Real-time influence of episodic memory retrieval on the sensorimotor network

Michael Freedberg; Rahmawati Rahmawati; Sara Hussain

doi:10.1016/j.brs.2024.12.187

Hippocampal and sensorimotor networks interact to support memory, but the biological dynamics of this relationship are unclear. It is commonly proposed that engagement of the hippocampus suppresses sensorimotor network activity and function. However, our recent work suggests that the sensorimotor network can shift resources away from motor/procedural processes to support hippocampal-mediated processes. We investigated how episodic memory retrieval affects the sensorimotor network in real time using single-pulse TMS (sp-TMS), electroencephalography (EEG), and electromyography (EMG). We measured sensorimotor network responses during episodic memory retrieval by delivering sp-TMS to the motor cortex and measuring ensuing motor-evoked potentials (MEPs). After cue onset, but before TMS delivery, we also measured event-related synchronization (ERS) in the mu (8-12 Hz) and beta (13-30 Hz) power bands, which are proxy measures of sensorimotor network inhibition. We predicted that hippocampal network engagement via episodic memory retrieval would suppress sensorimotor network MEPs and lower ERS after cue onset, suggesting reduced sensorimotor function, but enhanced sensorimotor network activity. Participants studied arbitrary pairs of words before either recalling these memories (episodic memory retrieval) or making perceptual judgements (control task). To equate effort required between the two tasks, perceptual task difficulty was anchored to memory task performance using an adaptive algorithm. Our results show that episodic memory retrieval (but not perceptual judgements) significantly reduced MEP amplitudes evoked from sp-TMS immediately after (<50 ms) cue onset, suggesting that sensorimotor network function is reduced during hippocampal engagement. However, contrary to prevailing views of hippocampal-sensorimotor network interactions, we found decreased mu band ERS, suggesting reduced sensorimotor inhibition. We did not identify changes in beta band activity. Collectively, our results show that sensorimotor responses to hippocampal engagement are measurable at sub-second timescales and support the hypothesis that the sensorimotor network can dynamically shift its processing resources to support hippocampal-mediated processes.

Real-time influence of episodic memory retrieval on the sensorimotor network

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