Magnetic fields of the human brain (Bereitschaftsmagnetfeld) preceding voluntary foot and toe movements

Voluntary movements are preceded by a slow electrical potential of the brain (Bereitschafts-potential, BP) or readiness potential. The BP is accompanied by a magnetic field shift of similar time characteristics (Bereitschaftsmagnetfeld, BM). The BM preceding volitional right foot or toe movements was recorded from anterior, posterior, and lateral positions of the scalp using a SQUID (Superconducting Quantum Interference Device) third-order gradiometer. Controls were implemented to reduce head movements, which were simultaneously recorded with a mechanograph. The results showed that movements of the lower extremities are also preceded by a BM. However, contrary to finger movements, BMs with field lines directed into the head were found predominantly for foot movements and exclusively for toe movements. The BM preceding foot movements was maximum over a position 2 cm left of the vertex, i.e., contralateral to the movement. Two centimeters right of the vertex it was smaller, thus exhibiting a normal contralateral preponderance and not sharing the paradoxical side preponderance of the electrical BP preceding foot or toe movements. The BM preceding toe movements was only apparent at the vertex and was smaller than the one preceding foot movement. This may suggest a source that is located still deeper in the brain than with foot movements.