Abstract
Evidence suggests that excitatory transcranial direct current stimulation (tDCS) may improve performance on a wide variety of cognitive tasks. Due to the non-invasive and inexpensive nature of the method, harnessing its potential could be particularly useful for the treatment of neuropsychiatric illnesses involving cognitive dysfunction. However, questions remain regarding the efficacious stimulation parameters. Here, using a double-blind between-subjects design, we explored whether 1 mA excitatory (anodal) left dorsolateral prefrontal cortex stimulation with a contralateral extracerebral reference electrode, leads to enhanced working memory performance across two days, relative to sham stimulation. Participants performed the 3-back, a test of working memory, at baseline, and during and immediately following stimulation on two days, separated by 24-48 hours. Active stimulation did not significantly enhance performance versus sham over the course of the experiment. However, exploratory comparisons did reveal a significant effect of stimulation group on performance during the first stimulation phase only, with active stimulation recipients performing better than sham. While these results do not support the hypothesis that dorsolateral prefrontal cortex tDCS boosts working memory, they raise the possibility that its effects may be greatest during early learning stages.
Highlights
Transcranial direct current stimulation has been utilised as a non-invasive brain stimulation methodology to improve performance on a variety of cognitive tasks in healthy volunteers, including decision-making1, planning2 and working memory3,4
The testing computer malfunctioned during the day 1 post-transcranial direct current stimulation (tDCS) assessment for 1 participant, approximately 40% through the task; these data were included in the model and the participant completed a further post-tDCS test, which was used only to determine progress to day 2
A significant difference between active sham stimulation was identified at the day 1 tDCS time point (F(1,13.373) = 10.747, P = 0.006; controlling for baseline performance and Bonferonni corrected for multiple comparisons), indicating a large effect size (Cohen’s d = 1.427, r2 = 0.337)
Summary
Transcranial direct current stimulation (tDCS) has been utilised as a non-invasive brain stimulation methodology to improve performance on a variety of cognitive tasks in healthy volunteers, including decision-making, planning and working memory. Tentative evidence far suggests that tDCS may be effective in ameliorating neurological and psychiatric illnesses associated with cognitive deficits. Prominent amongst these are recent developments in the study of the treatment of addiction, depression, schizophrenia and stroke. Questions remain over stimulation condition blinding, optimal stimulation frequency and appropriate electrode placement as these parameters can strongly influence the efficacy of the stimulation device, the induced neuronal activity and the interpretation of stimulation effects on cognitive performance. Questions have been raised about appropriate behavioural and stimulation controls
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