The purpose of eye movement desensitization and reprocessing (EMDR) is to reduce or eliminate the symptoms of post-traumatic stress disorder (PTSD). During EMDR, a patient is instructed to recall a traumatic memory while engaging in fast horizontal eye movements under the direction of a trained therapist. The patient is instructed to hold various aspects of the traumatic event in his/her mind and track with his/her eyes the therapist's hand as it moves back and forth across the patient's field of vision. The negative emotions associated with the trauma decrease, while positive emotions increase. The mechanism of action of EMDR is unclear, but several theories have been put forward, including processing of emotions, conditioning processes, and activation of brain structures involved in the modulation of emotions.