TMScogModTheta Burst Transcranial Magnetic Stimulation of Fronto-parietal Networks: Modulation by Mental State

We will test the broad hypothesis that when TBS is applied during a controlled mental state, network changes will be facilitated, compared to stimulation when mental state is uncontrolled. We will focus on the dorsolateral prefrontal cortex (dlPFC) and the associated fronto-parietal network (FPN), which subserves cognitive control -- the ability to flexibly adapt and regulate behavior, an ability known to be impaired in neuropsychiatric conditions such as depression and dementia. We will use an 'n-back' task tapping cognitive control and the FPN. We will employ a within-subjects design with 40 healthy subjects in 4 MRI sessions. Each MRI session will consist of blood oxygenation level-dependent (BOLD) fMRI during an n-back task, resting state BOLD fMRI to measure connectivity and resting state arterial spin labeling (ASL) MRI to measure cerebral blood flow (rCBF) and examine effects on resting activity level. BOLD activation during the n-back will identify the FPN and the target site for dlPFC TBS. After a baseline fMRI session, subsequent sessions over different days will entail TBS, immediately followed by an MRI session to assess the effects of stimulation. TBS will involve: 1) dlPFC stimulation by active iTBS (600 pulses) alone or 2) while simultaneously performing an n-back cognitive task or 3) vertex (control) iTBS stimulation, alone.