It's the thought that counts: examining the task-dependent effects of transcranial direct current stimulation on executive function

Background: Prior investigations employing transcranial direct current stimulation (tDCS) have shown that stimulation can elicit subsequent improvement on tests of various cognitive abilities, including working memory. While stimulation parameters such as intensity and duration are known to determine the effects of tDCS, the degree to which stimulation effects are influenced by the nature of cognitive activities during stimulation remains unclear.

Objective/hypothesis: To determine whether manipulating the working memory load of a task performed during stimulation would modulate tDCS-induced enhancement of performance on a different, related measure after stimulation.

Methods: In two separate but closely related sham-controlled experiments, two groups of healthy subjects underwent anodal tDCS (2 mA) of the left dorsolateral prefrontal cortex (DLPFC) for 20 min. In Experiment 1, subjects (n = 11) trained on a letter 3Back task during stimulation. In Experiment 2 subjects (n = 11) trained on a letter 1Back task, which resembled the 3Back task but featured a lower working memory load. In both experiments, before and after stimulation, subjects completed an adjusting Paced Auditory Serial Addition Task (A-PASAT). Both the experimenter and subjects were blind to stimulation conditions in both experiments.

Results: Subjects were both faster and more accurate on the A-PASAT task after receiving real tDCS paired with 3Back training (Experiment1) compared to sham+3Back, real+1Back, and sham+1Back conditions.

Conclusions: The cognitive demands of a task performed during tDCS can influence the effects of tDCS on post-stimulation performance. This finding has direct relevance to the use of tDCS as an investigative tool in cognitive neuroscience and as a therapy.