Transcranial direct current stimulation enhances delayed retention after 5 days of lower-limb motor skill learning

Transcranial direct current stimulation (tDCS) holds the potential to affect behaviour by modulating ongoing neural activity, and tDCS paired with hand motor practice can enhance motor learning. While augmenting the behavioural benefits of motor practice is relevant for neurorehabilitation following central and peripheral lesions to the motor system, as well as in sports, the effects of tDCS targeting the mesial motor cortex (M1-Leg) during multi-session lower-limb motor skill practice remain unexplored. We tested whether 5 days of anodal tDCS over M1-Leg during training of a sequential visuomotor tracking task improves within- and between-session learning and 1-week retention. Participants were randomized to skill practice with active tDCS, skill practice with sham stimulation, or volume-matched non-skilled ankle movements with sham stimulation. Changes in corticospinal excitability accompanying skill and non-skill motor practice with real and sham tDCS, were assessed as motor-evoked potential amplitudes recorded from the tibialis anterior muscle at rest. Compared with non-skill practice, motor skill practice yielded robust sequence-specific performance gains that transferred to the untrained leg and persisted for at least 1 week after practice ended. Concurrent tDCS did not increase learning within or between training sessions, but it did lead to improved one-week retention compared with sham stimulation. Corticospinal excitability did not increase after practice and was unaffected by tDCS. These findings suggest that combining lower-limb motor skill practice with tDCS over the M1-Leg can enhance retention of skill learning without measurable changes in resting corticospinal excitability. This is relevant for motor practice scheduling and potentially in neurorehabilitation. KEY POINTS: Transcranial direct current stimulation (tDCS) is a weak electrical brain stimulation that may boost learning when paired with motor practice; however, its effects during lower-limb skill training are not well known. We tested whether stimulation over the leg area of the motor cortex during 5 days of ankle skill training improves learning, consolidation and delayed retention. Ankle skill training produced clear sequence-specific improvements that transferred to the untrained leg and were still present 1 week later. Stimulation did not increase short-term learning, but it did improve 1-week retention compared with sham stimulation. Corticospinal excitability did not change with motor training or stimulation, consistent with the possibility that the observed retention effect relies on mechanisms that do not evoke sustained changes in resting corticospinal excitability (e.g. state-dependent or network-level processes).