sections include: organic amnesias, organic amnesia and the brain, functional amnesia, diagnosing and treating amnesia
Amnesia refers to a significant and selective impairment of a person’s memory, either in learning new information or in remembering the past. Since memory is a major brain function and one of the qualities that defines who we are, loss of that capacity can be devastating. Amnesia most commonly occurs as a symptom of another disease, such as Alzheimer’s or encephalitis, but can also occur by itself, in the absence of any other cognitive deficits.
There are many different types of amnesias, but they fall into two major categories, according to their cause:
- Organic amnesia involves memory loss caused by specific malfunctions in the brain.
- Functional amnesia refers to memory disorders that seem to result from psychological trauma, not injury or disease.
The memory loss of organic amnesia is typically produced by such brain disorders as tumors, strokes (ischemic and hemorrhagic), head injury, and degenerative diseases, such as Alzheimer’s. However, certain drugs affecting mood or behavior and alcoholism can also cause amnesia, as can temporal lobe surgery and electroconvulsive therapy for depression, although in this instance it is typically transient.
The best-known type of organic amnesia occurs when a person has problems recalling information from the past, a condition known as a retrograde amnesia. Doctors usually identify the disorder by asking an individual about past events, including personal landmarks (When did you graduate from high school?) and public occurrences he or she is likely to have heard about (Who won the last presidential election?). In most instances, people’s abilities to recall different facts vary by time. The information that they learned recently is the most likely to be disrupted, while they often retain knowledge acquired years earlier. Thus, adults with a retrograde amnesia generally remember details about their childhood and early schooling, but they may have great difficulty recalling personal and public events from the last few years.
People can also encounter difficulties in retaining new information. This, too, is a problem of memory, known as an anterograde amnesia. Such individuals have profound difficulty learning anything new. They may acquire information after hearing it repeated many times but then forget it shortly afterward. Doctors generally establish the presence of anterograde amnesia by seeing whether an individual can learn new information— either verbal, such as a story or a list of words, or nonverbal, such as pictures or geometric designs. Often the individual can learn some of this new information after much effort, but he or she retains little after a brief time elapses.
Amnesias of either kind can also be designated as temporary or permanent, depending on the type of brain injury the individual experiences. Permanent amnesia results from destruction of brain tissue. For such damage to disrupt a person’s memory but produce no major deficits in other cognitive domains, it must affect only the systems in the brain that are primarily responsible for normal memory. Damage to brain structures in the medial temporal lobe (such as the hippocampus and the entorhinal cortex) is most often the cause of permanent difficulty learning and retaining new information. Such damage can be caused by a variety of mechanisms, including head injury, infection, and stroke.
Temporary amnesia results from processes that disrupt the ability of the brain to function normally for a brief period of time but do not cause permanent brain damage. A blow to the head, as in a sports injury or an automobile accident, is the most common cause. A brief disruption of the blood supply to the brain—for example, from a narrowing of the blood vessels in the brain—can have the same effect. If these processes do not cause permanent injury, then the person will regain the ability to learn and retain new information, and to recall information from the past. The individual’s memory will also function normally in the future. However, memory for the period just preceding and following the damage will usually be lost. This is because it takes time for the brain to consolidate new information, and if this process is disrupted, even briefly, the new information that the individual should have acquired is lost forever.
Often brain damage, whether temporary or permanent, produces both anterograde and retrograde amnesia. However, the severity of the two need not be equal. For example, an individual can have a permanent and profound inability to learn new information but have a retrograde amnesia that goes back only a short period. Similarly, when an individual recovers from a temporary amnesia, the problems with learning may improve before the problems with remembering past events, or the reverse.
Organic Amnesia and the Brain
Studies of people with amnesia have provided surprising insights into how memory normally works. One remarkable finding is that even in severe amnesias, not all memory and learning abilities are impaired. Rather, only memories that are explicit and accessible are affected. These are sometimes called declarative memories, since we can bring them to mind as propositions or images. Declarative memory includes the facts, events, faces, and routes of everyday life. Nondeclarative, or implicit, memories can be expressed only in performance. These include motor skills and perceptual skills and are not affected by amnesia. Thus, a person with severe amnesia still knows how to tie a pair of shoes, even if he or she cannot remember buying those shoes the day before. In contrast, some people retain their memories of most things, but not how to perform certain tasks (apraxia) or recognize certain things (agnosia). This has led researchers to conclude that the brain has organized its memory functions around fundamentally different information-storage systems, some of which are impaired in amnesia and some of which are not.
Amnesia appears to result from damage to the medial temporal lobe (including the hippocampus and the entorhinal cortex) and the diencephalic midline. Studies have determined that damage to the hippocampus alone is sufficient to cause amnesia, but it appears that when several of these structures are damaged together, the severity of amnesia is worse than when only the hippocampus is damaged.
Problems with memory can also occur for psychological reasons. This is functional amnesia, also known as dissociative, or psychogenic, amnesia. The cause is usually an emotionally traumatic event. The pattern of memory loss in such cases is less systematic and predictable than in the amnesias described above. For example, individuals may not recall their names (which they learned as infants) but may retain a wide variety of skills or facts about the world that they learned as adults.
There are several types of functional amnesia. In dissociative amnesia, a person loses the memory of some important personal experience. This gap, or series of gaps, in memory is usually related to traumatic or extremely stressful situations. For example, a rape victim may lose his or her memory of the event, or a soldier may not be able to recall a battle.
Another type of functional amnesia, called a dissociative fugue, usually begins when a person suddenly, unexpectedly wanders away from home or work. This type of amnesia is also usually related to a stressful event. People with this disorder forget much more extensively; their whole past can become obscure, and they commonly lose all their memories of personal identity. The fugue state may last from a few hours to months or more. Often after recovery, the individual fails to remember anything that occurred during the fugue state.
Dissociative identity disorder is a type of amnesia in which a person appears to have two or more distinct personal identities that alternate control of his or her behavior. Such an individual may not only lose memory of periods from the past but also forget all personal information concerning some extended period of childhood. People with this disorder frequently have histories of severe physical and sexual abuse, especially during childhood.
Although functional amnesias are a popular theme in soap operas and movies, there are relatively few well-documented cases in the scientific literature. Most experts believe that these conditions do exist but that they are exceedingly rare.
Diagnosing and Treating Amnesia
The diagnosis of amnesia usually begins with a complete medical history. Doctors will ask questions to determine the extent and type of a person’s memory loss, and whether there are any aggravating or triggering factors, including head injury, emotionally traumatic events, recent surgery, use of drugs, or excessive use of alcohol. A physical examination may include a detailed neurological examination. Doctors test recent, intermediate, and long-term memory.
Treatment varies according to the type of amnesia and the suspected cause. One drug, donepezil, has been found to improve thinking and memory in patients with Alzheimer’s disease.
Researchers are still exploring how memory is organized and what structures and connections are involved. We hope that better understanding of the neurology of memory will lead to better diagnosis, treatments, and prevention of amnesia and other neurological diseases that affect memory.
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