Amnesia
Amnesia or amnæsia (from Greek ) (see spelling differences) is a condition in which memory is disturbed. The causes of amnesia are organic or functional. Organic causes include damage to the brain, through trauma or disease, or use of certain (generally sedative) drugs. Functional causes are psychological factors, such as defense mechanisms. Hysterical post-traumatic amnesia is an example of this. Amnesia may also be spontaneous, in the case of transient global amnesia. This global type of amnesia is more common in middle-aged to elderly people, particularly males, and usually lasts less than 24 hours.
Another effect of amnesia is the inability to imagine the future. A recent study published online in the Proceedings of the National Academy of Sciences shows that amnesiacs with damaged hippocampi cannot imagine the future. This is because when a normal human being imagines the future, they use their past experiences to construct a possible scenario. For example, a person who would try to imagine what would happen at a party that would occur in the near future would use their past experience at parties to help construct the event in the future.
Types of amnesia
- In anterograde amnesia, new events contained in the immediate memory are not transferred to the permanent as long-term memory. The sufferer will not be able to remember anything that occurs after the onset of this type of amnesia for more than a brief period following the event.
- Retrograde amnesia is the inability to recall some memory or memories of the past, beyond ordinary forgetfulness.
The terms are used to categorize patterns of symptoms, rather than to indicate a particular cause or etiology. Both categories of amnesia can occur together in the same patient, and commonly result from drug effects or damage to the brain regions most closely associated with episodic/declarative memory: the medial temporal lobes and especially the hippocampus.
An example of mixed retrograde and anterograde amnesia may be a motorcyclist unable to recall driving his motorbike prior to his head injury (retrograde amnesia), nor can he recall the hospital ward where he is told he had conversations with family over the next two days (anterograde amnesia).
- Traumatic amnesia is generally due to a head injury (fall, knock on the head). Traumatic amnesia is often transient, but may be permanent of either anterograde, retrograde or mixed type. The extent of the period covered by the amnesia is related to the degree of injury and may give an indication of the prognosis for recovery of other functions. Mild trauma, such as a car accident that could result in no more than mild whiplash, might cause the occupant of a car to have no memory of the moments just before the accident due to a brief interruption in the short/long-term memory transfer mechanism.
- Psychogenic amnesia results from a psychological cause as opposed to direct damage to the brain caused by head injury, physical trauma or disease, which is known as organic amnesia. This can include:
- Dissociative amnesia is used to refer to inability to recall information, usually about stressful or traumatic events in persons' lives, such as a violent attack or rape. The memory is stored in long term memory, but access to it is impaired because of psychological defense mechanisms. Persons retain the capacity to learn new information and there may be some later partial or complete recovery of memory. This contrasts with e.g. anterograde amnesia caused by amnestics such as benzodiazepines or alcohol, where an experience was prevented from being transferred from temporary to permanent memory storage: it will never be recovered, because it was never stored in the first instance. Also known as "Psychogenic Amnesia"
- Fugue state is also known as dissociative fugue. It is caused by psychological trauma and is usually temporary. The Merck Manual defines it as "one or more episodes of amnesia in which the inability to recall some or all of one's past and either the loss of one's identity or the formation of a new identity occur with sudden, unexpected, purposeful travel away from home" [2]. While popular in fiction, it is extremely rare.
- Posthypnotic amnesia is where events during hypnosis are forgotten, or where past memories are unable to be recalled.
- Childhood amnesia (also known as infantile amnesia) is the common inability to remember events from one's own childhood. Whilst Sigmund Freud attributed this to sexual repression, others have theorised that this may be due to language development or immature parts of the brain. This is often exploited by the use of false memories in child abuse cases.
- Transient Global Amnesia is a well described medical and clinical phenomenon. This form of amnesia is distinct in that it can sometimes be seen in a special form of MRI of the brain known as diffusion-weighted imaging (DWI). Symptoms typically last for less than a day and there is often no clear precipitating factor nor any other neurological deficits. The cause of this syndrome is not clear, hypothesis include transient reduced blood flow, possible seizure or an atypical type of migraine. Patients are typically amnestic of events more than a few minutes in the past, though immediate recall is usually preserved.
- Source amnesia is a memory disorder in which someone can recall certain information, but they do not know where or how they obtained the information.
- Excessive short-term alcohol consumption can cause a blackout phenomenon, with the amnesia being of the anterograde type.
- Long-term alcoholism or malnutrition can cause a type of memory loss known as Korsakoff's syndrome. This is caused by brain damage due to a Vitamin B1 deficiency and will be progressive if alcohol intake and nutrition pattern are not modified. Other neurological problems are likely to be present in combination with this type of Amnesia. Korsakoff's syndrome is also known to be connected with confabulation.
Amnesia in fiction
Amnesia is prevalent in many works of fiction. Global Amnesia is a common motif in fiction despite being extraordinarily rare in reality. Anterograde amnesia features in the movies Memento, Clean Slate, and 50 First Dates, and lacunar amnesia features in the movie Eternal Sunshine of the Spotless Mind. In the first season of 24, a prominent character has dissociative amnesia. In the first season of Lost, a character is kidnapped and has amnesia upon returning. The TV shows "Kyle XY" and "John Doe" are based on an amnesiacs who mysteriously appear in a forest, and in the middle of a sea. In the Bourne Identity, the main character has retrograde amnesia. In the 1966 motion picture "Mr. Buddwing", the protagonist enters an amnesial fugue state in response to distress in his marital relationship. In the 2004 film The Forgotten, adults struggle with memory loss about the existence of their children, who have been abducted for alien/government experiments. In the Marvel Comics series X-Men, Wolverine, one of the main characters, has retrograde amenesia due to brainwashing. In Season 4 of Smallville (season 4), Clark has his memory wiped by a Summerholt patient.
In movies and television, particularly sitcoms, it is often depicted that a second hit to the head (similar to the first one) cures the amnesia. In reality, however, a second concussion would have catastrophic consequences, a phenomenon known as Second Impact Syndrome. Dissociative Amnesia plays a critical role in the novel Mysterious Skin and movie of the same name.
Author Gene Wolfe addresses amnesia in the series "Soldier of the Mist", where the main character Latro is injured during battle, causing relatively long term (24 hour) anterograde amnesia.
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