Abstract
Anodal transcranial direct current stimulation (tDCS) to the left dorsolateral prefrontal cortex (DLPFC) has been shown to improve performance on a multitude of cognitive tasks. These are, however, often simple tasks, testing only one cognitive domain at a time. Therefore, the efficacy of brain stimulation for complex tasks has yet to be understood. Using a task designed to increase learning efficiency, this study investigates whether anodal tDCS over the left DLPFC can modulate both learning ability and subsequent long-term memory retention. Using a within-subject design, participants (N = 25) took part in 6 training sessions over consecutive days in which active or sham stimulation was administered randomly (3 of each). A computer-based task was used, containing flags from countries unknown to the participants. Each training session consisted of the repetition of 8 pairs of flag/country names. Subsequently, in three testing sessions, free, cued, and timed cued recall, participants were assessed on all 48 flags they had learnt. No difference in learning speed between active and sham tDCS was found. Furthermore, in the timed cued recall phase, flags learnt in the sham tDCS sessions were recalled significantly better than flags learnt in the active tDCS sessions. This effect was stronger in the second testing session. It was also found that for the flags answered incorrectly; thus, meaning they were presented more frequently, subsequent long-term retention was improved. These results suggest that for a complex task, anodal tDCS is ineffective at improving learning speed and potentially detrimental to long-term retention when employed during encoding. This serves to highlight the complex nature of brain stimulation, providing a greater understanding of its limitations and drawbacks.
Highlights
Cognitive enhancement using transcranial electrical brain stimulation has received much interest in the past decade, with a wide variety of methods showing improvements for memory (Javadi & Walsh, 2012; Katz et al, 2017), reaction time (Hill, Fitzgerald, & Hoy, 2016; Loftus, Yalcin, Baughman, London, UK 4 School of Rehabilitation, Tehran University of Medical Sciences, Tehran, IranVanman, & Hagger, 2015), and motor learning (Antal et al, 2004; Nitsche et al, 2003)
Contrary to previous literature showing potentially beneficial effects of transcranial direct current stimulation (tDCS) on long- and short-term memory performance, we found that anodal tDCS over the left dorsolateral prefrontal cortex (DLPFC) did not improve learning ability compared to sham tDCS
Using a variation of a retrieval-based learning task, we have shown that anodal tDCS to the left DLPFC has no effect on learning ability and is of detriment to subsequent retrieval of flag-country association
Summary
Cognitive enhancement using transcranial electrical brain stimulation has received much interest in the past decade, with a wide variety of methods showing improvements for memory (Javadi & Walsh, 2012; Katz et al, 2017), reaction time We investigated whether transcranial direct current stimulation (tDCS), previously shown to improve cognitive functioning (Coffman, Clark, & Parasuraman, 2014), can be beneficial for a more complex task utilising retrieval-based learning. Participants must provide a set number of correct responses in 1 min to complete the task This task will require substantial executive functioning across multiple cognitive domains. Previous research into retrieval has assessed working memory (WM; Rosen & Engle, 1997) and long-term memory (LTM; Karpicke & Roediger, 2007; Roediger & Karpicke, 2006) By employing this type of learning, the variability in encoding techniques can be reduced, allowing for a clearer understanding of how particular interventions can modulate cognition. As improvements have been shown for LTM following tDCS, it was hypothesised that subsequent long-term retention would be improved for active tDCS relative to the sham tDCS
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