Evidence for Medial to Lateral Somatotopy

Initial vetting of many brain imaging techniques derives from detecting somatosen-sory-driven activations attributable to anterior parietal cortex, and especially when results confirm medial to lateral sequences of activated foci from stimulating a small number of contralateral body regions (Table 2.1). For example, PET (reviewed in Reference 28) and fMRI (e.g., Reference 210) studies report a medial to lateral somatotopy for widely separate body representations (e.g., foot, hand, and face). Several studies describe activation of only a single peri-Rolandic site contralateral to stimulating the hand with high amplitude vibratory stimuli.26-40-71-213 These observations thus corroborate earlier reports of a single distorted contralateral representation spanning the postcentral gyrus and having an approximate medial to lateral topography with respect to successive segmental sacral to cervical to trigeminal levels of the body.181-233 Several studies also note ipsilateral activations even in the hand region of SI.28-90-130-138

Current imaging results must be tempered by recent findings of only 55% concordance between the centers of activated regions using fMRI and direct recordings in the same brains of anesthetized monkeys.55 Thus- discrepancies from expected SI map features may only be a consequence of inherent technical limitations.199 Further limiting possible findings is the crude spatial resolution of most imaging studies and the limited number of body representations studied.

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