Functionally specific effects of targeted noninvasive stimulation on hippocampal-cortical network connectivity
Kristen Warren1, Molly Hermiller1, Steven VanHaerents1, Joel Voss1; 1Northwestern University
Episodic memory is supported by the hippocampus and a distributed network of interacting cortical regions. We have shown that stimulation targeting this hippocampal-cortical network (HCN) increases resting-state fMRI correlations among network regions and influences episodic memory. However, connectivity of this network is also altered by memory retrieval. Here, we investigated the functional and regional specificity of stimulation effects on connectivity measured during a specific memory demand, autobiographical retrieval, relative to those observed via resting-state fMRI. Subjects (n=32) underwent resting-state and autobiographical retrieval-state fMRI scans following five days of high-frequency (20 Hz) transcranial magnetic stimulation to either a left lateral parietal cortex location of the HCN (n=16) or a left prefrontal cortex location that is not part of the HCN and not associated with autobiographical retrieval (n=16). We identified state-specific and network-specific effects of stimulation on connectivity. Parietal stimulation had a greater effect on retrieval-state relative to resting-state connectivity primarily in HCN regions, whereas prefrontal stimulation had a greater effect on resting-state relative to retrieval-state. fMRI connectivity changes in the medial temporal lobe due to stimulation predicted corresponding changes in episodic memory performance measured during a separate task, with greater modulation of retrieval-state connectivity relative to rest predicting greater context recollection improvement. These findings indicate that the expression of neuroplasticity generated by HCN-targeted stimulation is most robust when measured during memory processing. Additionally, recollection memory is causally related to memory-specific fMRI connectivity of the MTL. This result supports the utility of noninvasive stimulation for functionally selective modulation of brain networks.
Topic Area: LONG-TERM MEMORY: Episodic