Memory Loss & the Brain

Anterograde amnesia is a selective memory deficit, resulting from brain injury, in which the individual is severely impaired in learning new information. Memories for events that occurred before the injury may be largely spared, but events that occurred since the injury may be lost. In practice, this means that an individual with amnesia may have good memory for childhood and for the years before the injury, but may remember little or nothing from the years since. Short-term memory is generally spared, which means that the individual may be able to carry on a conversation; but as soon as he is distracted, the memory of the conversation fades.

It is now becoming apparent that while anterograde amnesia devastates memory for facts or events, it may spare memory for skills or habits. Thus, an individual with amnesia can be taught a new skill, such as how to play a game or how to write backwards. The next day, the amnesic individual will claim to have no memory of the prior session, but when asked to try executing the skill, can often perform quite well - indicating that some memories have been formed. It is an important area of current research to document exactly which kinds of memory can be formed in amnesia, and how this may be used to help rehabilitate amnesic individuals.

Anterograde amnesia can occur following damage to at least three distinct brain areas. The first, and most well-studied, is the hippocampus and associated areas in the medial temporal lobes of the brain. The hippocampus seems to act as a "gateway" through which new fact information must pass before being permanently stored in memory. If it is damaged, no new information can enter memory - although older information which has already passed through the gateway may be safe. Damage to the hippocampus (and medial temporal lobes) can occur following stroke or aneurysm to one of the arteries which supplies blood to these areas, as well as following epilepsy, encephalitis, hypoxia, carbon monoxide poisoning, near-drowning or near-suffocation, and the earliest stages of Alzheimer's disease. Some damage to the hippocampus also occurs in the course of normal aging.

Anterograde amnesia can also occur following damage to the basal forebrain, a group of structures which produce acetylcholine, a chemical which helps cells in the brain store new information during learning. The basal forebrain can be damaged by aneurysm of the anterior communicating artery, which supplies blood to the basal forebrain.

Finally, anterograde amnesia can sometimes occur following damage to the diencephalon - a set of structures deep in the brain including the medial thalamic nuclei. Currently, there is no good understanding of why damage to these brain areas should sometimes result in a selective memory deficit such as amnesia. Korsakoff's disease is a syndrome which can damage the diencephalon and cause anterograde amnesia.