cortical generator producing the early component is the so called supplementary motor area ("SMA"), whereas the late asymmetric (contralateral) component is generated by the primary (rolandic) motor area, MI. The early "SMA"-related BP component may have motivational, intentional or timing properties, while the late component is related to synaptic facilitation in the contralateral MI area in preparation for the final pyramidal tract volley. In motor learning, the frontal areas are activated in both DC-potential recordings and emission CT experiments. The complete role of the "SMA" is not yet clear. However, it obviously has to determine the right moment to start a movement including the timing of single movements in motor sequences. It is particularly strongly activated in bimanual tapping of difficult rhythms.