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
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.