Does tDCS over prefrontal cortex improve episodic memory retrieval? Potential importance of time of day

We report 4 experiments aiming to replicate and extend the finding that anodal transcranial direct current stimulation (tDCS) over dorsolateral prefrontal cortex after encoding and just prior to retrieval improves accuracy on an episodic recollection task. Our first 3 experiments failed to replicate the tDCS effect in planned analyses, but post-hoc analyses uncovered tDCS effects on recollection accuracy during morning sessions. To further investigate, Experiment 4 randomly assigned participants to morning or afternoon sessions. As predicted, tDCS (compared to sham stimulation) improved recollection accuracy in the morning. We hypothesize that tDCS effects are easier to detect during nonoptimal cognitive processing times (e.g., mornings for younger adults). Importantly, we found both significant and null tDCS results across our experiments, indicating that more research is needed to determine the extent that anodal tDCS to left prefrontal cortex reliably improves recollection accuracy at different stimulation times.