Abstract
Successful episodic memory involves dynamic increases in activity across distributed hippocampal networks, including the posterior-medial (PMN) and the anterior-temporal (ATN) networks. We tested whether this up-regulation of functional connectivity during memory processing can be enhanced within hippocampal networks by noninvasive stimulation, and whether such task-dependent connectivity enhancement predicts memory improvement. Participants received stimulation targeting the PMN or an out-of-network control location. We compared the effects of stimulation on fMRI connectivity during an autobiographical retrieval task versus during rest within the PMN and the ATN. PMN-targeted stimulation significantly increased connectivity during autobiographical retrieval versus rest within the PMN. This effect was not observed in the ATN, or in either network following control stimulation. Task-dependent increases in connectivity within the medial temporal lobe predicted improved performance of a separate episodic memory test. It is therefore possible to enhance the task-dependent regulation of hippocampal network connectivity that supports memory processing using noninvasive stimulation.
Highlights
There is substantial interest in treating memory disorders via brain stimulation (Suthana and Fried, 2014; Kim et al, 2016; Sreekumar et al, 2017)
The goal of this study was to determine whether noninvasive stimulation targeting the hippocampal-cortical network can enhance network connectivity measured during memory processing, and whether such enhancement is related to episodic memory improvement
We examined the task-dependent effects for posteriormedial network (PMN)-targeted versus prefrontal cortex (PFC)-targeted control stimulation on network-wide interconnectivity in the PMN and the anterior-temporal network (ATN) using a linear mixed effects model with factors stimulation condition, task, and stimulation location (PMN-targeted versus PFC-targeted), controlling for scan data quality
Summary
There is substantial interest in treating memory disorders via brain stimulation (Suthana and Fried, 2014; Kim et al, 2016; Sreekumar et al, 2017). We targeted specific portions of the posteriormedial network (PMN) and further hypothesized that stimulation would disproportionately impact the PMN rather than the anterior-temporal network (ATN). This question is of substantial mechanistic and practical significance, given that increased connectivity due to stimulation should manifest primarily during memory processing that depends on the network, and such task-dependent modulation would be essential for any effective intervention to improve memory ability. Hippocampal network regions can serve as a target for memory improvement
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