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, like 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. In the present study, we investigated whether tDCS combined with motor imagery above the primary motor cortex influences sequence-specific learning. Four groups of participants were involved: an anodal, cathodal, sham stimulation, 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. This task allows to estimate both non-specific learning and sequence-specific learning effects by comparing the execution of unfamiliar sequences, familiar imagined, familiar withheld, and familiar executed sequences in a test phase. Results showed that the effects of anodal tDCS were already developing during the practice phase, while no effects of tDCS on sequence-specific learning were visible during the test phase. Results clearly showed that motor imagery itself influences sequence learning, but we also revealed that tDCS does not increase the influence of motor imagery on sequence learning.