Targeting the Default Mode Network: A TMS-fMRI Study - PROJECT SUMMARY / ABSTRACT Post-traumatic stress disorder (PTSD) is a devastating illness in which traumatic autobiographical memories are intrusive and lead to anxiety symptoms. These symptoms align with the functions of the default mode network (DMN) and, in fact, PTSD patients have abnormalities within the DMN and in its interactions with other networks, notably the salience network and the frontoparietal or central executive network. Focal repetitive pulse transcranial magnetic stimulation (rTMS) enables neuromodulation of selected brain regions and connected networks to treat specific symptoms, but the brain targets to support this therapy in PTSD are under discovery. A recent analysis uncovered a brain circuit associated with improvement in anxiety and somatic symptoms following the rTMS treatment of depression. The left hemisphere region with the strongest fMRI functional connectivity with this circuit lies within anatomical area 8Av and the DMN. This association suggests that modulating the DMN through stimulation at left 8Av could be a novel rTMS approach for the treatment of anxiety and may help ameliorate anxiety symptoms in PTSD. This target would be novel since the vast majority of clinical trials of rTMS in PTSD have targeted the right frontal regions of the salience and frontoparietal networks instead of the DMN. One potential reason is that the most established nodes of the DMN do not lie directly below the scalp/skull and are thus unreachable by rTMS. In this proposal we test the overall hypothesis that left area 8Av can serve as a robust, direct brain target for the DMN thus facilitating therapy for PTSD and the many other disorders involving the DMN. We propose to use TMS-fMRI in 30 Veterans with PTSD to test the causal connections between left 8Av and other regions that could mediate a response. We will test the connectivity between 8Av and the inferior parietal lobe (IPL), a region in the DMN involved in context processing, and other nodes of the DMN (e.g., posterior cingulate, ventromedial prefrontal cortex). We have pilot data in which we found the functional connection between 8Av and the IPL to be abnormal relative to controls, and also that delivering rTMS to these regions ameliorates anxiety. We will also explore whether stimulation at 8Av modulates the anterior insula, a node of the salience network whose functional connectivity predicts benefit from prolonged exposure therapy in PTSD. We will measure the TMS induced BOLD response in these areas to stimulation of 8Av and compare this response to conventional seed-based resting-state fMRI functional connectivity analyses that could serve as an alternative marker for capacity for modulation. In addition we will deliver thetaburst rTMS stimulation and study how connectivity changes with respect to baseline. Our overall goal is to characterize left 8Av functional connectivity in PTSD, and explore the effects of rTMS stimulation parameters. This project will thus provide a mechanistic understanding of rTMS therapy at 8Av, and will reveal the effects of a novel connectivity-based atlas target.
