Amnesia is simply the standard term for a loss of memory, whether partial or complete. The condition is usually temporary, and it usually affects only a small part of a person’s experience, such as memory of the immediate past. Amnesia can be produced by a range of causes, including both psychological trauma and brain damage that may be caused by a blow to the head; but can also be due to such varied causes as stroke, brain tumor, encephalitis, or long-term damage by alcohol.
There are several distinct types of amnesia:
- Anterograde amnesia is the most common type to result from brain damage. Anterograde amnesia is an inability to form new memories, usually caused by damage to the temporal lobes and/or the hippocampus, a small structure located beneath the cerebral cortex. The person with anterograde amnesia has no difﬁculty remembering the past, or who he is; but may not be able to remember anything that happened after the injury. This can result in the victim’s permanently living in the present moment, and having to be re-introduced to people met since the injury, since no memory of them remains.
- Retrograde amnesia is a difﬁculty retrieving memories prior to an incident in which a head injury occurred. Contrary to the cliché presented on television, loss of memory is often limited to the seconds, minutes, or sometimes hours, leading up to the trauma. This is common in people who have been involved in serious automobile accidents: they frequently have no memory of either the accident or the moments leading up to it. Memory often returns eventually, though it is not unusual for the person to never recover the ﬁnal seconds leading up to the injury.
- Korsakoff’s syndrome is special kind of memory loss caused by large-scale, long-term alcohol abuse and often includes features of both anterograde and retrograde amnesia. The disorder is often accompanied by neurological symptoms, such as loss of feeling in the extremities. This is a progressive disorder, in the sense that it will continue to get worse if drinking continues. The damage tends to be irreversible; once symptoms occur, it is too late for the effects to be reversed, if one were to stop drinking.
Research on patients with anterograde amnesia and Korsakoff’s syndrome has led researchers to discover some interesting things about memory that might otherwise have remained poorly understood, speciﬁcally the differences between declarative and procedural memories. A declarative memory (also called semantic memory) is one that can be put into words—this would cover most of what we “know,” and most of our personal experiences (also called episodic or autobiographical memory). These are the memories that appear to be affected by anterograde amnesia. There is another category, however, of procedural (or implicit) memories—these are the things we know but cannot express verbally, and may not even have a clear, conscious awareness of knowing. The amnesic patient rarely forgets how to speak, or walk, or use silverware, for example.
In a famous experiment with HM, perhaps the best-known anterograde amnesia patient in the psychological literature, a psychologist introduced a new, difﬁcult task one day: mirror writing. This involved a simple hinged apparatus that set up so that the subject could only see his own hand and the writing paper in a mirror. The task is to write something neatly, which is very challenging for most people the ﬁrst time they try it. HM was no exception; he was extremely frustrated the ﬁrst time he tried it, and remained agitated every time it was introduced to him for the next two weeks (since it was, in his view, always the ﬁrst time). After two weeks, however, he did the task extremely well, showing a clear practice effect. With no explicit memory of ever encountering the task, he nonetheless improved steadily in his performance.
A similar effect was discovered with Korsakoff’s patients by Clarapéde, a physician who greeted a patient one day with a handshake. Unknown to the patient, there was a pin concealed in the proffered hand, which of course resulted in pain and unpleasantness. The next day, Clarapéde again offered a handshake, but the patient hesitated. When asked why, the patient offered only a vague suspicion that “sometimes, people conceal sharp things in their hands.” This suspicion was unaccompanied by any explicit memory of it having actually happened. These and similar cases make up a fairly clear body of evidence suggestive of different neurological systems in the brain handling the different types of memory—damage that affects formation of declarative memories leaves implicit memories unaffected.
One other type of amnesia deserves some mention here: dissociative amnesia, also known as psychogenic amnesia or fugue amnesia. This refers to the extremely rare phenomenon of amnesia produced by psychological trauma rather than physical injury to the brain. Although the rarest form of amnesia, this is the type favored by television writers as a plot device, in which the primary symptom is a pervasive loss of memory of signiﬁcant personal information, including identity. What the television writers usually get wrong is the fact that dissociative amnesia, unlike other types of amnesia, does not result from medical trauma, such as a blow to the head. Dissociative amnesia is often confused with a related disorder, dissociative fugue, which can be succinctly described as “dissociative amnesia plus travel.” An individual with dissociative fugue suddenly and unexpectedly sets off on a journey, which may only last hours, but may also last days, weeks, or even months. In the fugue state, the individual becomes confused about his identity, and may actually assume a new identity in a location thousands of miles from home (some psychologists are skeptical about the authenticity of such extreme cases of fugue, especially when the person involved is conveniently absolved, by the fugue, of serious responsibilities and accountability for his actions).
- Parkin, A. J. Memory and Amnesia: An Introduction. 2nd ed. Malden, MA: Blackwell, 1997.