Effects of transcranial direct current stimulation over prefrontal cortex on encoding of verbal episodic memory

: a multi-model approach

Abstract

There is growing interest in the effects of transcranial direct current stimulation (tDCS) on episodic memory and on underlying neural activity. Memory enhancement induced by the technique has potential clinical applications. A few recent studies suggest that left prefrontal tDCS (anode over left dorsolateral prefrontal cortex, DLPFC) during encoding may enhance verbal episodic memory. This thesis adopted a multi-modal approach, investigating behavioural and neural effects of this stimulation montage via experimental, meta-analytic, and neuroimaging methods.

Chapters 1 and 2 explored the effect of left prefrontal tDCS (active vs sham) during verbal encoding in a group of healthy young adults 20 minutes and 1 day after stimulation. Results did not show an enhancement effect, irrespective of different learning instruction and memory test (recognition or recall).

The Network Meta-Analysis described in Chapter 3 explored the efficacy of commonly used electrode montages relative to sham. Based on synthesis of previously published results, the effect of the left prefrontal setup (anode over left DLPFC) was not statistically significant, with small negative to moderate positive population-level effects within the estimated 95% confidence interval. Significant effects with other anodal electrode locations have not yet been successfully replicated.

Using the same protocol of Chapter 1, the event-related fMRI study described in Chapter 4 evaluated neural activity related to verbal memory encoding, and its modulation due to active vs sham tDCS. Results corroborated earlier findings of a widely distributed left-lateral cortical network supporting verbal episodic memory 2 encoding. A potential novel finding induced by tDCS was identified, as insular activity supporting memory encoding appeared to be decreased in the active stimulation group. Whilst potential tDCS effects on neural activity were uncovered, the memory enhancement effect was not observed.

Alternative explanations, limitations and potential other avenues of future research are discussed

Date of Award	6 May 2021
Original language	English
Awarding Institution	University of Roehampton
Sponsors	Roehampton VC Scholarship
Supervisor	Paul Allen (Director of Studies), Marco Sandrini (Co-Supervisor) & Margot Crossman (Co-Supervisor)