Improving working memory: the effect of combining cognitive activity and anodal transcranial direct current stimulation to the left dorsolateral prefrontal cortex

Background Transcranial direct current stimulation (tDCS), applied to the left dorsolateral prefrontal cortex (DLPFC) has been found to improve working memory (WM) performance in both healthy and clinical participants. However, whether this effect can be enhanced by cognitive activity undertaken during tDCS has not yet been explored. Objective This study aimed to explore whether tDCS applied to the left DLPFC during the persistent performance of one WM task would improve performance on a subsequent WM task, to a greater extent than either tDCS or cognitive activity alone. Methods Ten healthy participants took part in three counterbalanced conditions. The conditions involved 10 minutes of either anodal tDCS while completing an n-back task, anodal tDCS while at rest, or sham tDCS while completing an n-back task. The n-back that was used in this study was a computer-based letter WM task that involved 5 minutes of two-back, followed by 5 minutes of three-back. Digit span forward and backward was administered immediately before and after each treatment, and performance change (pre- to posttreatment) calculated and compared across conditions. The digit span tasks involved a series of numbers being read to the participant, and the participant was required to repeat them back, either in the same order (Digits forward) or in the reverse order (Digits backward). Results tDCS applied during completion of the n-back task was found to result in greater improvement in performance on digit span forward, compared with tDCS applied while at rest and sham tDCS during the n-back task. This finding was not evident with digit span backward. Conclusions These results indicate that there may be potential for the use of adjunctive cognitive remediation techniques to enhance the effects of tDCS. However, further research needs to be undertaken in this area to replicate and extend this finding.