MEG Studies of Sensorimotor Function

The first measurements of magnetic fields accompanying voluntary movements using single channel magnetometers [10, 15] indicated that slow pre-movement shifts, similar to the electrically recorded readiness potential, or Bereitschaftspotential [20], could be recorded over sensorimotor areas of the brain. In subsequent studies, my colleagues and I found that these slow magnetic field shifts began approximately a half a second prior to voluntary movement, were widely distributed over both hemispheres, and were typically followed by three larger phasic responses following the onset of electromyographic activity in the involved muscles, which we termed movement evoked fields I, II and III [6, 22]. Fig 1A shows a typical readiness field (RF) preceding a self-paced finger movement, beginning approximately 600 ms prior to movement reaching peak amplitude just prior to movement onset. Movement-evoked fields accompany the onset and execution phase of the movement, the first component (MEFI) being the largest in amplitude and occurring at a latency of about 30 ms following movement onset, or approximately 100 ms following onset of EMG activity in the active muscles.