This project seeks to identify causal neural mechanisms underlying unwanted, repetitive behaviors (compulsions). Using non-invasive brain stimulation coupled with practice in a computer task, we will modulate activity in a target brain region in two directions (up or down) and measure effects on compulsive behaviors and related measures. This work could ultimately lead to the ability to treat compulsions more effectively by targeting the regions of the brain that can help or hinder attempts to overcome compulsions.