What is amnesia?
The primary attribute of amnesia is a loss of memory for a specific time period. The extent, duration, and type of that memory loss can vary greatly. In anterograde amnesia, the formation of new memories is impaired, while in retrograde amnesia, the retrieval of previously formed memories is impaired. In rare cases, anterograde amnesia is continuous, in which there is great impairment of memory formation for the remainder of a person’s life. Retrograde amnesia in rare cases may be generalized, in which the totality of an individual’s personal memory preceding the onset of amnesia is lost. More commonly, both anterograde and retrograde amnesias are localized, in which the memory of a period of time ranging from seconds to minutes (although occasionally days or longer) is lost. The memory loss may be selective, with only some aspects of a particular time period being absent. In all these forms of amnesia, the most common type of information lost is episodic; rarely are procedural memories destroyed.
Amnesia is typically caused by either psychological circumstances, in which case it is termed psychogenic, or by biological processes, in which it is referred to as biogenic or organic. Sometimes, however, the cause involves both psychological and biological factors. Psychogenic amnesias are usually caused by some sort of emotional trauma. Emotional trauma is the common thread that runs through the amnesia associated with the following disorders: dissociative amnesia (the inability to recall significant personal information); fugue (memory loss accompanied by sudden, unexpected travel from home); dissociative identity disorder (the presence of two or more distinct personalities, with the inability to recall extensive time periods); and post-traumatic stress disorder (significant distress and memory disturbances following an extreme traumatic event). Emotional trauma is typically absent in posthypnotic amnesia, which is induced by hypnotic suggestion, and childhood amnesia, in which adult memories of early childhood experiences before the age of five are typically vague and fragmentary. Psychogenic memories, while principally involving episodic information, may extend to semantic information, which is rarely seen in biogenic amnesia.
Biogenic amnesia is usually the result of trauma to the brain or disease processes. Anterograde and retrograde amnesia are common following a concussion or brain surgery, particularly involving the temporal lobe. Electroconvulsive treatments induced by a current passed through electrodes on the forehead (sometimes used to treat depression) tend to have a more anterograde effect. A diversity of toxic and infectious brain illnesses can lead to Korsakoff’s syndrome , first described in chronic alcoholics. The primary feature is anterograde disturbance, with the ability to store new information limited to a few seconds. Dementia typically begins with the loss of recent memories and gradually spreads retrograde into the person’s more distant past as the condition progresses. Hardening of the brain arteries, Alzheimer disease, and numerous infectious agents can lead to dementia. Transient global amnesia is an abrupt anterograde and retrograde loss leaving some degree of permanent memory loss; it is thought to be caused by temporary reductions in the blood supply to specific brain regions.
As time passes from the emotional and physiological traumas that precipitate amnesia, there is usually some degree of memory recovery. The less severe the trauma, the better the prognosis. Psychological interventions involving the use of careful interrogation, the use of emotionally significant stimuli, or hypnosis can help the amnesic fill in the gaps of memory deficits. Drugs that affect levels of neurotransmitters such as acetylcholine, aspartate, glutamate, norepinephrine, and serotonin can also have an impact on memory recovery. For example, John Krystal reported in 1993 that Vietnam War veterans given yohimbine, a drug that activates norepinephrine, experienced vivid flashbacks of combat trauma.
Some amnesias can be either prevented—using electrodes on only one side of the head in electroconvulsive treatment lessens the likelihood of amnesia—or significantly ameliorated by psychological and/or biological intervention. Severe amnesias, such as dementia, have no effective cure. However, drugs to may be prescribed to help manage symptoms, such as cholinesterase inhibitors and memantine, which help boost chemical messengers involved in memory. Major memory loss in the elderly has a particularly poor prognosis for recovery and is often indicative of imminent death. Where memory recovery is poor, optimizing the use of the remaining mental abilities and available environmental resources can help those with amnesia better adapt to their living environments.
The first scientific explanations of amnesia came in the late nineteenth century. Théodule-Armand Ribot (1839–1916) proposed a “law of regression,” in which memory loss was thought to progress from the least stable to the most stable memories. Sergey Korsakoff (1853–1900) was one of the first to demonstrate that amnesia need not be associated with the loss of reasoning abilities found in dementia. While Ribot and Korsakoff focused on organic causes of amnesia, Pierre Janet (1859–1947) described amnesics who apparently had no underlying biological disease. He explained these cases in terms of mental fragmentation that he called dissociation.
Prevention, rather than treatment, of amnesia became the focus of attention as the twentieth century drew to a close. Two promising research areas in the twenty-first century are drugs to limit the effects of brain damage and the beneficial impact of a stimulating environment in staving off the effects of aging on the brain.
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