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The Neuroscientist Comments

Reorganization of the somatosensory cortex in animal models has now been demonstrated following limb or digit amputation, denervation, or central lesions. The effects of sensory and motor deprivation in the human brain, however, have been less well studied. Now Langer and others (2012) present a longitudinal study that has examined the effects of limb immobilization on the organization of the human cortex. In this study, they tested the hypothesis that immobilization leads to a reduction in cortical thickness in the contralateral primary and somatosensory motor cortex and in the integrity of the corticospinal tract, which are responsible for fine motor movements. They studied 10 righthanded human patients with injuries to the right upper extremity that required at least 14 days of immobilization. Patients underwent two magnetic resonance imaging (MRI) examinations, initially within 48 hours postinjury, and then after 16 days of immobilization. They also carried out behavioral analysis. Most of the patients demonstrated significant enhancements of performance using the left (nonimmobilized) hand. MRI revealed a decrease in cortical thickness in the left primary motor and somatosensory areas, together with decreased fractional anisotropy of the left corticospinal tract. Notably, motor skill of the uninjured left hand tended to improve, and this improvement was correlated with increased cortical thickness and fractional anisotropy in the right motor cortex. This interesting study provides evidence, in human subjects, that thickness of the somatosenory cortex and integrity of the corticospinal tract change rapidly during immobilization of one arm, suggesting that these changes may be associated with transfer of skills from the immobilized to the nonimmobilized hand. The authors suggest that immobilization induces rapid reorganization of the somatosensory system and that the results may have implications for neuropsychological therapies such as constraint-induced rehabilitation therapy. Whether the changes demonstrated in this study are long-lasting is not yet known. It is hoped that future studies will address this and related questions.