Amnesia and organic memory impairment are commonly seen after many types of brain injury including degenerative disorders, head injury, anoxia and infections of the brain. If severe, amnesia is often more handicapping in everyday life than severe physical problems.

People with the classic amnesic syndrome show an anterograde amnesia (AA), i.e. they have great difficulty learning and remembering most kinds of new information. Immediate memory, however, is normal when this is assessed by forward digit span or the recency effect in free recall. There is usually a period of retrograde amnesia (RA), that is a loss of information acquired before the onset of the amnesia. This gap or period of RA is very variable in length and may range from a few minutes to decades. Previously acquired semantic knowledge about the world and implicit memory (remembering without awareness or conscious recollection) are typically intact in amnesic subjects. As the majority of patients with severe memory disorders present with additional cognitive problems such as attention deficits, word finding problems or slowed information processing, those with a classic amnesic syndrome are relatively rare.

Nevertheless, people with a ‘pure’ amnesic syndrome and people with more widespread cognitive deficits tend to share certain characteristics. In both cases, immediate memory is reasonably normal; there is difficulty in remembering after a delay or distraction; new learning is difficult and there is a tendency to remember things that happened a long time before the accident or illness better than things that happened a short time before.