Abstract
Working memory (WM) is a fundamental cognitive ability that supports complex thought but is limited in capacity. Thus, WM training interventions have become very popular as a means of potentially improving WM-related skills. Another promising intervention that has gained increasing traction in recent years is transcranial direct current stimulation (tDCS), a noninvasive form of brain stimulation that can modulate cortical excitability and temporarily increase brain plasticity. As such, it has the potential to boost learning and enhance performance on cognitive tasks. This study assessed the efficacy of tDCS to supplement WM training. Sixty-two participants were randomized to receive either right prefrontal, left prefrontal, or sham stimulation with concurrent visuospatial WM training over the course of seven training sessions. Results showed that tDCS enhanced training performance, which was strikingly preserved several months after training completion. Furthermore, we observed stronger effects when tDCS was spaced over a weekend break relative to consecutive daily training, and we also demonstrated selective transfer in the right prefrontal group to nontrained tasks of visual and spatial WM. These findings shed light on how tDCS may be leveraged as a tool to enhance performance on WM-intensive learning tasks.
INTRODUCTION
Working memory (WM) is a fundamental cognitive ability that is limited in capacity and supports complex thought. It is highly predictive of academic and professional success (Alloway & Alloway, 2010; Gathercole, Pickering, Knight, & Stegmann, 2004), and thus, interventions to improve WM are highly sought. Training of WM typically leads to substantial improvements on the trained task and has also been shown by many studies to enhance various aspects of cognitive functioning, from improving performance on nontrained WM and executive function tasks (Schwaighofer, Fischer, & Buhner, 2015; Melby-Lervåg & Hulme, 2013) to broader tests such as those indexing fluid intelligence (see Weicker, Villringer, & Thone-Otto, 2015; Au et al., 2014; Karbach & Verhaeghen, 2014, for recent meta-analyses). However, obtaining reliable results often requires extensive training on the order of weeks or even months, thereby rendering participant compliance difficult and research costs high. These practical constraints have often led to underpowered studies (Bogg & Lasecki, 2014) and inconsistent results in the literature. Therefore, the field would benefit from a catalyst to intensify or expedite the effects of WM training. Herein, we evaluated the efficacy of transcranial direct current stimulation (tDCS) to boost the effects of training on both trained and untrained measures of WM and executive function over a short period of 7 days. In contrast with previous investigations, the design of the current study included both a long-term follow-up as well as a training schedule that permitted us to explore the impact of spacing on training performance. Thus, the present research not only adds to the growing literature in support of the effects of tDCS on WM, but it also offers novel insights with regard to the cumulative efficacy of multisession stimulation, the effects of intersession spacing, and the long-term durability of stimulation- enhanced training.
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