Evidence for a rostral-to-caudal somatotopic organization in human primary somatosensory cortex with mirror-reversal in areas 3b and 1.

Medial-to-lateral somatotopy is a well-established feature of the human primary somatosensory cortex (SI); however, it is unknown whether, similarly to non-human primates, a rostral-to-caudal somatotopic arrangement exists as well. Therefore, in this functional magnetic resonance imaging (fMRI) study on eight healthy human subjects, five circumscribed skin areas sequentially located on the third finger and the palm of the hand were stimulated with innocuous electrical pulses. Within area 3b of contralateral SI, successive cortical representation sites ordered in a rostral-to-caudal fashion were seen in the group analysis and in six individual subjects. The fingertip was located most rostrally, whereas the proximal parts of the finger as well as the distal palm were represented at more caudal locations. Within area 1, the group analysis revealed a similar pattern of discrete representations. However, in contrast to area 3b, the fingertip was located most caudally, whereas the more proximal parts of the finger were found to be represented rostrally within area 1. Thus, the representation pattern of area 1 appeared as a 'mirror image' of that of area 3b. In comparison to the representations of the finger and the distal palm, the proximal palm was found to be represented at a more medial position of the postcentral gyrus.