Classical lesion studies led to a consensus that episodic and procedural memory arises from segregated networks identified with the hippocampus and the caudate nucleus, respectively. Neuroimaging studies, however, show that competitive and cooperative interactions occur between networks during memory tasks. Furthermore, causal experiments to manipulate connectivity between these networks have not been performed in humans. Although nodes common to both networks, such as the precuneus and ventrolateral thalamus, may mediate their interaction, there is no experimental evidence for this. We tested how network-targeted noninvasive brain stimulation affects episodic–procedural network interactions and how these network manipulations affect episodic and procedural memory in healthy young adults. Compared to control (vertex) stimulation, hippocampal network-targeted stimulation increased within-network functional connectivity and hippocampal connectivity with the caudate. It also increased episodic, relative to procedural, memory, and this persisted one week later. The differential effect on episodic versus procedural memory was associated with increased functional connectivity between the caudate, precuneus, and ventrolateral thalamus. These findings provide direct evidence of episodic–procedural network competition, mediated by regions common to both networks. Enhanced hippocampal network connectivity may boost episodic, but decrease procedural, memory by co-opting resources shared between networks.
Repetitive transcranial magnetic stimulation (rTMS) of the inferior parietal cortex (IPC) increases resting-state functional connectivity (rsFC) of the hippocampus with the precuneus and other posterior cortical areas and causes proportional improvement of episodic memory. The anatomical pathway responsible for the propagation of these effects from the IPC is unknown and may not be direct. Using diffusion tensor imaging, we examined whether individual differences in fractional anisotropy (FA), a tensor-derived quantity related to white matter properties, in pathways between the IPC and medial temporal lobe (MTL), via the parahippocampal cortex and the precuneus, accounted for individual differences in hippocampal rsFC and memory change after rTMS. FA in the IPC-parahippocampal pathway was associated with rsFC change in a few small cortical clusters, while FA in the IPC-precuneus pathway was strongly linked to widespread changes in rsFC. FA in both pathways was related to episodic memory, but not to procedural memory. These results implicate pathways to the MTL and to the precuneus in the enhancing effect of parietal rTMS on hippocampal rsFC and memory.
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