Increased transcranial direct current stimulation after effects during concurrent peripheral electrical nerve stimulation.

In this study we tested the hypothesis whether a lasting change in the excitability of cortical output circuits can be obtained in healthy humans by combining a peripheral nerve stimulation during a concomitant depolarization and/or hyperpolarization of motor cortex. To reach this aim we combined two different neurophysiological techniques each of them able to induce a lasting increase of cortical excitability by them self: namely median nerve repetitive electrical stimulation (rEPNS) and transcranial direct current stimulation (tDCS). Ten normal young volunteers were enrolled in the present study. All subjects underwent five different protocols of stimulation: (1, 2) tDCS alone (anodal or cathodal); (3) Sham tDCS plus rEPNS; (4, 5) anodal or cathodal tDCS plus rEPNS. The baseline MEP amplitude from abductor pollicis brevis (APB) and flexor carpi radialis (FCR) muscle, the FCR H-reflex were compared with that obtained immediately after and 10, 20, 30, 60 min after the stimulation protocol. Anodal tDCS alone induced a significant transient increase of MEP amplitude immediately after the end of stimulation while anodal tDCS + rEPNS determined MEP changes which persisted for up 60 min. Cathodal tDCS alone induced a significant reduction of MEP amplitude immediately after the end of stimulation while cathodal tDCS + rEPNS prolonged the effects for up to 60 min. Sham tDCS + rEPNS did not induce significant changes in corticospinal excitability. Anodal or cathodal tDCS + rEPNS and sham tDCS + rEPNS caused a lasting facilitation of H-reflex. These findings suggest that by providing afferent input to the motor cortex while its excitability level is increased or decreased by tDCS may be a highly effective means for inducing an enduring bi-directional plasticity. The mechanism of this protocol may be complex, involving either cortical and spinal after effects.