Selective and coherent activity increases due to stimulation indicate functional distinctions between episodic memory networks

Posterior-medial and anterior-temporal cortical networks interact with the hippocampus and are thought to distinctly support episodic memory. We causally tested this putative distinction by determining whether targeted noninvasive stimulation could selectively affect neural signals of memory formation within the posterior-medial network. Stimulation enhanced the posterior-medial network's evoked response to stimuli during memory formation, and this activity increase was coherent throughout the network. In contrast, there was no increase in anterior temporal network activity due to stimulation. In addition, control stimulation of an out-of-network prefrontal cortex location in a separate group of subjects did not influence memory-related activity in either network. The posterior-medial network is therefore a functional unit for memory processing that is distinct from the anterior temporal network. These findings suggest that targeted stimulation can lead to network-specific increases in excitability during memory formation and hold promise for efforts to fine-tune network involvement in episodic memory via brain stimulation © 2018 The Authors, some rights reserved

Supplementary Materials

Supplementary Material

Data from: Network-targeted stimulation engages neurobehavioral hallmarks of age-related memory decline

Objective: To test whether targeting hippocampal-cortical brain networks with high-frequency transcranial magnetic stimulation (TMS) in older adults influences behavioral and neural measures characteristic of age-related memory impairment. Methods: Fifteen adults aged 64 – 80 years (mean = 72 years) completed a single-blind, sham-controlled experiment. Stimulation targets in parietal cortex were determined based on functional magnetic resonance imaging (fMRI) connectivity with the hippocampus. Recollection and recognition memory were assessed after five consecutive daily sessions of full-intensity stimulation versus low-intensity sham stimulation using a within-subjects crossover design. Neural correlates of recollection and recognition memory formation were obtained via fMRI, measured within the targeted hippocampal-cortical network versus a control frontal-parietal network. These outcomes were measured ~24 hours after the final stimulation session. Results: Recollection was specifically impaired in older adults compared to a young-adult control sample at baseline. Relative to sham, stimulation improved recollection to a greater extent than recognition. Stimulation increased recollection fMRI signals throughout the hippocampal-cortical network, including at the targeted location of the hippocampus. Effects of stimulation on fMRI recollection signals were greater than those for recognition and were greater in the targeted network compared to the control network. Conclusions: Age-related recollection impairments were causally related to hippocampal-cortical network function in older adults. Stimulation selectively modified neural and behavioral hallmarks of age-related memory impairment, indicating effective engagement of memory intervention targets in older adults