Rehabilitation can induce cortical reorganization in chronic stroke patients. In this study we investigated the mechanisms underlying treatment-associated plasticity. Eight patients with a stroke >6 months earlier participated in a 4-week period of physiotherapy based on a forced use concept. Before and after treatment, focal transcranial magnetic stimulation over the affected hemisphere was used to assess the motor output map of the paretic first dorsal interosseous muscle. Using a paired pulse paradigm, intracortical inhibition was investigated at the center of the cortical output map (CoG) and one cm anterior, posterior, lateral and medial of that position. Motor function was evaluated with the Motor Activity Log and the Wolf Motor Function Test. After therapy, the cortical representation size of the affected hand muscle was increased. In each patient, the CoG moved in the direction where intracortical inhibition had been lowest prior to therapy. Significant correlations were found between motor function tests and changes of output map size and CoG shifts, respectively. We conclude that treatment-associated cortical reorganization is influenced by the distribution of inhibitory properties within the representation area prior to therapy, since the CoG moved in the direction of lowest inhibition. The correlations between motor functions and electrophysiological results indicate a functional relevance of the observed reorganization pattern.
