Enhancement of motor consolidation by post-training transcranial direct current stimulation in older people.

Consolidation, by which performance increments after a training intervention are secured and sometimes generated, is reduced in elderly humans. The present study addressed the question whether transcranial direct current stimulation (tDCS) applied after motor training improves consolidation of explicit motor sequence learning in healthy older humans. In the first experiment, anodal or cathodal tDCS to the left primary motor cortex, anodal tDCS to premotor cortex, or sham tDCS was applied immediately after completion of a finger sequence learning task. Performance was retested at 8 and 22 hours after the initial training session. Whereas all groups achieved similar performance at the end of training, off-line improvements differed between groups at later retesting, depending on the type of intervention. Relative to the other 3 interventions, anodal tDCS to primary motor cortex (M1) led to performance improvements already at retesting 8 hours after initial learning and were maintained on the next day. In the second experiment, effects of the timing of post-training anodal tDCS to M1 with respect to the end of training were studied. Participants received anodal tDCS of M1 immediately or 60 or 120 minutes after training and were retested on sequence performance 8 hours post training. Only application of tDCS immediately after the end of training, but not after 1 or 2 hours, enhanced off-line consolidation. These findings suggest that anodal tDCS applied off-line immediately post training to M1 interacts specifically with early processes promoting consolidation of motor sequence learning in healthy older individuals.