administration of surveys, the extent and severity of flashbacks that occur in prisoners of war. this study concluded that the persistence of severely traumatic autobiographical memories can last up to 65 years. until recently, the study of flashbacks has been limited to participants who already experience flashbacks, such as those suffering from ptsd, restricting researchers to observational/exploratory rather than experimental studies. there have also been treatments based on theories about the inner workings of the involuntary memory. the procedure involves changing the content of the intrusive memories and restructuring it so the negative connotations associated with it is erased. the patients are encouraged to live their lives and not focus on their disruptive memories, and are taught to recognize any stimulus that may start the flashbacks. the events related to the flashbacks still mostly exist in their mind, but the meaning and the way the person perceives it is now different. according to ehlers, this method has a high success rate with patients who have suffered from trauma. neuroimaging investigations neuroimaging techniques have been applied to the investigation of flashbacks. using these techniques, researchers attempt to discover the structural and functional differences in the anatomy of the brain in individuals who suffer from flashbacks compared to those who do not. neuroimaging involves a cluster of techniques, including computerized tomography, positron emission tomography, magnetic resonance imaging (including functional), as well as magnetoencephalography. neuroimaging studies investigating flashbacks are based on current psychological theories that are used as the foundation for the research. one of theories that is consistently investigated is the difference between explicit and implicit memory. this distinction dictates the manner in which memories are later recalled, namely either consciously (voluntarily) or unconsciously (involuntarily). these methods have largely relied on subtractive reasoning, in which the participant first voluntarily recalls a memory before recalling the memory again through involuntary means. involuntary memories (or flashbacks) are elicited in the participant by reading an emotionally charged script to them that is designed to trigger a flashback in individuals who suffer from ptsd. the investigators record the regions of the brain that are active during each of these conditions, and then subtract the activity. whatever is left is assumed to underpin the neurological differences between the conditions. imaging studies looking at patients with ptsd as they undergo flashback experiences have identified elevated activation in regions of the dorsal stream including the mid-occipital lobe, primary motor cortex, and supplementary motor area. the dorsal stream is involved in sensory processing, and therefore these activations might underlie the vivid visual experiences associated with flashbacks. the study also found reduced activation in regions such as the inferior temporal cortex and parahippocampus which are involved in processing allocentric relations. these deactivations might contribute to feelings of dissociation from reality during flashback experiences. relations to mental illness and drug use flashbacks are often associated with mental illness as they are a symptom and a feature in diagnostic criteria for ptsd, acute stress disorder, and obsessive-compulsive disorder (ocd). flashbacks have also been observed in people suffering from bipolar disorder, depression, homesickness, near-death experiences, epileptic seizures, and substance abuse. some researchers have suggested that the use of some drugs can cause a person to experience flashbacks. users of lsd sometimes report "acid flashbacks." the effect of certain drugs on flashbacks may involve a variety of factors. for example, a study on the use of the nabilone for the treatment of nightmares in ptsd patients found that, in some cases, the use of the synthetic cannabinoid reduced daytime flashbacks. however another study found subjects previously exposed to cannabinoids (non-synthetic), could experience cannabinoid "flashbacks" when thc stored in fat cells re-enters the bloodstream through lipolysis. culture the psychological phenomenon has frequently been portrayed in film and television. some of the most accurate media portrayals of flashbacks have been those related to wartime, and the association of flashbacks to ptsd caused by the traumas and stresses of war. one of the earliest screen portrayals of this is in the 1945 film mildred pierce.