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Numerous brain stimulation studies have focused on WM and the majority of the studies suggest that tDCS can improve performance and thus brain stimulation techniques offer promise as a possible tool to remediate or enhance WM (Fregni et al., 2005; Ohn et al., 2008). The majority of the research investigating the effects of tDCS on WM task performance, often utilise the n-back task (e.g., Andrews et al., 2011; Berryhill & Jones, 2012; Fregni et al., 2005; Gill et al., 2015; Mylius et al., 2012; Mulquiney, Hoy, Daskalakis, & Fitzgerald, 2011; Lally, Nord, Walsh, & Roiser, 2013; Ohn et al., 2008; Teo, Hoy, Daskalakis, & Fitzgerald, 2011; Zaehle, Sandmann, Thorne, Jäncke, & Herrmann, 2011; see Berryhill, Peterson, Jones, & Stephens, 2014; Brunoni & Vanderhasselt, 2014 for reviews).

Fregni and colleagues (2005) sought to establish whether anodal tDCS, would have an effect on the performance on a 3-back letter WM task. The study entailed 15 participants who undertook a 3- back WM task based on letters, with anodal stimulation applied on the DLPFC (left dorsolateral prefrontal cortex) at a current of 1 mA for 10 minutes. The findings indicated that left prefrontal cortex anodal stimulation led to increases in the accuracy of task performance, suggesting effects on WM. Additionally, they investigated whether the observed enhancement was due to the focality by applying anodal stimulation to the motor cortex instead of the left DLPFC or polarity by applying cathodal stimulation to the left DLPFC. The findings suggest that the enhancement was dependent on focality and polarity as there was no effect in either of the conditions. As a result, the

researchers concluded that left prefrontal anodal stimulation enhanced WM performance, suggesting possibilities for clinical application.

In a similar study, Richmond, Wolk, Chein, and Olson (2014) explored the extent of tDCS learning enhancement on WM training regime, as well as the level to which the learning gains made could transfer beyond the primary WM training task. Fifty-eight participants took part in an adaptive WM training task lasting 10 sessions taken over 2 weeks. The training was concurrent with dorsolateral PFC stimulation and sandwiched by tests measuring various domains associated with WM abilities. The researchers found that tDCS could enhance learning on the verbal aspect of the training,

besides enhancing near transfer of learning to other untrained WM tasks. The results indicate that tDCS may apply in bolstering training and transfer gains among people with compromised WM abilities. These findings are consistent with the conclusions made by Boggio, Ferrucci, Rigonatti,

Covre, Nitsche, Pascual-Leone, and Fregni (2006), whose study established that tDCS may have a beneficial impact on WM in Parkinson’s disease patients.

Andrews, Hoy, Enticott, Daskalakis, and Fitzgerald (2011) noted the consensus that tDCS applied to the DLPFC could improve WM performances among healthy and clinical subjects. The

researchers then sought to establish whether the aforementioned effect of tDCS on WM could be enhanced through cognitive activity during the tDCS procedure. The study involved 10 participants taking part in three situations; an n-back task during anodal tDCS, anodal tDCS while at rest, and an n-back task during sham tDCS. The findings indicated that applying tDCS during an n-back task led to greater improvement WM performance when compared to tDCS at rest and sham tDCS with an n-back task. These findings suggest that tDCS can not only be employed in improving WM but can also be improved through employing adjunctive cognitive remediation tasks. It is important to note that in this study, tDCS did not affect performance on WM tasks during rest.

Multiple systematic reviews and meta-analyses investigated the effect of tDCS on WM. Brunoni and Vanderhasselt (2014) performed a systematic review and meta-analyses of the effects of Non- Invasive Brain Stimulation (NIBS) sham-controlled, randomised studies over the DLPFC. They assessed whether NIBS improved the performance in the n-back task, which is a reliable index for WM. They found that rTMS of the DLPFC significantly improved all measures of WM

performance whereas tDCS significantly improved reaction time, but not the percentage of correct and error responses. Moreover, they reported that NIBS effects were greater in clinical samples as compared to healthy volunteers. In another systematic reviews and meta-analysis, Hill, Fitzgerald and Hoy (2016) performed a meta-analysis investigating the effects of anodal tDCS, compared to sham, on WM, as assessed using the n-back, Sternberg and digit-span tasks in healthy and

neuropsychiatric cohorts. They also separated the results from tasks performed ‘online’ (during stimulation) and ‘offline’ (following stimulation) and assessed the effects of current density and stimulation duration on WM performance. Their findings demonstrated that anodal tDCS enhanced offline WM reaction times in healthy populations, with a trend towards improvement for accuracy, while online WM accuracy in neuropsychiatric populations was improved. Finally, Mancuso, Ilieva, Hamilton and Farah (2016) conducted a meta-analytic review to explore reasons why meta-analyses may have underestimated the effect of tDCS on WM and report a more comprehensive and

arguably more sensitive meta-analysis. Their analyses revealed a small but significant effect of left DLPFC stimulation coupled with WM training. Left DLPFC stimulation alone also enhanced WM performance, however, the effect was reduced to insignificant after correction for publication bias.

They conclude that the primary WM enhancement potential of tDCS probably lies in its use during training.

As evident by the literature, the effect of anodal tDCS over the left DLPFC on WM has been found not only in healthy young individuals but also older adults (Seo et al., 2011), people with

Parkinson’s disease (Boggio, et al., 2006), major depressive disorder and bipolar depressive disorder (Brunoni et al., 2001), depression (Oliveira et al., 2013), ADHD (Nejati et al., 2017), and stroke patients (Jo et al., 2009) as well. It is important to note that tDCS is approved as a

rehabilitation method for depression by UK clinical guidance bodies. However, due to a lack of robust evidence of its efficacy, mainly due to small sample sizes and differences in protocols (e.g. montages, current strength, duration), NICE encourages and recommends further research (NICE, 2015).