A neurophysiological study of mirror movements in adults and children

The mechanism underlying mirrored activity/movements in normal individuals is unknown. To investigate this, we studied 11 adults and 39 children who p erformed sequential finger-thumb opposition or repetitive index finger abdu ction. Surface electromyographic (EMG) activity recorded from the left and right first dorsal interosseous muscles (1DI) during unilateral sequential finger-thumb opposition (voluntarily activated muscle, 1DIvol) showed mirro red EMG activity (homologous muscle of the opposite hand, 1DImm) that decre ased with increasing age. The time of onset of involuntary compared with vo luntary EMG activity was variable but could start at the same time. A signi ficant increase in E2 (transcortical component) size of the cutaneomuscular reflex recorded from the 1DImm indicated increased excitability of the mot or cortex ipsilateral to the 1DIvol during active index finger abduction co mpared with the 1DIvol relaxed. Transcranial magnetic stimulation, using th e Bistim technique, indicated that the transcallosal inhibitory pathway in children may not operate in the same way as in the adult. Cross-correlation analysis did not detect shared synaptic input to motoneuron pools innervat ing homologous left and right hand muscles. We conclude that the mirrored m ovements/activity observed in healthy adults and children are produced by s imultaneous activation of crossed corticospinal pathways originating from b oth left and right motor cortices.