EEG and MEG responses following stimulation of unmyelinated C fibers

Cerebral processing of first pain, associated with A-delta fibers, has been studied intensively, but the cerebral processing associated with C-fibers, relating to second pain, remains to be investigated. This is the first study to clarify the primary cortical processing of second pain by magnetoencephalography (MEG). We selectively activated C-fibers by the stimulation of a tiny area of skin with a CO2 laser. As for the primary component (1M), in the hemisphere contralateral to the stimulation, two regions in the hand area of the primary somatosensory cortex (SI) and secondary somatosensory cortex (SII)-insular were activated. The onset and peak latency of the two sources in SI and SII-insular were not significantly different. In the hemisphere ipsilateral to the stimulation, only one source was estimated in SII-insular, and its peak latency was significantly longer than that of the SII-insular source in the contralateral hemisphere, probably through corpus callosum. Our findings suggest that parallel activation of SI and SII-insular contralateral to the stimulation represents the first step in the cortical processing of C-fiber-related activities. In addition to SI and SII-insular, cingulate cortex and medial temporal area (MT) around amygdala and hippocampus in bilateral hemispheres were also activated for the subsequent component, 2M. All components of EEG and MEG responses were significantly reduced in amplitude during distraction and diminished during sleep, particularly 2M component. These findings indicate that these regions are related to the cognitive aspect of second pain perception, particularly activities in cingulate cortex.