Transcranial Direct Current Stimulation Studies Investigating the Role of the DLPFC in Memory and Metamemory

Metamemory, broadly defined as knowledge about one’s own memory, is thought to consist of two separate processes, monitoring, the ability to assess the contents of your own memory, and control, altering behavior in service of memory, that work together to aide in encoding and retrieval processes. Prior work has shown that metamemory monitoring accuracy can be improved in a semantic memory task via high-definition transcranial direct current stimulation (HD-tDCS), a form of non-invasive brain stimulation, over the left dorsolateral prefrontal cortex (DLPFC). Over three experiments, the current dissertation sought to replicate and extend this work by: 1) testing the effects of HD-tDCS over the DLPFC in memory and metamemory monitoring for an episodic memory task, 2) testing of the effects of conventional tDCS over the DLPFC in memory and metamemory monitoring for both a semantic and episodic task using remotely supervised (rs) tDCS, 3) using HD-tDCS to examine the role of the DLPFC in memory and metamemory monitoring and additionally metamemory control. There were some effects of brain stimulation on memory performance, with greater cued recall when HD-tDCS was applied over the DLPFC right before cued recall compared to sham. Additionally, there were rs-tDCS induced improvements in recognition accuracy, but only in certain task orders. However, across three experiments there was no evidence that either HD or conventional rs-tDCS over the left DLPFC results in improved metamemory monitoring accuracy. A follow-up behavioral study suggested that, in one experiment, the lack of improvement in metamemory monitoring accuracy was related to changes made in the behavioral task. Turning to metamemory control, there were no effects of brain stimulation, but there was evidence that individuals use their metamemory monitoring judgments to control their memory. Although there were some interesting findings, there were mostly null effects of tDCS, suggesting a need for more systematic testing of tDCS as an experimental tool, specifically regarding the parameters in which tDCS does and does not produce changes in cognition

Differential effects of remotely supervised transcranial direct current stimulation on recognition memory depending on task order

BackgroundPrior work has shown positive effects of High Definition transcranial direct current stimulation (HD-tDCS) over the dorsolateral prefrontal cortex (DLPFC) on semantic memory performance and metamemory monitoring accuracy. However, HD-tDCS requires setup by a trained researcher, which is not always feasible. Few studies have used remotely supervised (rs) tDCS in healthy populations, and remote supervision has strong practical benefits.Objective/hypothesisThe goal of the current study was to test if previously shown effects of HD-tDCS over the left DLPFC on semantic memory performance and metamemory monitoring accuracy extended to conventional rs-tDCS, which is less focal than HD-tDCS, and to episodic memory and metamemory tasks.Materials and methodsA total of 36 healthy participants completed 6 weeks of rs-tDCS sessions, with either active left or right anodal DLPFC stimulation, or sham. Participants completed semantic and episodic memory and metamemory tasks, which each lasted for three consecutive sessions, and session order was counterbalanced across participants.ResultsOverall, there were no main effects of rs-tDCS on metamemory monitoring accuracy or memory performance for either the semantic or the episodic tasks. However, there were effects of rs-tDCS that depended on the order of completing the episodic and semantic task sessions. When participants completed the semantic task sessions after the episodic task sessions, semantic recognition was greater in the left anodal DLPFC condition. In a parallel effect, when participants completed the episodic task sessions after the semantic task sessions, episodic recognition was greater in the right anodal DLPFC condition.ConclusionPrior experience with tDCS is a factor for effects of rs-tDCS on cognition. Additionally, the current experiment provides evidence for the feasibility of fully remotely supervised tDCS in healthy participants