Causal Role of Frontostriatal Circuitry in Goal-directed Behavior

This study is a pilot, four-session, crossover study with transcranial magnetic stimulation (TMS), electroencephalography (EEG), and magnetic resonance imaging (MRI) to understand the causal role of delta-beta coupling in goal-directed behavior in the dorsolateral prefrontal cortex (dlPFC) to dorsal striatum circuit. Participants that request to be in the experiment will provide verbal, documented consent to undergo a phone screening to assess that the participant meets initial exclusion/inclusion criteria. Participants complete an MRI and TMS screening form over the phone to ensure eligibility.

The first session will be an EEG session with the reward-based decision-making task. At the start of the session, the investigators will acquire written informed consent. Then, the investigators will administer a pregnancy test if applicable. Participants will complete five assessments: the Snaith Hamilton Pleasure Scale, Behavioral Activation System and Behavioral Inhibition System, Temporal Experience of Pleasure Scale, the State-Trait Anxiety Inventory, and Ruminative Responses Scale. Note that the participants are from a convenient sample and are not required to be diagnosed with major depressive disorder. Thus, these assessments were selected as they survey various personality traits that might be relevant to performance in the task.

The scalp dimensions of each participant are calculated and an EEG net is applied. Next, the participants complete an eyes-open and eyes-closed resting-state recording of EEG. Then, the streamlined version of the Expenditure of Effort for Reward Task (S-EEfRT) is completed. These data serve as baseline measurement of brain activity without any form of stimulation. This session takes approximately 1.5 hours to complete. After each block of the task, the task difficulty will increase or decrease based on performance. At the end of the session, if the participant chose to perform the hard task greater than 85% of the time or less than 15% of the time, then the participant will not be invited to the next session of the experiment. Participants that do not dynamically change their response based on the incentive are not engaged with the relevant cognitive constructs under investigation in this study.

The second session takes place at the MRI facility. In the 24 hours prior to this session, participants complete an MRI screening form to ensure eligibility based on common contraindications for MRI. During the 60 minutes of scanning, a 5-minute structural MR is acquired and the remaining time is used to complete as many blocks of the S-EEfRT as possible. The minimal number of sessions required to use the data is 5 blocks, which requires approximately 25 minutes to collect. If a participant was unable to complete the requisite number of sessions, then they will be excluded from the study. Functional MRI data is analyzed before the 3rd and 4th session to localize the regions of dorsolateral prefrontal cortex (dlPFC) and medial prefrontal cortex (mPFC) for stimulation. In the localization analysis, a region of interest mask in the head of the left caudate and in left nucleus accumbens are drawn and the region in dlPFC and mPFC with peak functional connectivity in task-based functional connectivity to these regions. In a pilot dataset collected by the investigators, it was found that the contrast of trails in which there was a decision to perform the hard task versus trials in which the easy task was selected was sufficient to localize the anterior middle frontal gyrus. The contrast of trials in which high versus low incentive was offered was sufficient to localize the medial prefrontal cortex. Thus, the investigators will choose regions in these anatomical areas with maximal connectivity to their respective nucleus in the striatum.

The order of regions (dlPFC then mPFC, or mPFC then dlPFC) targeted by TMS in the third and fourth session will be randomized and counter-balanced. In the third and fourth session, participants will complete a TMS contraindications screening form. The same TMS screening form will be administered over the phone and at the start of each of the TMS session. If there is any ambiguity in the contraindications for the TMS form, then the medical monitor who is an epileptologist is consulted and final approval is acquired. Participants will be fitted with a low-profile EEG net. In the third session, the motor threshold of each participant will be calculated using single-pulse TMS to the hand knob of the left primary motor cortex with real-time monitoring of the motor-evoked potential using electrodes on the first dorsal interosseus muscle. Researchers may also use visible twitch to calculate the motor threshold. The motor threshold is defined as the percent stimulator output when a motor-evoked potential or visible twitch is observed approximately 50% of the time. For the fourth session, the same stimulator intensity will be used as in the third session. The structural MRI and regions of interest (dlPFC and mPFC) are imported into neuronavigation software. The participant wears a three-dimensional stereotaxic tracking headband and their head is registered to their structural MRI using canonical coordinates on the scalp. Then, the TMS coil is targeted to either mPFC or dlPFC and the position of the coil relative to the head is recorded throughout the session. The participant performs the S-EEfRT as the patterned trains of TMS are delivered on every trial. Each block of the study is randomized to receive either delta-beta patterned (triplets of TMS pulses at 20 Hertz every 3 Hertz), theta-gamma patterned (triplets of pulses at 50 Hertz every 5 Hertz), or an arrhythmic pattern (same number of pulses and duration with a random inter-pulse interval). After stimulation, a questionnaire is provided with common side effects of TMS. Based on the results of the stimulation side effects questionnaire, a structured adverse events interview is conducted to acquire more information regarding any side effects that were selected to be "very high" by the participant. The third and fourth session will each take approximately two hours.