The effect of parietal stimulation on hippocampal-cortical rsFC and episodic memory is associated with fractional anisotropy in multiple stimulation site pathways.

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.