The Effects of High Definition Transcranial Direct Current Stimulation (HD-tDCS) on Memory and Metamemory by Task Difficulty

Abstract

Sometimes when individuals don’t know the answer to a question, they may nevertheless have a “feeling of knowing” (FOK) that the answer is in their memory. FOK reflects ‘metamemory’, or knowledge of one’s own memory. Previous work showed that high definition transcranial direct current stimulation (HD-tDCS) over the dorsal lateral prefrontal cortex (DLPFC) enhanced FOK accuracy, whereas HD-tDCS over the anterior temporal lobes (ATL) showed no effect on memory or metamemory. However, other studies show that the effects of tDCS on cognition vary based on task difficulty. We hypothesized that difficulty moderates the effects of HD-tDCS on memory and metamemory. Thirty-six adults completed 3 sessions with DLPFC, ATL, and sham stimulation during 3 tasks: recall, recognition, and an FOK task using 100 general knowledge questions of varying difficulty (easy, medium, and hard). Recall tests showed effects of difficulty. During the medium block, there was greater recall with ATL stimulation compared to sham (p<0.005) and DLPFC stimulation (p<0.02), but no effects of HD-tDCS during easy and hard blocks. In contrast, during recognition, DLPFC stimulation led to better recognition compared to sham (p<0.05) and ATL stimulation (p<0.005), but there was no interaction with difficulty. A similar pattern was seen for metamemory accuracy; DLPFC stimulation led to more accurate FOK judgments compared to sham (p<0.00005) and ATL stimulation (p<0.0003), and no interaction with difficulty. These results indicate that HD-tDCS can dissociate the roles of the ATL and DLPFC, and that the effects of HD-tDCS on behavior can be moderated by task difficulty.