Within-arm somatotopy in human motor areas determined by positron emission tomography imaging of cerebral blood flow.

Within-arm somatotopy was identified in multiple motor areas of six normal human subjects who performed a visuomotor tracking task during positron emission tomography (PET) measurement of relative cerebral blood flow (relCBF). A randomly moving target, presented on a computer monitor, was continuously followed with the index finger (movement at the metacarpophalangeal joint), thumb, fist (movement at the wrist), forearm (movement at the elbow), elbow (movement at the shoulder), and eyes alone (control task) during sequential imaging. Segmental limb movements were associated with relCBF responses in the contralateral motor, supplementary motor, cingulate, and parietal cortex, and in the ipsilateral cerebellum. Localization of responses after stereotaxic transformation into Talairach atlas space, as well as within-subject analysis without anatomic deformations, demonstrated an overlapping somatotopic distribution in the motor cortex, with thumb responses most ventrolateral and shoulder responses most dorsomedial. Proximal limb movements induced relCBF responses of greater magnitude than distal movements. Somatotopy was also identified in the supplementary motor area, with index finger responses dorsal and anterior to shoulder responses. An additional set of somatotopic responses were located in the cingulate cortex, also with finger responses anterior to shoulder responses. Somatotopy was not identified in the anterior cerebellum. The distribution of relCBF responses is concordant with electrophysiologic studies in nonhuman primates that demonstrate a fractured somatotopy on a fine scale and a general somatotopic scheme of the limb on a large scale in multiple discrete motor areas.