Topographic organization of the human primary and secondary somatosensory areas: an fMRI study.

The topographical organization of SI and SII somatosensory areas was investigated using fMRI at 1.5 T and electrical sensory stimulation. Electrical stimuli were delivered unilaterally to the median nerve at the wrist and to the tibial nerve at the medial malleolus, during a block paradigm study. In all subjects, activation was observed, contralaterally to the stimulated side, in the post-central gyrus, in the posterior parietal cortex, in the mesial pre-frontal region and, bilaterally, in the supratemporal region at the level of the Sylvian fissure. The latter region, corresponding presumably to SII, showed a rough but clearcut topographical organization, with the median nerve areas located more posteriorly. In addition, weaker activations were observed in some subjects in the ipsilateral mesial prefrontal region and in the ipsilateral posterior parietal cortex. Information contained in the present study represent an interesting database for future investigations on the effects of sensorimotor learning in normal individuals on plastic reorganization following a lesion of the primary sensorimotor centers, i.e. in stroke patients, on the topography and balance between upper and lower limb representations in primary and secondary somatosensory cortices.