Dual effects of cortical polarization on peripheral motor activity in the rabbit

Anodal direct currents at intensities ranging from 0.3 to 30.0 microA were unilaterally applied for 30 min once a day to the premotor area of the rabbit cerebral cortex. The anodal polarization was repeated 10 times at intervals of 2-3 days, and the effect on the motor activity of the forelimbs during and after each polarization trial was compared with that before polarization. Peripheral motor activity was classified as either gentle flexion of forelimbs or struggle with violent movement of forelimbs. A current of 0.3 microA caused no change in motor behavior. Flexion of the forelimb contralateral to the polarized cortex was clearly increased when a polarizing current of 1.0 or 3.0 microA was applied, and peak flexion was observed between the third and seventh polarization trials. A current of 10 or 30 microA had no effect on forelimb flexion. Conversely, forelimb struggle on both sides was decreased when 10.0 or 30.0 microA, but not 1.0 or 3.0 microA, was applied. These results show that anodal polarization of the cerebral cortex exerts dual effects on peripheral motor activity, probably through changes in cortical excitability associated with the current intensity.