Exposure and Response Prevention (ERP) is an effective form of psychotherapy(talk therapy) for OCD. ERP helps even those on medications and the combined benefit from both is better than either of them alone. ERP works by habituation. Habituation is a process wherein individuals exposed to the same stimulus repeatedly respond with lesser intensity.
In the context of OCD, if an individual has obsessive thoughts of contamination/dirt and engages in repeated compulsive acts of washing, real or imagined exposure in a therapeutic setting results in habituation, and the individual is able to ignore these obsessive thoughts (dirt) without indulging in the compulsions (washing). As difficult and threatening as it may sound, the patient is under the supervision of the therapist while the entire process takes place. The tasks are chosen from one that provokes little anxiety to higher levels of anxiety ( ie. fear ladder). Once the patient learns the skill, it is easier for him/her to extrapolate the skills to other stimuli. ERP is usually done over multiple sessions lasting for several weeks.
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