Portraits and memory: Investigating HF-rTMS effects on episodic recall through a sham-controlled fMRI study

Abstract

Background: The dorsolateral prefrontal cortex (DLPFC) is implicated in cognitive control and memory processes, but its causal role in episodic retrieval, especially for complex abstract stimuli, remains unclear. This study examined whether high-frequency repetitive transcranial magnetic stimulation (HF-rTMS) over the left DLPFC modulates memory performance and associated brain activation patterns.

Methods: In a within-subject, sham-controlled fMRI design, 20 healthy female participants viewed and encoded abstract portrait images. Following a single session of active or sham HF-rTMS, they completed a recognition memory task during scanning. Behavioral measures (reaction times, accuracy) were recorded, and whole-brain and region-of-interest (ROI) analyses assessed activation differences related to stimulation and stimulus type (seen vs. unseen).

Results: Seen portraits elicited faster responses overall. However, active HF-rTMS reduced accuracy specifically for seen stimuli, indicating a speed-accuracy trade-off. fMRI analyses revealed a significant interaction between stimulation and stimulus type: active HF-rTMS increased BOLD activity in the left hippocampus (notably the dentate gyrus and subiculum) and right calcarine cortex during recognition of seen stimuli. No comparable effects were observed for unseen images.

Conclusions: A single session of HF-rTMS over the left DLPFC selectively altered the neural dynamics of episodic memory retrieval for abstract portrait stimuli. Although behavioral performance did not improve-and showed reduced accuracy for previously seen items-stimulation elicited increased activation in hippocampal subregions and early visual cortex during successful retrieval. These findings demonstrate that prefrontal neuromodulation can reshape hippocampal-visual network engagement during episodic recall without concomitant behavioral enhancement, highlighting not only the needed engagement of the left DLPFC, but also the potential dissociation between neural recruitment and memory accuracy when processing complex abstract stimuli.

Keywords: Abstract visual stimuli; Dorsolateral prefrontal cortex; Episodic memory; HF-rTMS; Hippocampus; Neuromodulation; fMRI.