The Journal of Neurophysiology Vol. 88 No. 1 July 2002, pp. 464-474
Copyright ©2002 by the American Physiological Society

Differentiating Noxious- and Innocuous-Related Activation of Human Somatosensory Cortices Using Temporal Analysis of fMRI

 ¹Department of Neurology and Neurosurgery, Faculty of Graduate Studies & Research, McGill University, Montreal, Quebec H3A 2B4;  ²Department of Physiology, Faculty of Graduate Studies & Research, McGill University, Montreal, Quebec H3G 1Y6;  ³Centre de recherche en sciences neurologiques, Université de Montréal, Montreal, Quebec H3C 3J7;  ⁴Department of Neurology and Neurosurgery, and  ⁵Department of Anesthesiology, Faculty of Medicine, and  ⁶Faculty of Dentistry, McGill University, Montreal, Quebec H3G 1Y6;  ⁷McConnell Brain Imaging Centre, Montreal Neurological Institute, Montreal, Quebec H3A 2B4; and  ⁸Département de stomatologie, Faculté de médicine dentaire, Université de Montréal, Montreal, Quebec H3C 3J7, Canada

Chen, Jen-I, Brian Ha, M. Catherine Bushnell, Bruce Pike, and Gary H. Duncan. Differentiating Noxious- and Innocuous-Related Activation of Human Somatosensory Cortices Using Temporal Analysis of fMRI. J. Neurophysiol. 88: 464-474, 2002. The role of the somatosensory cortices (SI and SII) in pain perception has long been in dispute. Human imaging studies demonstrate activation of SI and SII associated with painful stimuli, but results have been variable, and the functional relevance of any such activation is uncertain. The present study addresses this issue by testing whether the time course of somatosensory activation, evoked by painful heat and nonpainful tactile stimuli, is sufficient to discriminate temporal differences that characterize the perception of these stimulus modalities. Four normal subjects each participated in three functional magnetic resonance imaging (fMRI) sessions, in which painful (noxious heat 45-46°C) and nonpainful test stimuli (brushing at 2 Hz) were applied repeatedly (9-s stimulus duration) to the left leg in separate experiments. Activation maps were generated comparing painful to neutral heat (35°C) and nonpainful brushing to rest. Directed searches were performed in SI and SII for sites reliably activated by noxious heat and brush stimuli, and stimulus-dependent regions of interest (ROI) were then constructed for each subject. The time course, per stimulus cycle, was extracted from these ROIs and compared across subjects, stimulus modalities, and cortical regions. Both innocuous brushing and noxious heat produced significant activation within contralateral SI and SII. The time course of brush-evoked responses revealed a consistent single peak of activity, approximately 10 s after the onset of the stimulus, which rapidly diminished upon stimulus withdrawal. In contrast, the response to heat pain in both SI and SII was characterized by a double-peaked time course in which the maximum response (the 2nd peak) was consistently observed ~17 s after the onset of the stimulus (8 s following termination of the stimulus). This prolonged period of activation paralleled the perception of increasing pain intensity that persists even after stimulus offset. On the other hand, the temporal profile of the initial minor peak in pain-related activation closely matched that of the brush-evoked activity, suggesting a possible relationship to tactile components of the thermal stimulation procedure. These data indicate that both SI and SII cortices are involved in the processing of nociceptive information and are consistent with a role for these structures in the perception of temporal aspects of pain intensity.