Online transcranial direct current stimulation (i.e., tDCS concurrent to the task) and priming tDCS are proposed to have cumulative effects on motor performance. However, the impact of the tDCS polarity for priming remains unclear. The aim of this study was to enhance more motor learning and retention of a motor task with multiple online anodal tDCS (atDCS) sessions using cathodal tDCS (ctDCS) priming as compared to atDCS or sham.

In a double blind randomized and sham controlled study design, 22 participants separated in 3 independent groups underwent for 3 consecutive days high definition-atDCS (20min, 2mA) training sessions targeting the left sensorimotor cortex, preceded by a baseline measurement (d0) and followed by two retention tests (d4 and d18, one day and two weeks after training, respectively). A circular tracing-movement task of 5 trials of 1min intersected by 1min rest was performed at pre-, during and post- atDCS for each training session and for d0, d4 and d18.

The motor performance increased significantly at the end of training (d3) for both atDCS and ctDCS priming (P<0.001) but not for sham. This increase was also observed at d4 for atDCS (P=0.05) and for ctDCS at d4 and d18 (P<0.001).

The combination of priming tDCS and multiple sessions of motor task-concurrent anodal tDCS was beneficial for improving performance during and after training without being superior to sham. The cumulative effects of priming with training persisted only for ctDCS priming two weeks after the end of the training.