Somatotopic Organization of the Ventral and Dorsal Finger Surface Representations in Human Primary Sensory Cortex Evaluated by Magnetoencephalography

Cortical reorganization of the subtly differentiated hand map after peripheral nerve injury might be better understood if there was a topographic conception of the homuncular representation of the dorsal finger surfaces in humans, in addition to the well-established sequential rostrocaudal array of the ventral finger aspects in cortical area 3b. In the present magnetoencephalographic study, tactile pneumatic stimulation was delivered to the fingertip and to the ventral and dorsal proximal phalanx of each digit of the dominant hand in 20 right-handed volunteers. Source localization of equivalent current dipoles underlying the recorded somatosensory evoked magnetic field was performed using a Cartesian coordinate system established by the anatomical landmarks nasion and preauricular points. Of the first major peak of each somatosensory evoked field, the region with the maximum field power (root-mean-square across channels) was selected for source reconstruction. Analysis of variance for repeated measures yielded significant results with respect to the arrangement of digits along the vertical coordinate axis, demonstrating a sequential array from the most inferiorly located D1 to the most superiorly located D5 for all different stimulus positions. This is the first study providing evidence for a sequential topographical arrangement of not only the ventral but also the dorsal surface representations of the individual digits in the human somatosensory cortex. The study contributes to a better understanding of the somatosensory hand representation in human primary cortex and provides useful information with regard to cortical plasticity studies in patients with peripheral nerve injuries at the upper extremity.