Can high definition transcranial direct current stimulation (HD-tDCS) enhance cognitive training and transfer?

The INSIGHT project combines cognitive training with high-definition transcranial direct current stimulation (HD-tDCS) with the goal of enhancing fluid intelligence (Gf) and adaptive problem solving in healthy adults. 129 young to middle-aged subjects completed 20 1hr cognitive training sessions with a videogame called Mind Frontiers while receiving HD-tDCS. Mind Frontiers was developed for the INISGHT project and comprised six adaptively difficult cognitive training tasks. HD-tDCS was applied for 30 minutes (active) or 30 seconds (sham) at 2.0 mA at the start of each training session, with anodes placed bilaterally over the prefrontal cortex. Before and after cognitive training, participants completed a battery of tasks to assess Gf, executive function, working memory, and episodic memory. Participants who received active tDCS showed improved performance on the Mind Frontiers games compared to those who received sham. Transfer to untrained tasks, however, was unaffected; no comparable improvements in transfer from pre- to post-training were observed. Such results provide strong evidence that tDCS can enhance cognitive training performance but no evidence for enhanced transfer of learned skills to untrained tasks. When individual differences were examined, Gf was found to be an effective predictor of trained task improvement and age moderated the beneficial effect of HD-tDCS on training performance, with younger subjects benefitting more than old. An alternative explanation of the results, an acute effect of tDCS on vigilance decrement, was evaluated but evidence was insufficient to draw firm conclusions. To summarize, tDCS appears to be more effective at enhancing performance of specific skills than improving general cognitive abilities, and this effect is stronger in younger adults