Magnetoencephalographic responses to painful impact stimulation

Magnetoencephalographic (MEG) field recordings are unique to detect current dipoles in SI and SII. Few devices are available for painful mechanical stimulation in magnetically shielded MEG rooms. The aim of the present MEG (dual 37-channel biomagnetometer) study was to investigate the location of the cortical generators evoked by painful impact stimuli of different intensities. An airgun was placed outside the shielded MEG room, and small plastic bullets were fired at the arm and trunk of the subjects in the room. The velocity of the bullet was measured and related to the evoked pain intensity. Stimuli were delivered for each of the following three conditions: strong pain intensity elicited from the upper arm and upper trunk; weak pain intensity elicited from the upper trunk. The evoked MEG responses had a major component with the characteristically polarity-reversal deflections indicating a dipole located beneath the coils. The response could be estimated by a single current dipole. When the estimated locations of the dipoles were superimposed on the individual magnetic resonance images (MRIs), consistent bilateral activation of areas corresponding to the secondary sensory cortices (SII) was found.