Amnesia

A partial or total loss of memory.

There are numerous causes of amnesia, including stroke, injury to the brain, surgery, alcoholism, encephalitis, and electroconvulsive therapy. Contrary to the popular notion of amnesia—in which a person suffers a severe blow to the head, for example, and cannot recall his or her past life and experiences—the principal symptom of amnesia is the inability to retain new information, beginning at the point at which the amnesia began. The capacity to recall past experiences may vary, depending on the severity of the amnesia.

There are two types of amnesia: retrograde and anterograde. Retrograde amnesia refers to the loss of memory of one's past, and can vary from person to person. Some retain virtually full recall of things that happened prior to the onset of amnesia; others forget only their recent past, and still others lose all memory of their past lives. Anterograde amnesia refers to the inability to recall events or facts introduced since the amnesia began.

Amnesiacs often appear perfectly normal. Motor skills such as tying laces and bows and bike riding are retained, as is the ability to read and comprehend the meaning of words. Because of this phenomenon, researchers have suggested that there is more than one area of the brain used to store memory. General knowledge and perceptual skills may be stored in a memory separate from the one used to store personal facts.

The most famous study of amnesia involves a patient called H.M., who in 1953 underwent brain surgery designed to treat his epilepsy. Following the surgery, he could recall all the events of his past life up until three weeks before the operation. However, H.M. could no longer function normally because he had lost the ability to learn new facts and associations. For example, he could not recognize his doctor from day to day or hour to hour.

Childhood amnesia, a term coined by Anna Freud in the late 1940s, refers to the fact that most people cannot recall childhood experiences during the first three to five years of life. It has been suggested that this type of amnesia occurs because children and adults organize memories in different ways based on their brain's physical development. Others believe children begin remembering facts and events once they have accumulated enough experience to be able to relate experiences to each other.