Poster A87, Saturday, March 25, 5:00 – 7:00 pm, Pacific Concourse
Frequency dependence of noninvasive brain stimulation effects on hippocampal-cortical networks
Molly S. Hermiller1, Zainab Fatima1, Jonathan O'Neil1, Robert Palumbo1, Stephen VanHaerents1, Tommi Raij1,2, Donna Bridge1, Joel L. Voss1; 1Northwestern University Feinberg School of Medicine, 2Rehabilitation Institute of Chicago
The hippocampus serves as a hub within hippocampal-cortical networks that are thought to contribute to distinct memory processes, such as item versus source memory. Transcranial magnetic stimulation (TMS) can be used to manipulate brain networks and test their causal roles in various memory expressions. We previously showed that multi-day 20-Hz repetitive TMS (rTMS) of lateral parietal areas can increase fMRI connectivity among distributed hippocampal-cortical network regions and enhance associative memory. rTMS using patterns such as theta-burst stimulation (TBS) could improve targeting of the hippocampal-cortical network. Here, we compared within-subject effects of single-session intermittent TBS, continuous TBS, and 20-Hz rTMS relative to sham (counterbalanced across four days) on memory performance and fMRI connectivity. On each experimental day, subjects (N=12) studied 96 unique words presented in one of eight colors. Words were later tested in an old/new recognition test followed by forced-choice associative color recognition. During the retention interval, rTMS was delivered, followed immediately by a resting-state fMRI and then memory testing. Whereas one session of 20-Hz rTMS had no robust effects on hippocampal connectivity, intermittent and continuous TBS both significantly modulated hippocampal-cortical fMRI connectivity. Changes were more constrained to the posterior/medial hippocampal cortical network following intermittent TBS, whereas changes were more cortically distributed following continuous TBS. Recollection-related performance measured in the memory test increased significantly only after intermittent TBS. These findings suggest that stimulation effects vary for different stimulation frequencies and patterns. This research has implications for distinguishing hippocampal-cortical network contributions to item versus source memory.
Topic Area: LONG-TERM MEMORY: Episodic