Inclusion Criteria:
* Adults aged 18-50
* Medically healthy
* U.S. citizen or holding permanent resident status
* English-speaking
Exclusion Criteria:
* Any current or past history of neurological disorders or acquired neurological disease (e.g. stroke, traumatic brain injury), including intracranial lesions (including clinically significant findings identified in first MRI)
* History of inpatient psychiatric hospitalization
* History of head trauma resulting in prolonged loss of consciousness; or a history of greater than 3 grade I concussions
* Current history of poorly controlled headaches including intractable or poorly controlled migraines
* Any systemic illness or unstable medical condition that may cause a medical emergency in case of a provoked seizure (cardiac malformation, cardiac dysrhythmia, asthma, etc.)
* History of seizures, diagnosis of epilepsy, history of abnormal (epileptiform) EEG, or family history of treatment resistant epilepsy except for a single seizure of benign etiology (e.g. febrile seizures) in the judgment of a board-certified neurologist
* Possible pregnancy or plan to become pregnant in the next 6 months (self reported)
* Any metal in the head
* Any medical devices or implants (i.e. cardiac pacemaker, medication infusion pump, cochlear implant, vagal nerve stimulator)
* Dental implants
* Permanent retainers
* Any hair braid, dreadlocks, hair pieces, or extensions which cannot be taken out before the study sessions
* Any head coverings or headdress that participant feels uncomfortable removing for the purposes of study sessions
* Any medication that may alter seizure threshold taken during the study i.e., Attention-deficit/hyperactivity disorder (ADHD) stimulants (Adderall, amphetamine); Tricyclic/atypical antidepressants (amitriptyline, doxepin, imipramine, maprotiline, nortriptyline, bupropion); Antipsychotics (chlorpromazine, clozapine), Bronchodilators (theophylline, aminophylline); Antibiotics (fluoroquinolones, imipenem, penicillin, cephalosporins, metronidazole, isoniazid); Antivirals (valacyclovir, ritonavir); over the counter antihistamines (diphenhydramine, Benadryl)
* Claustrophobia (a fear of small or closed places)
* Back problems that would prevent lying flat for up to two hours
* Regular night-shift work (second or third shift)
* Sleep apnea or other sleep disorder (self-reported)
