Does transcranial direct current stimulation affect the learning of a fine sequential hand motor skill with motor imagery?

Learning a fine sequential hand motor skill, comparable to playing the piano or learning to type, improves not only due to physical practice, but also due to motor imagery. Previous studies revealed that transcranial direct current stimulation (tDCS) and motor imagery independently affect motor learning. Other studies showed that training with motor imagery combined with anodal tDCS might generally enhance motor performance. In the present study, we investigated whether tDCS combined with motor imagery above the primary motor cortex influences sequence-specific learning of a fine hand motor skill. Four groups of participants were involved: an anodal, cathodal, and sham stimulation group, and a control group without stimulation. A modified discrete sequence production (DSP) task was employed: the Go/NoGo DSP task. After a sequence of spatial cues, a response sequence had to be either executed, imagined or withheld. The task allows to estimate general learning effects in a practice phase, and sequence-specific learning effects in a test phase by comparing the execution of unfamiliar sequences, familiar imagined sequences, familiar withheld and familiar executed sequences. Results showed that the effects of anodal tDCS were already developing during the practice phase, while no sequence-specific effects were visible during the test phase. These findings confirm that anodal tDCS affects motor performance but they also reveal that it does not facilitate the influence of motor imagery on sequence learning.