Anodal motor cortex stimulation paired with movement repetition increases anterograde interference but not savings

Retention of motor adaptation is evident in savings, where initial learning improves subsequent learning, and anterograde interference, where initial learning impairs subsequent learning. Previously, we proposed that use-dependent movement biases induced by movement repetition contribute to anterograde interference, but not to savings. Here, we evaluate this proposal by limiting or extending movement repetition while stimulating the motor cortex (M1) with anodal transcranial direct current stimulation (tDCS), a brain stimulation technique known to increase use-dependent plasticity when applied during movement repetition. Participants first adapted to a counterclockwise rotation of visual feedback imposed either abruptly (extended repetition) or gradually (limited repetition) in a first block (A1), during which either sham or anodal tDCS (2 mA) was applied over M1. Anterograde interference was then assessed in a second block (B) with a clockwise rotation, and savings in a third block (A2) with a counterclockwise rotation. Anodal M1 tDCS elicited more anterograde interference than sham stimulation with extended but not with limited movement repetition. Conversely, anodal M1 tDCS did not affect savings with either limited or extended repetition of the adapted movement. Crucially, the effect of anodal M1 tDCS on anterograde interference did not require large errors evoked by an abrupt perturbation schedule, as anodal M1 tDCS combined with extended movement repetition within a gradual perturbation schedule similarly increased anterograde interference but not savings. These findings demonstrate that use-dependent plasticity contributes to anterograde interference but not to savings.