Stress-Related and Transient Amnesic States

In addition to instances of organic amnesia, psychological stresses can result in impairments to and distortions of memory. One of the main types of nonorganic memory loss occurs as part of the syndrome termed posttraumatic stress disorder and in many circumstances can be linked to the experience of a precipitating traumatic event such as a natural disaster, accident, or war.

The phenomenon was first recognized during World War I, when soldiers returned from the front affected by shell shock, characterized by recurring nightmares and flashbacks to their traumatic experiences. The disorder can also manifest as the loss of one’s identity and of autobiographical memories from around the time of the precipitating event, referred to as psychogenic or dissociative amnesia. For example, studies of soldiers conducted following World War II suggested that up to 5% had no memory for the traumatic combat events they had recently experienced.

One possible explanation for these types of amnesia may be that they reflect a psychological defense mechanism that attempts to repress or inhibit retrieval of the traumatic memory and prevent it from reaching conscious awareness. Evidence suggests that such retrieval inhibition processes are supported by regions in the frontal lobes, emphasizing the important role for this brain area in memory. Stress-related forgetting may be transient in nature, and patients can sometimes recover their memories after the appropriate treatment or therapy.

The recovery of traumatic memories following therapy has proven controversial, however, with some commentators questioning the accuracy of such recovered recollections and suggesting that the techniques sometimes used in therapy sessions might lead patients to remember false traumatic events (ones that had not in fact previously occurred). This debate remains largely unresolved, with evidence suggesting that retrieval inhibition processes do certainly exist but, at the same time, that one’s memory of past events can be highly susceptible to suggestibility biases and distortions at the time of recall.

Although one of the diagnostic criteria for psychogenic amnesia is that there is no evidence of structural brain damage, the increasing use of sophisticated neuroimaging techniques that measure levels of blood flow in the brain suggest that there can be altered brain function in some cases, although the reasons for this are currently unclear. For example, one study examined brain activity in a patient with a persistent psychogenic amnesia for the whole of his past life. When the patient was thinking about his autobiographical memories, blood flow was found to be greater in the same brain regions that control subjects engaged during the processing of nonpersonal memories.

This suggests that some patients with psychogenic amnesia may treat previously personally salient episodic memories in a neutral, semantic way, as if they no longer belonged to them, perhaps in order to escape the emotional associations of the traumatic episode(s) that triggered the amnesia. Alternatively, it is possible that the brain regions typically involved in processing autobiographical recollections, which are similar to those involved in processing emotional information, may be temporarily disrupted as a result of experiencing the stressful event, leaving the brain regions used for processing neutral memories as the only means by which autobiographical memories can be processed.

The important link between stress and memory can be further revealed in a syndrome called transient global amnesia (TGA). Although not classified as a psychogenic amnesia (patients are rarely confused about their own identity), the disorder is characterized by abrupt confusion and a complete anterograde amnesia, which eventually shrinks after (typically) 4-6 hours.

Although there are known risk factors for TGA, such as a history of epilepsy or migraine, in as many as 30% of cases attacks can be directly linked to precipitating stresses, such as strenuous exertion, pain, immersion in water, and emotional events. Recent neuroimaging studies have revealed physiological changes (low blood flow) in the hippocampal complex during an attack.

It is possible, therefore, that extreme stress may result in memory impairment because the hippocampal complex is part of a complicated network in the human brain that mediates fear-related behavior and stress responses, as well as aspects of memory function. Highly stressful situations may overstimulate the hippocampus and related structures, leading to decreased blood flow in this region of the brain and subsequent memory loss.