Normal and Impaired Cooperative Hand Movements: Role of Neural Coupling

Recent research indicates that a task-specific, interhemispheric neural coupling is involved in the control of cooperative hand movements required for activities of daily living. This neural coupling is manifested in bilateral electromyographic reflex responses in the arm muscles to unilateral arm nerve stimulation. In addition, fMRI recordings show a bilateral task-specific activation and functional coupling of the secondary somatosensory cortical areas (S2) during the cooperative, but not during bimanual control tasks. This activation is suggested to reflect processing of shared cutaneous input during the cooperative task in both cortical areas. In chronic poststroke patients, arm nerve stimulation of the unaffected arm also leads to bilateral electromyographic responses, similar to those seen in healthy subjects in the cooperative task. However, stimulation of the affected side is frequently followed only by ipsilateral responses. The presence/absence of contralateral electromyographic responses correlates with the clinical motor impairment measured by the Fugl-Meyer score. The observations suggest impaired processing of afferent input from the affected side leading to defective neural coupling during cooperative hand movements after stroke. In moderately affected patients, movement execution seems to rely on the involvement of the ipsilateral corticospinal tract arising in the non-damaged hemisphere. According to these results, hand rehabilitation of stroke patients, currently focused on reach and grasp movements of the affected side, should be supplemented with the training of cooperative hand movements required during activities of daily living.