Functional reorganization of the brain in humans following spinal cord injury: evidence for underlying changes in cortical anatomy.

Loss of somatosensory drive results in functional reorganization of the primary somatosensory cortex (SI). While the phenomenon of functional cortical reorganization is well established, it remains unknown whether in humans, functional reorganization results from changes in brain anatomy, or simply reflects an unmasking of already existing dormant synapses. In 20 subjects with complete thoracic spinal cord injuries (SCIs) and 23 controls, we used functional and structural magnetic resonance imaging to determine whether SI reorganization was associated with changes in SI anatomy. SCI resulted in a significant SI reorganization, with the little finger representation moving medially toward the lower body representation (i.e., area of sensory loss). Furthermore, although SCI was associated with gray matter volume loss in the lower body representation, this loss was minimized as reorganization increased. That is, the greater the medial shift in little finger representation, the greater the gray matter preservation in the lower body representation. In addition, in the region of greatest SI reorganization (little finger), fractional anisotropy was correlated with SI reorganization. That is, as SI reorganization increased, the extent of aligned structures decreased. Finally, although thalamocortical fibers remained unchanged, the ease and direction of water movement within the little finger representation was altered, being directed more toward the midline in SCI subjects. These data show that SI reorganization following SCI is associated with changes in SI anatomy and provide compelling evidence that SI reorganization in humans results from the growth of new lateral connections, and not simply from the unmasking of already existing lateral connections.