Amnesia is total or partial loss of the ability to recall experiences or events that happened in the preceding few seconds, in the preceding few days, further back in time, or after the event that caused the amnesia.
Because many areas of the brain are involved in memory, damage almost anywhere in the brain can cause amnesia.
How amnesia is caused is only partly understood.
How long memory loss lasts depends on the severity of the damage that caused it.
Doctors evaluate memory loss by asking simple questions and doing formal tests of memory.
The cause of memory loss is treated if possible.
Memory loss can be classified as follows:
Retrograde: Amnesia for the events before the event that caused the amnesia
Anterograde: Amnesia for the events after the event that caused the amnesia
Sense-specific: Amnesia for events processed by one sense, such as hearing
Memory loss involves facts more commonly than learned skills.
How far back in time memories are lost varies from a few seconds before the amnesia occurred to a few days, to further back in time, affecting more distant past (remote, or long-term) memories.
Processing of memories involves the following:
Taking in new information (registration)
Linking new information with memories already stored in the brain, with mental pictures, or with other things that can help with retrieval (encoding)
Recalling the memory (retrieval)
The brain’s mechanisms for storing information and recalling it from memory are located primarily in the temporal lobe and frontal lobe, but many areas of the brain are involved in memory. For example, the hippocampus, located deep within the brain, is involved in the formation of new memories and retrieval of stored memories. The hippocampus is part of the limbic system, which controls the experience and expression of emotions. Thus, the hippocampus helps connect memories to the emotions experienced when the memories form.
Emotions originating from the limbic system can influence the storing of memories and their retrieval. The limbic system includes parts of the cerebrum and some structures deep within the brain. Areas that are responsible for alertness and awareness in the brain stem also contribute to memory.
Causes of Amnesia
How amnesia is caused is only partly understood. It may result from
A nutritional disorder, particularly thiamin deficiency, which can cause Wernicke-Korsakoff syndrome
A head injury that affects the brain
Disorders that reduce the supply of blood or nutrients to the brain (including strokes and cardiac arrest)
A brain infection (encephalitis)
Dementia such as Alzheimer disease
Severe mental stress (as occurs in dissociative amnesia)
Depending on the cause, amnesia may be
Temporary, as occurs after head injury
Permanent and unchanging (as occurs after a serious disorder such as encephalitis or a stroke that affects a large part of the brain)
Progressive (as occurs with disorders that cause progressive degeneration of the brain, such as Alzheimer disease)
Thanks for the Memories
Amnesia is a popular theme for many movies and television shows. Characters often appear with no identity and no memories of the past. They are essentially starting over, but they, for the most part, are fully equipped mentally to do so. However, this cinematic portrayal has little in common with the reality of amnesia. |
In the movies: The amnesia may be unrelated to any abnormality or injury of the brain. People just forget. The reason may be unclear. Sometimes sleep seems to wipe the memory clean of the previous day’s events—an improbable scenario but full of comic possibility. Or the cause may be a blow to the head, a head injury in a crash of some sort, or a psychologic trauma, such as witnessing a murder or being raped. Or memories may be removed by a special erasing device, as used in Men in Black or Eternal Sunshine of the Spotless Mind. In reality: Amnesia usually has less glamorous causes, such as a brain infection, alcoholism, a stroke, drugs, a brain tumor, or an injury due to brain surgery. Psychologic trauma occasionally causes amnesia—a disorder called dissociative amnesia. However, psychologic trauma often has the opposite effect on memory loss. People cannot forget what happened to them. They frequently replay and relive the traumatic event, even though they would rather forget it. |
In the movies: People with amnesia have few if any problems with everyday activities. They may readily get a new job and make new (or new-old) friends. In reality: Most people have great difficulty learning and retaining new information (because the brain has been damaged). As a result, they struggle with everyday activities. People have difficulty remembering names and where they are going and why. These problems cause frustration, and people with amnesia often feel very confused and get lost. |
In the movies: People often go through a complete personality change. Values and behaviors are transformed. Bad people become good. In reality: Amnesia affects personality or identity only rarely, when the specific areas of the brain that controls these functions malfunction. |
In the movies: People with amnesia due to trauma have stored memories of the trauma, intact and accurate, deep in their unconscious. With the right trigger, they can replay the memories of the trauma like a video camera. In reality: The way the brain recalls memories is dynamic. When people remember an event, they reconstruct it, pulling bits from different places in the brain. No memory, traumatic or otherwise, is ever frozen and immune from reconstruction over time. |
In the movies: Amnesia can be cured mechanically. That is, amnesia caused by a blow to the head can often be reversed by another blow. Or amnesia, regardless of its cause, can be cured by looking at a familiar object or by being hypnotized. In reality: Most of these cures are dubious. A second blow to the head is more likely to cause further damage. Hypnosis is useful only when the cause of amnesia is a disturbing event. Then, when done gently and carefully, it is often successful. Treatment and its chances of success depend on the cause. |
In the movies: Memories are not really lost, just temporarily inaccessible. In reality: Whether memories can be recovered depends on the severity and cause of the damage. Often, the damage is not severe, or the cause is temporary. In such cases, the amnesia often lasts for only minutes or hours, and most people recover their memory without treatment. However, when damage is substantial, memory often cannot be recovered. |
Symptoms of Amnesia
Depending on the severity of the damage, amnesias can last for minutes, hours, or longer. Sometimes memory is lost suddenly but temporarily (called transient global amnesia).
Some people recover their memory without treatment. However, if brain damage is severe, the ability to form new memories may be lost. Affected people are more likely to remember things from the distant past. For example, people may remember their spouse from their first marriage but not the current marriage.
Diagnosis of Amnesia
A doctor's evaluation
Formal tests of memory
Doctors evaluate memory loss by asking simple questions (such as repeating a list of three items) and by doing formal tests of memory. Results of this evaluation and the person's symptoms often suggest a cause and other tests that may need to be done.
Treatment of Amnesia
Treatment of the cause if possible
If a cause of amnesia is identified, it is treated if possible. Such treatment may or may not lessen the amnesia.
