A systematic review of transcranial electrical stimulation combined with cognitive training

Background: Transcranial Electrical Stimulation (tES) methods have been shown to enhance performance across a range of cognitive tasks. It is thought that tES can be used to enhance the treatment-effects of cognitive training (CT), leading to lasting improvements in neurocognitive function. Recently, a small number of studies have investigated the effects of tES combined with CT in healthy and cognitively impaired subjects. Objective: To evaluate the effects of tES + CT on both CT task performance and on non-trained cognitive outcomes. Methods: A systematic review was conducted in accordance with PRISMA guidelines. Databases (PsycINFO, EMBASE, PubMed and Medline) were searched for all randomized, controlled and naturalistic prospective studies up until June 2014, combining tES and CT. Results: 13 studies comprising 465 participants met the inclusion criteria. Findings indicated that tES + CT enhanced performance on the majority of CT tasks. The effects on non-trained tasks were mixed, with some evidence for improvements in working memory, cognitive control, approximate number sense and arithmetic processing. Conclusions: tES + CT enhances performance on CT tasks across a range of cognitive functions. Preliminary evidence suggests that tES may also increase transfer effects to non-trained tasks in some domains. Recommendations for future studies are provided.