Effects of stimulus intensity on fMRI and MEG in somatosensory cortex using electrical stimulation

To clarify the relationship between neuronal activity and the hemodynamic response, activations of the primary somatosensory cortex were studied using MEG (magnetoencephalogram) and fMRI (functional magnetic resonance imaging). Effects of stimulus intensity by electrical stimulation to the right thumb was investigated. The authors focused on the number of activated pixels, time courses of hemodynamic response obtained by fMRI, and dipole moments of neuromagnetic activity. For fMRI experiments, a number of activated pixels and signal power, which was defined as the integrated value of MR signal, greatly increased when an increase in stimulus intensity generated pain. This trend, however, did not correspond to the observed MEG responses. When the stimulus intensity changed, fMRI responses showed the different tendency of MEG responses.