The hippocampus is a brain structure which lies under the medial temporal lobe, one on each side of the brain. It is sometimes grouped with other nearby structures including the dentate gyrus and called the "hippocampal formation." The hippocampus is critical for the formation of new autobiographical and fact memories. It may function as a memory "gateway" through which new memories must pass before entering permanent storage in the brain. Hippocampal damage can result in anterograde amnesia: loss of ability to form new memories, although older memories may be safe. Thus, someone who sustains an injury to the hippocampus may have good memory of his childhood and the years before the injury, but relatively little memory for anything that happened since.

Some memories, such as the memory for new skills or habits, can sometimes be formed even without the hippocampus. A current research area is to determine exactly what kinds of learning and memory can survive hippocampal damage, and how these kinds of learning can be used to guide rehabilitation.

The hippocampus is especially sensitive to global reductions in oxygen level in the body. Thus, periods of oxygen deprivation (hypoxia) which are not fatal may nonetheless result in particular damage to the hippocampus. This could occur during a heart attack, respiratory failure, sleep apnea, carbon monoxide poisoning, near-drowning, etc. The hippocampus is also a common focus site in epilepsy, and can be damaged through chronic seizures. It is also sometimes damaged in diseases such as herpes encephalitis, and is one of the first brain areas to show damage in Alzheimer's disease.

Further Reading:

L. Squire & E. Kandel (2000) Memory: From Mind to Molecules. New York: Scientific American Library.

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