Magnetic field of the human sensorimotor cortex

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Abstract
A magnetic field associated with voluntary finger flexion was found to be confined over a well-defined region of the scalp overlying the sensorimotor cortex contralateral to the finger. The magnetic field had opposite directions over two regions of the scalp superior and inferior to the classical finger area of the cortex, implying that the field was generated by a source or sources in this area. The field whose onset begins approximately 50 ms prior to muscle activation was associated with a cortical source lying about 2.8 cm beneath the scalp and the active area shifted posteriorly 3–6 mm into the somatosensory area during the finger movement. Consistent with this shift, the early component of the magnetic field prior to flexion of a finger was absent when it was passively moved by the finger of the opposite hand, but the later components were present.