Augmentation of working memory training by transcranial direct current stimulation (tDCS)

Abstract

Transcranial direct current stimulation (tDCS) to the dorsolateral prefrontal cortex (dlPFC) can modulate working memory (WM) performance. However, evidence regarding the enhancement of WM training, its sustainability and transferability is ambiguous. Since WM functioning appears to be lateralized in respect to stimulus characteristics, this study examined the difference between task-congruent (spatial-right, verbal-left), task-incongruent (spatial-left, verbal-right) and sham tDCS in regards to the efficacy of WM training. In a randomized, sham-controlled experiment, 71 healthy adults trained on a spatial or verbal adaptive n-back task. After a baseline session, anodal or sham tDCS (1 mA) to the right or left dlPFC was applied during the next three training sessions. Sustainability of training gains and near-transfer (verbal or spatial 3-back task) were tested in a fourth training and a follow-up session. Compared to sham stimulation, we found a steeper learning curve when WM training was combined with task-congruent tDCS. This advantage was also present compared to task-incongruent tDCS. Moreover, these effects lasted for up to nine months and transferred to the respective untrained task. These long-lasting, transferable, task-specific effects demonstrate a behaviorally relevant and sustainable facilitation of neuroplastic processes by tDCS that could be harnessed for the treatment of disorders associated with deficient WM.

Expectations May Influence the Effects of Transcranial Direct Current Stimulation

“…Specifically, we asked: i) whether people tend to have neutral, optimistic, or pessimistic expectations of NIBS; ii) whether expectations of NIBS outcomes can be modified based on information indicating that the procedure either can or cannot enhance cognitive function; and iii) whether expectations of NIBS interact with the effects of anodal tDCS during performance of a cognitively challenging task. Our study represents a conceptual replication and extension of previous research reporting enhanced WM performance following a single session of anodal tDCS applied to the left dlLFC, and showing greater within-and between-session improvements following concurrent (i.e., "online") rather than sequential (i.e., "offline") task performance (Martin, Liu, Alonzo, Green, & Loo, 2014; see also Ruf et al, 2017).…”

Working memory training mostly engages general-purpose large-scale networks for learning

“…Training can be based on a single task or multiple tasks (Cheng et al 2012, Binder et al 2016 Clair and Thompson 2008), and the training outcomes can be measured in various ways ranging from cognitive tasks to subjective ratings, symptom measures, and daily activities. In addition to training regimes simply focusing on repetitive performance of the same tasks, some studies have examined joint effects of cognitive training and conventional academic tasks (Nemmi et al 2016), aimed for improvement of everyday attention control functions (see Lilienthal et al 2013), or examined the effects of concomitant brain stimulation (Richmond et al 2014, Nilsson et al 2017, Ruf et al 2017, Stephens et al 2017. Brain imaging evidence is still lacking for many of these approaches developed in behavioral research.…”

Cognitive function assessment during 2 mA transcranial direct current stimulation in DLPFC in healthy volunteers

“…Au et al (2016), for example, showed an active tDCS enhanced cognitive performance (7 days of working memory training) compared to sham tDCS to a sample of healthy individuals and lasted for many months after cessation of training (Au et al 2016). In a similar research study by Ruf et al (2017 showed improvement for spatial and verbal working memory after active tDCS in healthy adults for three consecutive training sessions.The idea of the performing task during stimulation to control the variability and the expected effects of this anodal stimulation might have increased cognitive performance in the present study.…”