I use head-mounted miniaturized microscopes combined with chronically implanted microendoscopes to image calcium signaling in vivo in striatal output neurons and midbrain dopamine neurons of freely-behaving rats to visualize how motivational behavior is encoded in the brain, specifically the shift from goal-directed to habitual behavior. In addition, I use this technique to image cellular activity in genetically engineered mice that show compulsive-like behaviors to understand the neuronal mechanisms of compulsivity that could potentially lead to a better understanding of psychiatric disorders such as addiction, OCD, and trichotillomania.