Stimulation of the left dorsolateral prefrontal cortex with slow rTMS enhances verbal memory formation.

Mircea Van Der Plas,Simon Hanslmayr,Verena Braun,Benjamin Johannes Stauch,Martin Dresler

doi:10.1371/journal.pbio.3001363

Abstract

Encoding of episodic memories relies on stimulus-specific information processing and involves the left prefrontal cortex. We here present an incidental finding from a simultaneous EEG-TMS experiment as well as a replication of this unexpected effect. Our results reveal that stimulating the left dorsolateral prefrontal cortex (DLPFC) with slow repetitive transcranial magnetic stimulation (rTMS) leads to enhanced word memory performance. A total of 40 healthy human participants engaged in a list learning paradigm. Half of the participants (N = 20) received 1 Hz rTMS to the left DLPFC, while the other half (N = 20) received 1 Hz rTMS to the vertex and served as a control group. Participants receiving left DLPFC stimulation demonstrated enhanced memory performance compared to the control group. This effect was replicated in a within-subjects experiment where 24 participants received 1 Hz rTMS to the left DLPFC and vertex. In this second experiment, DLPFC stimulation also induced better memory performance compared to vertex stimulation. In addition to these behavioural effects, we found that 1 Hz rTMS to DLPFC induced stronger beta power modulation in posterior areas, a state that is known to be beneficial for memory encoding. Further analysis indicated that beta modulations did not have an oscillatory origin. Instead, the observed beta modulations were a result of a spectral tilt, suggesting inhibition of these parietal regions. These results show that applying 1 Hz rTMS to DLPFC, an area involved in episodic memory formation, improves memory performance via modulating neural activity in parietal regions.

Highlights

We are able to encode and store episodes that are rich in detail, filled with information, and highly associative [1]
We reanalysed their repetitive transcranial magnetic stimulation (rTMS)-EEG dataset and found that 1 Hz rTMS applied to the left dorsolateral prefrontal cortex (DLPFC) during encoding of verbal material enhances memory performance
We demonstrated in 2 experiments that 1 Hz rTMS delivered to the left DLPFC during episodic memory encoding boosts memory performance

Summary

Introduction

We are able to encode and store episodes that are rich in detail, filled with information, and highly associative [1]. Stimulation at the DLPFC during encoding has been shown to reduce performance on verbal episodic memory tasks [7,8]. These reductions in performance have been mainly achieved with facilitative stimulation protocols We here report an incidental finding from the dataset of an existing study [14] in which the authors examined the role of the left DLPFC in voluntary forgetting. We reanalysed their rTMS-EEG dataset and found that 1 Hz rTMS applied to the left DLPFC during encoding of verbal material enhances memory performance. To ensure that the memory enhancing effects of rTMS are replicable, we conducted a second experiment that confirmed the memory enhancing effect of left DLPFC stimulation (experiment 2)

Results

Discussion

Conclusions

Material and methods