Amnesia

Amnesia, loss or impairment of memory. Amnesia can be a symptom of a variety of brain injuries or degenerative brain conditions resulting from infection or substance abuse. Amnesia may be total, with complete loss of recall; or partial, occurring only immediately after a traumatic event such as head injury; or systematic.

Anterograde amnesia occurs after brain injury when short-term memory is affected but long-term memory for events before the trauma remains intact. In retrograde amnesia the opposite occurs and the person forgets what happened before the trauma but their short-term memory is less affected. In transient amnesia the person experiences short periods of confusion and memory loss. Memory loss due to alcohol abuse, Korsakoff’s psychosis, results in severe difficulty in remembering simple stories, lists of words, or faces even though the general short-term memory is unaffected. This condition is progressive and is accompanied by increasing neurological disorders.

In recent years the study of amnesiacs has provided important clues as to how memory works. The method of examining single case studies was developed to draw inferences about memory from observations of people with specific brain injury. The most famous case study is that of a patient known as HM, who had brain surgery for epilepsy in 1953. The operation resulted in his loss of the ability to learn new facts or associations; he could not even recognize his doctor from one hour to the next.

In particular, the idea that memory works as a single, central, organized store has been challenged by the fact that people with amnesia often have very specific forms of memory loss which indicates that memory is not located in a single area of the brain but distributed across regions of the brain.