Amnesia or amnaesia (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.
Types of amnesia
- In anterograde amnesia, new events are not transferred to long-term memory, so the sufferer will not be able to remember anything that occurs after the onset of this type of amnesia for more than a few moments. The complement of this is retrograde amnesia, where someone will be unable to recall events that occurred before the onset of amnesia. The terms are used to categorise 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 damage to the brain regions most closely associated with episodic/declarative memory: the medial temporal lobes and especially the hippocampus.
- Traumatic amnesia is generally due to a head injury (fall, knock on the head). Traumatic amnesia is often transient; the duration of 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.
- Dissociative amnesia is used to refer to long-term repressed memory that is the result of psychological or emotional trauma.
- Long-term alcoholism 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.
- Lacunar amnesia is the loss of memory about one specific event.
- 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" .
- 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.
- Global amnesia is total memory loss. This may be a defence mechanism which occurs after a traumatic event. Post-traumatic stress disorder can also involve the spontaneous, vivid retrieval of unwanted traumatic memories.
- Posthypnotic amnesia is where events during hypnosis are forgotten, or where past memories are unable to be recalled.
- 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.
- Source amnesia is a memory disorder in which someone can recall certain information, but they do not know where or how they obtained it.
- Memory distrust syndrome is a term invented by the psychologist Gisli Gudjonsson to describe a situation where someone is unable to trust their own memory.
- Excessive short-term alcohol consumption can cause a blackout phenomenon with similar symptoms to amnesia.
Amnesia in fiction
Amnesia is a common motif in fiction. 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 traumatic amnesia. In the Bourne Identity, the main character has retrograde amnesia. In movies, 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 a catastrophic consequences: a phenomenon known as Second Impact Syndrome.