Electrical stimulation of the dorsolateral prefrontal cortex improves memory monitoring

Abstract

The ability to accurately monitor one's own memory is an important feature of normal memory function. Converging evidence from neuroimaging and lesion studies have implicated the dorsolateral prefrontal cortex (DLPFC) in memory monitoring. Here we used high definition transcranial direct stimulation (HD-tDCS), a non-invasive form of brain stimulation, to test whether the DLPFC has a causal role in memory monitoring, and the nature of that role. We used a metamemory monitoring task, in which participants first attempted to recall the answer to a general knowledge question, then gave a feeling-of-knowing (FOK) judgment, followed by a forced choice recognition task. When participants received DLPFC stimulation, their feeling-of-knowing judgments were better predictors of memory performance, i.e., they had better memory monitoring accuracy, compared to stimulation of a control site, the anterior temporal lobe (ATL). Effects of DLPFC stimulation were specific to monitoring accuracy, as there was no significant increase in memory performance, and if anything, there was poorer memory performance with DLPFC stimulation. Thus we have demonstrated a causal role for the DLPFC in memory monitoring, and showed that electrically stimulating the left DLPFC led people to more accurately monitor and judge their own memory.

Effects of HD-tDCS on memory and metamemory for general knowledge questions that vary by difficulty

“…This is important because tDCS effects on cognition have been shown to vary based on task difficulty, at least in working memory tasks [10,11]. The present study tested whether our previous results were replicable [9] and also examined the effects of HD-tDCS over the DLPFC and ATL on a semantic long-term memory and metamemory task where general knowledge questions varied based on difficulty.…”

“…In terms of the brain regions supporting metamemory and semantic memory, the frontal lobes are thought to be critical for metamemory [4,5] and the anterior temporal lobes are thought to be critical for semantic memory [6–8]. We recently showed the accuracy of FOK judgments (i.e., how related the judgments are to memory accuracy) can be enhanced by increasing excitability of the dorsolateral prefrontal cortex (DLPFC) via high definition transcranial direct current stimulation (HD-tDCS), whereas there was no significant effect of increasing excitability of the anterior temporal lobe (ATL) via HD-tDCS on semantic long-term memory or metamemory [9]. However, it is unknown whether these effects are consistent across different levels of general knowledge question difficulty.…”

“…In the present study, we targeted the ATL and DLPFC to: 1) replicate our previous work [9], and 2) based on previous work showing their roles in memory and metamemory [15]. We chose to stimulate the ATL because ERP studies using the general knowledge task have shown memory effects over inferior-temporal electrode sites [16–18].…”

“…In another example"anodal” tDCS over the prefrontal cortex led to improvements in sentence processing and n-back tasks, but only in the more difficult conditions [43]. There have been numerous published papers using conventional tDCS [for review, see 25, 29], but there are fewer studies that have tested the effects of HD-tDCS on cognition [9, 44–47]. We previously showed that HD-tDCS over the DLPFC improved metamemory accuracy [9], but did not show the expected increase in recall with ATL stimulation.…”

“…There have been numerous published papers using conventional tDCS [for review, see 25, 29], but there are fewer studies that have tested the effects of HD-tDCS on cognition [9, 44–47]. We previously showed that HD-tDCS over the DLPFC improved metamemory accuracy [9], but did not show the expected increase in recall with ATL stimulation. Therefore, the aims of this experiment were to replicate those findings and examine whether the effects of HD-tDCS were moderated by question difficulty.…”