What is the position of an arm relative to the body? Neural correlates of body schema and body structural description.

Neuropsychological studies suggest that the human brain is endowed with two body representations: the body schema (BS), coding the orientation of one's body parts in space, and the body structural description (BSD), coding the location of body parts relative to a standard body. We used fMRI to disentangle the neural mechanisms underlying these putatively distinct body representations. Participants saw an arm or a pot's handle (stimulus: arm, handle) rotated at different angles (angle: 30-150 degrees). If the stimulus was an arm, subjects were instructed to imagine (1) rotating their own arm until it matched the stimulus orientation (comparing the seen arm to their own) or (2) seeing the stimulus moving toward its appropriate position on a simultaneously presented human body [comparing the arm to the one of a standard body (strategy: motor, visual imagery)]. If the stimulus was a handle, subjects were instructed to imagine (1) placing the handle on its appropriate position on a simultaneously presented pot or (2) seeing it moving toward its pot's position. The analysis of the interaction stimulus x strategy revealed activation of left secondary somatosensory cortex (SII), specifically when comparing the stimulus arm to one's own. The analysis of the parameters describing the linear effect of angle revealed that neural activity of left posterior intraparietal sulcus was modulated by the stimulus's rotation, but only when relating the arm to a standard body. The results associate BS and BSD with differential neural substrates, thereby suggesting that these are independent body representations, and furthermore extend current concepts of SII function.