If someone has had a traumatic experience in her life, she may develop post-traumatic stress disorder (PTSD), a type of anxiety disorder in which she re-experiences that traumatic event. In the United States, about 7.7 million adults have PTSD, according to the National Institute of Mental Health (NIMH). This anxiety disorder can also affect children. The U.S. Department of Veterans Affairs notes that between 15 to 43 percent of girls experience at least one traumatic event, and of those girls, between 3 and 15 percent of them will develop PTSD.

Several different events can cause PTSD to occur. In the Diagnostic and Statistical Manual of Mental Disorders Fourth Edition Text Revision(DSM-IV-TR), which mental health professionals use to diagnose PTSD, traumatic experiences listed include experiencing a diagnosis of a life-threatening condition, torture, kidnapping, disasters, severe motor vehicle accidents, hostage situations, combat or a violent personal assault, such as sexual assault, mugging or a physical attack. The time frame for the onset of PTSD symptoms can range from right after the trauma to six months or more afterward. MedlinePlus, a service of the U.S. National Library of Medicine and the National Institutes of Health, notes that if a person develops PTSD soon after her traumatic experience, the disorder improves after three months; however, some people can have PTSD symptoms that last for years.

With PTSD, patients can relive the experience, which interferes in their daily lives. One such symptom is a flashback, in which the PTSD patient feels like she is living the event again. These flashbacks can occur often. In a study conducted by the University of Minnesota and the Minneapolis Veterans Affairs Medical Center, researchers found that the brains of PTSD patients have increased activity in the right side of the brain, which may have an association with flashbacks. HealthDay News reports that the study included 80 PTSD patients, 18 PTSD patients who were in remission and 284 controls. To detect changes in the brain, the researchers used magnetoencephalography (MEG), which picks up on magnetic charges from neural communication.

The researchers found that patients with PTSD had more activity in the temporal cortex of the brain, which may be the region of the brain associated with flashbacks, as well as two additional regions of the brain on the right hemisphere. In comparison, the PTSD patients in remission also had this increase brain activity, but not to the same degree as the PTSD patients who were not in remission.