Steven Phillipson, PhD is one of the leading cognitive-behavioral psychologists in the United States treating Obsessive-Compulsive Disorder (OCD). Phillipson has an unusually high success rate, with many clients reporting 80-100% remission of OCD symptoms without the use of medications. Phillipson makes use of the psychological principles of extinction and habituation in treating OCD and emphasizes the role of the brain's amygdala in the experience of anxiety. Phillipson encourages clients to contrast experience and perception in grappling with OCD, which he classifies as a psychological disorder, but not a disease or illness. According to Phillipson, OCD symptoms are reinforced through patterns of learned behavior that directly encourage fight-or-flight responses from the brain's amygdala (primitive emotional center). These patterns of behavior can be altered (unlearned) through exposure and response treatments in which clients systematically bring on anxiety-provoking situations but consciously choose to tolerate the resultant anxiety and experience of ambiguity and uncertainty. Phillipson's particular success is in the treatment of pure-obsessional ("Pure-O") OCD, which often involves anxious rumination (repetitive thought geared toward problem solving, question answering and/or relief seeking). Dr. Phillipson has written numerous articles on OCD, which he distributes for free on his website.