Central representation of somatic sensations in the parietal operculum (SII) and insula.

Four subjects with small restricted cerebral cortical infarcts have been examined. One had a lesion confined to the parietal operculum (SII), while in the second the SII lesion also encroached on the posterior insula; in the third subject, both banks of the sylvian fissure and the dorsal insula were involved, while in the fourth the lesion involved the upper bank of the sylvian fissure. In all cases, the postcentral gyrus (SI) was intact. Subjects 1 and 2 had mild spontaneous pain, but subjects 3 and 4 had never had spontaneous pain. In the affected areas, none could feel mechanical (skinfold pinch) pain. The 2 subjects with spontaneous pain could not discriminate sharpness (pinprick), but this was unimpaired in the third and fourth subjects. Warmth, cold, and heat pain were impaired in the 2 subjects with spontaneous pain, but not in those without; however warm-cold difference was greater in the affected regions of all subjects. The possibility must nevertheless be considered that the presence of central pain in some way alters the cortical mechanisms for the perception of thermal stimuli. Certainly, as we had earlier observed, spontaneous pain only occurs when there is interference with thermal sensation. Functional MRI (fMRI) studies following thermal stimulation in subjects 1 and 2 showed these areas, particularly SII, to be concerned with the reception of innocuous and noxious thermal stimuli, mechanical (skinfold pinch) pain and sharpness (pinprick), implying that SI is principally concerned with the reception of low-intensity mechanical stimuli, although it was activated in 1 of our fMRI-studied subjects by innocuous cooling.