SOMATOSENSORY CORTEX - A COMPARISON OF THE RESPONSE TO NOXIOUS THERMAL, MECHANICAL, AND ELECTRICAL STIMULI USING FUNCTIONAL MAGNETIC-RESONANCE-IMAGING

In the present study, functional magnetic resonance imaging (fMRI) was used to examine pain perception in humans. Three types of noxious sti muli were presented: electric shock (20.8 mA, 2 Hz), heat (48 degrees C), and mechanical, as well as a control tactile stimulus. The signifi cance of activation at the level of the voxel was determined using cor relation analysis. Significant region of interest (ROI) activation was determined by comparing the percentage of active voxels in each ROI t o activation in a control ROI in the visual cortex. In response to tac tile and shock stimuli, consistent activation was seen in the postcent ral gyrus, parietal operculum, and ipsilateral cerebellar cortex. No s ignificant cortical activation was detected in response to noxious hea t or mechanical stimulation when compared to nonpainful intensity leve ls. The data did not indicate adaptation, although further study in th is area is necessary. Stationary noxious thermal and mechanical stimul ation are ''pure'' noxious stimuli, while electrical stimulation influ enced nociceptive and nonnociceptive receptors. Lack of detectable act ivation in response to pure noxious stimuli supports the idea that noc iceptive and nonnociceptive fibers are interspersed in the somatosenso ry cortex. Conflicting results from recent functional imaging studies of pain perception regarding cortical activation indicate that it is e ssential to consider both the tactile and nociceptive components of th e stimuli used, the spatial extent of stimulation, and the possibility of adaptation to the response. Furthermore, these results suggest tha t subtractive or correlative methods may not be sufficiently sensitive to image the activity of nociceptive cells, which are sparsely distri buted throughout the somatosensory cortex. (C) 1998 Wiley-Liss, Inc.