Focal brain stimulation in healthy humans: motor maps changes following partial hand sensory deprivation

Cortical motor maps of first dorsal interosseus (FDI), abductor digit minimi (ADM) and wrist flexors muscles were produced following transcranial focal magnetic stimulation of the contralateral motor areas in seven volunteers. Motor evoked potentials (MEPs) were recorded before and during median and radial nerves anaesthetic block at wrist. Results indicated that the FDI muscle cortical representation was significantly reduced; this muscle was entirely "enveloped' in the hand region deprived of its cutaneous sensory information, despite maintaining its usual proprioceptive feed-back and strength via the ulnar nerve. On the contrary, the ADM (serving as "control condition' because outside the anaesthetised hand area) cortical representation was unchanged, even if showing a tendency to enlarge in the topographic maps. No amplitude changes of compound muscle action potentials of ADM and FDI during peripheral stimulations were observed, while changes in F-wave responses were detected in both muscles. No significant topographic changes were found for the wrist flexors muscles. A possible explanation for these "short term' rearrangements of brain motor maps is given on the basis of neural plasticity mechanisms due to the loss of tonic cutaneous inputs on cortical and spinal motoneurons.