Transcranial direct current stimulation to the left dorsolateral prefrontal cortex enhances early dexterity skills with the left non-dominant hand: a randomized controlled trial

Abstract

Background: The left dorsolateral prefrontal cortex (DLPFC) is involved in early-phase manual dexterity skill acquisition when cognitive control processes, such as integration and complexity demands, are required. However, the effectiveness of left DLPFC transcranial direct current stimulation (tDCS) on early-phase motor learning and whether its effectiveness depends on the cognitive demand of the target task are unclear. This study aimed to investigate whether tDCS over the left DLPFC improves non-dominant hand dexterity performance and determine if its efficacy depends on the cognitive demand of the target task.

Methods: In this randomized, double-blind, sham-controlled trial, 70 healthy, right-handed, young adult participants were recruited. They were randomly allocated to the active tDCS (2 mA for 20 min) or sham groups and repeatedly performed the Purdue Pegboard Test (PPT) left-handed peg task and left-handed assembly task three times: pre-tDCS, during tDCS, and post tDCS.

Results: The final sample comprised 66 healthy young adults (mean age, 22.73 ± 1.57 years). There were significant interactions between group and time in both PPT tasks, indicating significantly higher performance of those in the active tDCS group than those in the sham group post tDCS (p < 0.001). Moreover, a greater benefit was observed in the left-handed assembly task performance than in the peg task performance (p < 0.001). No significant correlation between baseline performance and benefits from tDCS was observed in either task.

Conclusions: These results demonstrated that prefrontal tDCS significantly improved early-phase manual dexterity skill acquisition, and its benefits were greater for the task with high cognitive demands. These findings contribute to a deeper understanding of the underlying neurophysiological mechanisms of the left DLPFC in the modulation of early-phase dexterity skill acquisition.

Trial registration: This study was registered in the University Hospital Medical Information Network Clinical Trial Registry in Japan (UMIN000046868), Registered February 8, 2022 https://center6.umin.ac.jp/cgi-open-bin/ctr_e/ctr_view.cgi?recptno=R000053467.

Keywords: Cognitive aspects; Dorsolateral prefrontal cortex; Fine motor; Motor learning; Non-dominant hand; Transcranial direct current stimulation.

Fig. 1

Schematic representation of the experimental procedure and design. A Time course of the study design. This study consisted of the following three phases: Pre-assessment, Assessment, and Post-assessment. The assessment phase consisted of three sessions: baseline, online, and offline. Navy blue shows the 20-min tDCS stimulation period. B The protocol in each of the three assessment sessions consisted of four repetitions of the two subtests of the PPT, the left-handed peg task and left-handed assembly task. Data from the first trial was removed from analysis, considering the possibility of generating large variation in task performance. C Schematic illustration of active and sham stimulation. The left figure shows active stimulation, a sustained 2-mA current delivered for 20 min. The right figure shows sham stimulation, a ramp up to 2 mA followed by an immediate ramp down. PPT Purdue Pegboard Test, tDCS transcranial direct current stimulation

Fig. 2

Left DLPFC (F3) and orbitofrontal cortex (Fp2) electrode placement (international 10/20 system). The left image shows the electrode configurations with the anode (red) over F3 and cathode (blue) over Fp2. The right image shows the underlying cortical electric field on different directions of the brain map. The horizontal color bar indicates the electric field magnitude expressed in norm E (V/m). DLPFC dorsolateral prefrontal cortex

Fig. 3

CONSORT flow diagram. Flow diagram summarizing the steps and number of participants excluded with implementation of each eligibility criteria culminating in the final data analysis. PPT Purdue Pegboard Test, SD standard deviation

Fig. 4

Effects of tDCS stimulation on the number of completions for the two PPT subtests. A The results of the number of completions in the simple peg task. The left panel shows the results of 2 × 3 mixed-design analysis of variance for the left-handed simple peg task. The middle and right panels show the individual performance of the simple peg task across the three assessment sessions in the active and sham groups, respectively. The bold lines show the mean number of completions in each group performance. B The results of the number of completions in the assembly task. The left panel shows the results of 2 × 3 mixed-design analysis of variance for the left-handed assembly task. The middle and right panels show the individual performance of the assembly task across the three assessment sessions in the active and sham groups, respectively. The bold lines show the mean number of completions in each group performance. Error bars indicate the standard error of the mean. * = Corrected p < 0.05. PPT Purdue Pegboard Test, tDCS transcranial direct current stimulation

Fig. 5

Effects of tDCS stimulation on the normalized Z-scores of the two PPT subtests. The results of 2 × 2 × 2 mixed-design analysis of variance of the normalized Z-scores with group as the between-participant factor and time and task type as the within-participant factors. The displayed points show the individual Z-scores. Error bars indicate the 95% confidence interval; the bottom and top of each box, the 25th and 75th percentiles; and the line and square inside the box, the 50th percentile (median) and the mean, respectively. ** = p < 0.001. PPT Purdue Pegboard Test, tDCS transcranial direct current stimulation

Fig. 6

Scatterplots for relationship between PPT task performance at baseline and online and offline session changes. Left, scatterplots of each of the two tasks (simple peg task and assembly task) in the online session. Right, scatterplots of each of the two tasks in offline assessment. The straight and curved lines indicate the mean and 95% confidence interval, respectively. PPT Purdue Pegboard Test