Objective: Although recent neuroimaging studies have shown that painful stimuli can produce activity in multiple cortical areas, the question remains as to the role of each area in particular aspects of human pain perception. To solve this problem we used transcranial magnetic stimulation (TMS) as an 'interference approach' tool to test the consequence on pain perception of disrupting activity in several areas of cortex known to be activated by painful input.
Methods: Weak CO(2) laser stimuli at an intensity around the threshold for pain were given to the dorsum of the left hand in 9 normal subjects. At variable delays (50, 150, 250, 350 ms) after the onset of the laser stimulus, pairs of TMS pulses (dTMS: interpulse interval of 50 ms, and stimulus intensity of 120% resting motor threshold) were applied in separate blocks of trials over either the right sensorimotor cortex (SMI), midline occipital cortex (OCC), second somatosensory cortex (SII), or medial frontal cortex (MFC). Subjects were instructed to judge whether or not the stimulus was painful and to point to the stimulated spot on a drawing of subject's hand.
Results: Subjects judged that the stimulus was painful on more trials than control when dTMS was delivered over SMI at 150-200 ms after the laser stimulus; the opposite occurred when dTMS was delivered over MFC at 50-100 ms. dTMS over the SII or OCC failed to alter the pain threshold.
Conclusions: These results suggest that TMS to SMI can facilitate whereas stimulation over MFC suppresses central processing of pain perception. Since there was no effect of dTMS at any of the scalp sites on the localization task, the cortical locus for point localization of pain may be different from that for perception of pain intensity or may involve a more complex mechanism than the latter.
Significance: This is the first report that TMS of SMI facilitates while that of MFC suppresses the central processing of pain perception. This raises the possibility of using TMS as a therapeutic device to control pain.