NEUROMAGNETIC FIELDS OF THE BRAIN EVOKED BY VOLUNTARY MOVEMENT AND ELECTRICAL-STIMULATION OF THE INDEX FINGER

Neuromagnetic fields from the left cerebral hemisphere of five healthy , right-handed subjects were investigated under two different experime ntal conditions: (1) electrical stimulation of the right index finger (task somatosensory evoked fields, task SEF's), and (2) voluntary move ment of the same finger referred to as movement-related fields, (MRFs) . The two conditions were, performed in random order every 5-8 s. In a ddition, the task SEF's were compared to control SEF's recorded at the beginning of the experiment in order to find the optimal dewar positi on for localizing the central sulcus. The magnetic signals of the sour ces corresponding to the main components of the somatosensory evoked f ields (early ones at 24 ms and at 34 ms, and late ones after 50 ms) an d movement-related fields (motor field, MF and movement-evoked field I -MEF I) were mapped and localized by means of a moving dipole model. I n four out of five subjects the MEF I dipoles were found to be located deeper than the early task SEF dipoles. In addition, all of the task SEF's components were found to exhibit larger amplitudes than the cont rol SEF's components. The results are discussed in respect to the abil ity to selectively analyze contributions of mainly proprioceptive (are a 3a) and cutaneous (area 3b) areas in the primary somatosensory corte x using magnetoencephalography. An additional finding of the study was that all of the task SEF's components were found to exhibit larger am plitudes than the control SEF's components.