Slight lapses in memory, such as forgetting to return a call from a friend or to pick up milk at the grocery store, are normal and to be expected in healthy individuals. Amnesia is not a normal lapse of memory; rather, it is memory loss due to brain damage or psychological trauma. Amnesias caused by surgery, an accident, drugs, or disease are called organic amnesias. Organic amnesia can devastate one’s ability to learn new information, to remember information from the past, or both. Amnesia differs from dementias (such as Alzheimer’s disease), which involve more than simple memory loss. Amnesic patients do not have deficits in immediate recall (e.g., repeating a string of numbers immediately after hearing them) and have preserved overall intelligence, whereas patients with dementia exhibit continuous deterioration of memory ability along with other cognitive skills. Further, dementias progress as a result of ongoing neuronal loss, whereas amnesia has an abrupt onset.
Anterograde amnesia refers to the inability to learn and remember new information. It is often seen in individuals who have suffered a stroke, although a wide variety of medical problems that inflict damage on the medial temporal lobe of the brain can cause anterograde amnesia. Usually, these amnesics have difficulty learning all varieties of new information, although it is possible to have problems learning specific varieties of information (e.g., amnesia may be limited to learning spatial locations). The most famous anterograde amnesic is a patient known as H. M., who underwent radical surgery that removed tissue from his medial temporal lobe in order to help control his life-threatening seizures. From the time he awoke following surgery until the present (more than 50 years), H. M. has been unable to learn and remember any new information, including what year it is, where he now lives, and who his caregivers are.
An inability to remember information that was previously known is called retrograde amnesia. This can result from damage to the cortex, with the extent of brain damage strongly related to the density of the resulting amnesia. Even in cases of severe retrograde amnesia, there may be “islands” of intact memory for salient events. Generally, retrograde amnesia occurs only for a brief period of time preceding the injury. For example, if a person in a car crash sustains brain damage, he or she might be brought to the hospital unconscious and awaken a few hours later. Although the patient may be able to understand and remember that he or she is now in the hospital, the last thing the patient might remember is leaving the house that morning. Interestingly, most cases of anterograde amnesia are accompanied by some degree of retrograde memory loss, although the severity of the anterograde and retrograde deficits is not always strongly correlated.
Psychological amnesia is sometimes also called functional, psychogenic, traumatic, or dissociative amnesia. Psychological amnesia can result when an individual is under extreme stress or experiences a traumatic event (in the absence of brain injury). The amnesic individual may report lapses in memory for information related to a traumatic or stressful event or period in his or her life. Memory loss can last anywhere from an hour to a period of years. Memories may be recalled after being triggered by stimuli related to the trauma. Psychological amnesia sometimes occurs in individuals involved in combat or victims of childhood abuse. A rare subtype of psychological amnesia is dissociative fugue, in which an individual completely forgets who he or she is and often creates a new identity. Dissociative fugue can last anywhere from a few hours to years, and patients emerging from the fugue state often regain their true identity but have amnesia for events from the fugue period.
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- Gluck Lab Online, http://www.gluck.edu/memory/
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