Cerebral potentials preceding voluntary toe, knee and hip movements and their vectors in human precentral gyrus

In this study, the brain potentials related to voluntary, self-paced plantarflexions of the left toes, flexion of the right knee and isometric extensions of the left hip were examined in 3 groups of 10 subjects each. In the first half of the foreperiod, the Bereitschaftspotential (BP) or readiness potential for all movements was symmetrically distributed over both hemispheres. For toe and knee movements, an ipsilateral preponderance of the BP developed in the later foreperiod, which was statistically significant for toe movements. For hip movements, BP topography was symmetrical during the entire foreperiod including its second half. Since finger, hand or shoulder movements of previous experiments show a strong contralateral preponderance of the BP, the results are discussed as further support of the hypothesis that the lateralisation of the BP is due to the orientation of the precentral electrical field vector generated by an active source in the MI motor cortex. Thus, part of the somatotopic representation of the human precentral gyrus can be mapped by this non-invasive means: upper limb movements are located on the convexity; toe, foot and knee movements are generated on the mesial cortex between the hemispheres; and hip movements seem to be located at the mantle edge.