Anatomical localization revealed by MEG recordings of the human somatosensory system

A 14-channel cryogenic magnetometer system (BTi) was used to record the magnetic fields over the left hemisphere of 3 human subjects in order to locate the sources of responses to tactile stimulation of the index, the thumb and the little finger of the right hand. The locations of the active dipole sources determined using the spherical model were then projected onto the magnetic resonance image (MRI) of the individual subjects providing an anatomical localization. The MRI slices were also used to construct a 3-dimensional image to enhance visualization of the area of the calculated sources. The locations of the dipole sources from the 3 fingers were distinct from one another in all subjects. An analysis of variance ('ANOVA') showed the most significant (P less than 0.05) difference in source location between the little finger and the thumb with the former being superior to the sources of the other 2 fingers in all of the subjects. In all cases, the sources were found to be located on the postcentral gyrus. The strength of the equivalent dipole sources and the amplitudes of the responses to stimulation for all 3 fingers showed a consistent trend among all of the 3 subjects, with the thumb having the largest response. In general, no signs of habituation were found.