Single-Session Anodal tDCS with Small-Size Stimulating Electrodes Over Frontoparietal Superficial Sites Does Not Affect Motor Sequence Learning

Abstract

Due to the potential of anodal transcranial direct current stimulation (a-tDCS) for enhancement of fine sequenced movements and increasing interest in achieving high level of fine movements in the trained and untrained hands especially at initial stage of learning, we designed this study to investigate whether the application of single-session a-tDCS with small-size stimulating electrodes over FPN sites, such as dorsolateral prefrontal cortex (DLPFC), primary motor cortex (M1) or posterior parietal cortex (PPC) could enhance sequence learning with the trained hand and these effects are transferred into the untrained hand or not. A total of 51 right-handed healthy participants were randomly assigned to one of the four stimulation groups: a-tDCS of left M1, DLPFC, PPC, or sham. Stimulation was applied for 20 min during a sequential visual isometric pinch task (SVIPT). Eight blocks of training using SVIPT were completed with the right hand during stimulation. Two blocks of sequence training with each hand were performed by participants as assessment blocks at three time points: baseline, 15 min and one day following the intervention. Behavioral outcomes including movement time, error rate and skill were assessed in all assessment blocks across three time points. We also measured corticospinal excitability, short-interval intracortical inhibition, and intracortical facilitation using single- and paired-pulse transcranial magnetic stimulation. The results indicated that the behavioral outcomes were significantly improved with the right trained hand, but this learning effect was not modulated by a-tDCS with small-size stimulating electrodes over the FPN. Transfer of learning into the untrained hand was observed in all four groups for movement time but not for the error rate or skill. Our results suggest that sequential learning in SVIPT and its transfer into the untrained hand were not sensitive to a single-session a-tDCS with small-size stimulating electrodes over left M1, DLPFC or PPC in young healthy participants.

Keywords: dorsolateral prefrontal cortex; motor sequence learning; non-invasive brain stimulation; posterior parietal cortex; primary motor cortex; transcranial magnetic stimulation; transfer of learning.

FIGURE 1

Experimental set-up. Participants held a force transducer between their thumb and index finger and altered their precision force on the force transducer to move a cursor on the computer screen to reach different target forces. During eight blocks of training, a-tDCS (left M1, DLPFC, PPC, and sham) were applied over the left hemisphere contralateral to the performed hand. Each block consisted of eight trials and each trial included seven target forces from 10 to 30 % of MVC which appeared on the computer screen. Cortical and behavioral changes were assessed over three time points at baseline, post 15 min and post 24 h after intervention (A,B). Participants were required to squeeze the force transducer to reach the target force in a range of 5% below or above the target force (C,D). SVIPT: Sequential visual isometric pinch task, A-tDCS: Anodal transcranial direct current stimulation, M1, Primary motor cortex; DLPFC, Dorsolateral prefrontal cortex; PPC, Posterior parietal cortex; S, Sequence block; R, Random block; CSE, Corticospinal excitability; SICI, Short-interval intracortical inhibition; ICF, intra-cortical facilitation (ICF); maximum voluntary contraction (MVC).

FIGURE 2

The mean of peak-to-peak amplitudes of MEPs in the right FDI among the four groups over three time points. No significant difference was found in the main factors of Time or Group. Data are presented at Mean ± SEM.

FIGURE 3

The mean of SICI in M1 after a-tDCS stimulation among the four condition groups over three time points. No significant differences were found in main factors of Time or Group. Data are presented at Mean ± SEM.

FIGURE 4

The mean of ICF in M1 after a-tDCS stimulation among four condition groups over three time points. No significant differences were found in main factors of Time or Group. Data are presented at Mean ± SEM.

FIGURE 5

Changes in movement time in blocks of right trained hand (A) and left untrained hand (B). The results showed significant improvement in movement times for all four stimulation groups over three time points. ^∗∗P < 0.01.

FIGURE 6

Results of Friedman test and distribution of error rate by rank in both right trained hand (A), left untrained hand (B) over three time points. Asterisks indicate significant differences in mean rank across time points. ^∗∗P < 0.01.

FIGURE 7

Effects of a-tDCS and training on error rate among the four stimulation groups in the right trained hand (A) and left untrained hand (B) at three time points. No significant effects were found between all stimulation conditions.

FIGURE 8

Results of Friedman test and distribution of skill by rank in blocks of the right hand (A) and left hand (B) over three time points. Asterisks indicate significant differences in mean rank across time points. ^∗∗P < 0.01.

FIGURE 9

Effects of a-tDCS and training on skill among the four stimulation groups in the right trained hand (A) and left untrained hand (B) at three time points. No significant effects were found between all stimulation conditions.