Shapiro has spent the last 12 years developing the EMDR method and training more than 30,000 practitioners around the world. Yet she still isn't sure precisely why it's effective. She believes, along with others who have studied her technique, that the eye movements might be helping the brain to process memories in a way similar to what happens during the rapid eye movement (REM) phases of sleep. "That's what the REM sleep is for," Shapiro explains, "it's for the learning to take place of the day's experience."
Shapiro thinks traumatic memories that continue to feel as intense as the original experience are stored in "the wrong form of memory." They are "still hot," as she puts it, because they're still directly linked to the emotions and physical sensations that accompanied the original experience. "When we work with EMDR, Shapiro explains, "we target those earlier experiences and we set in motion this information-processing system that for some reason didn't allow that experience to be appropriately digested at the time. And then what's useful from that experience is learned, it's absorbed, it's stored with appropriate emotion, it can guide the person in the future, and what's useless, the negative emotion, physical sensation, all of that's able to be let go."
In 1998, a Boston University trauma specialist, Dr. Bessel A. van der Kolk, attempted to trace the process by performing photon emission computed tomography (PET scans) on the brains of 12 PTSD sufferers before and after EMDR therapy. Following three EMDR sessions, the patients' PET scans showed increased activity in the frontal lobes of the cerebral cortex, the so-called seat of "reason," and decreased activity in the limbic system, the more primitive area of the brain associated with fear and emotion. Dr. van der Kolk theorized at the time that stress-response chemicals flooding the body and brain during a traumatic event interfere with information processing, causing memories to be stored as episodic fragments and sensory impressions. EMDR, he concluded, somehow allows the frontal lobes to act as a filter, stripping the memories of their "emotional valence."