No evidence of tDCS placebo-effect on associative and working memory performance in healthy adults

Abstract

Transcranial Direct Current Stimulation (tDCS) is a valuable tool in cognitive research as sham condition enables causal conclusions about neural substrates of cognitive functions. Since tDCS studies often yield inconsistent findings, there has been a growing interest in factors that may moderate the effects, one of which being the participants’ awareness of the tDCS condition (real or sham) they received. Here we explore if participants’ beliefs about received stimulation types impacted their task performance in tDCS experiments on associative (AM) and working memory (WM). We analysed data from four within-subject, sham-controlled tDCS memory experiments. Eighty-two young, healthy volunteers took part in four experiments. Two AM experiments included 20 minutes of anodal 1.5mA tDCS over posterior-parietal cortex (PPC) – left in Experiment 1; right in Experiment 2. WM experiments targeted PPC and dorsolateral prefrontal cortex (DLPFC) – left side in Experiment 3; right side in Experiment 4, with 20 minutes of 1.8mA anodal tDCS. The participants completed memory tasks immediately following the stimulation. Results revealed that correct sham guessing had no effects on AM improvement in Experiments 1 and Experiment 2. Similarly, the group that accurately guessed sham condition did not have higher enhancement of verbal and spatial WM performance in neither Experiment 3 nor Experiment 4. Therefore, we found no evidence that the awareness of the tDCS-condition leads to better memory performance in within-subject experiments.