The Dorsolateral Prefrontal Cortex Plays a Role in Self-Initiated Elaborative Cognitive Processing during Episodic Memory Encoding: rTMS Evidence

Colin Hawco,Martin Lepage,Marcelo T Berlim,Antonio Verdejo García

doi:10.1371/journal.pone.0073789

Abstract

During episodic memory encoding, elaborative cognitive processing can improve later recall or recognition. While multiple studies examined the neural correlates of encoding strategies, few studies have explicitly focused on the self-initiation of elaborative encoding. Repetitive transcranial magnetic stimulation (rTMS), a method which can transiently disrupt neural activity, was administered during an associative encoding task. rTMS was either applied to the left dorsolateral prefrontal cortex (DLPFC) or to the vertex (a control region not involved in memory encoding) during presentation of pairs of words. Pairs could be semantically related or not related. Two encoding instructions were given, either cueing participants to analyze semantic relationships (cued condition), or to memorize the pair without any specific strategy cues (the self-initiated condition). Participants filled out a questionnaire regarding their use of memory strategies and performed a cued-recall task. We hypothesized that if the DLPFC plays a role in the self-initiation of elaborative encoding we would observe a reduction in memory performance in the self-initiated condition, particularly for related. We found a significant correlation between the effects of rTMS and strategy use, only in the self-initiated condition with related pairs. High strategy users showed reduced performance following DLPFC stimulation, while low strategy users tended to show increased recall following DLPFC stimulation during encoding. These results suggest the left DLPFC may be involved in the self-initiation of memory strategy use, and individuals may utilize different neural networks depending on their use of encoding strategies.

Highlights

There has recently been a growing interest in the role of the frontal lobes in long-term memory formation
In order to insure there was no effect of the order of Repetitive transcranial magnetic stimulation (rTMS) stimulation blocks, participants were separated into 2 groups according to the order of encoding blocks (DLPFC followed by vertex, or vice-versa) and we compared the mean response for each question within the four types of trials
For each of the four conditions in the study, a difference score was calculated to determine the effects of rTMS applied over dorsolateral prefrontal cortex (DLPFC) versus vertex

Summary

Introduction

There has recently been a growing interest in the role of the frontal lobes in long-term memory formation. Neuroimaging studies of item memory encoding have often observed greater activity in the ventrolateral prefrontal cortex (VLPFC) for items which are later remembered over those which are not later remembered [1,2]. Given that associative encoding may involve greater elaboration or executive control than item encoding, Blumenfeld and Ranganath [4] examined this issue utilizing an effortful, elaborative encoding task for both item and associative encoding. They observed greater activity in the DLPFC for remembered over forgotten stimuli for associative encoding only, while VLPFC activity was greater for subsequently remembered stimuli during both item and associative encoding. This suggests that DLPFC activity was not the result of increased task demands and elaboration but was instead related to forming associations between the items

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