Dynamic changes in prefrontal cortex involvement during verbal episodic memory formation

- We investigated encoding-related post-stimulus brain activity in the prefrontal cortex with r-TMS.
- We delivered r-TMS during encoding while a word appeared on the screen, and at different points in time after its offset.
- r-TMS over the ventrolateral prefrontal cortex shortly after stimulus offset decreased subsequent memory accuracy.
- No effects were found when r-TMS was delivered over the dorsolateral prefrontal cortex at any time point.
- The involvement of the ventrolateral prefrontal cortex may reflect offset-specific processes.

During encoding, the neural activity immediately before or during an event can predict whether that event will be later remembered. The contribution of brain activity immediately after an event to memory formation is however less known. Here, we used repetitive Transcranial Magnetic Stimulation (rTMS) to investigate the temporal dynamics of episodic memory encoding with a focus on post-stimulus time intervals. At encoding, rTMS was applied during the online processing of the word, at its offset, or 100, 200, 300 or 400 ms thereafter. rTMS was delivered to the left ventrolateral (VLPFC) or dorsolateral prefrontal cortex (DLPFC). VLPFC rTMS during the first few hundreds of milliseconds after word offset disrupted subsequent recognition accuracy. We did not observe effects of DLPFC rTMS at any time point. These results suggest that encoding-related VLPFC engagement starts at a relatively late processing stage, and may reflect brain processes related to the offset of the stimulus.