Transcranial Direct Current Stimulation and Modulation of Affect and Pain

This double-blind randomized crossover study aims to examine the effects of left primary motor cortex (M1) tDCS on evoked thermal pain perception, pain-related decision making, and placebo effects by manipulated pain and monetary offers accepted by others. The investigators plan to collect data for up to 50 participants who undergo four separate sessions: an initial pain calibration session and three experimental sessions with either anodal, cathodal, or sham tDCS. Each session includes pain testing before and after a single tDCS protocol (either anodal, cathodal, or sham), a pain-monetary decision-making task, and social placebo pain tests. All tasks following tDCS will take place inside the MR scanner. The investigators hypothesize that anodal M1 tDCS will decrease pain ratings, and that cathodal tDCS will increase them, both relative to the sham condition. The investigators will also examine the effects of M1 tDCS on social decision-making to examine whether it affects participants' valuation of pain and their altruistic behavior, as well as how these same factors impact pain sensation in the placebo task. The investigators hypothesize that participants will behave hyperaltruistically during the decision-making task, and that the same factors that influence choice will also impact pain perception in the same context.

Inclusion Criteria: * ages 18-55 * Participants must be capable of performing experimental tasks (e.g., are able to read) * Fluent or native speakers of English Exclusion Criteria: * No self-reported current or history of depression, bipolar disorder, or other psychiatric diagnosis. * No self-reported current seizure disorder (i.e., seizure within past 10 years) * No history of stroke or other major neurological diagnosis. * No self-reported current chronic pain, or acute pain within three months of the study period. * No current migraine disorder (i.e., 15 headache days or more in 1 month). * No use of central nervous system-effective medication or other medication for neurological/psychiatric treatment. No self-reported substance abuse within the last six months. * No contraindication to MRI or tDCS (e.g., pregnancy, claustrophobia, pacemakers, ear/cochlear implants, shrapnel injuries, clips, or other ferromagnetic/electrical objects/devices, diagnosed brain abnormality such as tumor, or skin lesions on the scalp.). * No contraindications for induced pain (e.g., no heart disease, high blood pressure, heart surgery, heart problems of any kind, severe asthma, respiratory problems of any kind, fibromyalgia, Raynaud's Syndrome or Disease, chronic pain, diabetes).