The hippocampus is a part of the forebrain, located in the medial temporal lobe. It belongs to the limbic system and plays major roles in short term memory and spatial navigation. Humans and other mammals have two hippocampi, one in each side of the brain. In rodents, where it has been studied most extensively, the hippocampus is shaped something like a banana. In humans it has a curved and convoluted shape that reminded early anatomists of a seahorse. The name, in fact, derives from the Greek word for seahorse (Greek: ιππος, hippos = horse, καμπος, kampos = sea monster).

In Alzheimer's disease the hippocampus is one of the first regions of the brain to suffer damage; memory problems and disorientation appear among the first symptoms. Damage to the hippocampus can also result from oxygen starvation (anoxia), encephalitis, or mesial temporal lobe epilepsy. People with extensive hippocampal damage may experience amnesia, that is, inability to form or retain new memories.

Functions of the hippocampus

Perhaps the earliest idea was that the hippocampus is involved in olfaction: this seems to have been suggested mainly by its location in the brain, next to the olfactory cortex. There continues to be some interest in hippocampal olfactory responses, but few people believe today that olfaction is the primary function of the hippocampus.

Over the years, three main ideas of hippocampal function have dominated the literature: inhibition, memory, and space. The behavioral inhibition theory (caricatured by O'Keefe and Nadel as "step on the brakes!") was very popular up to the 1960s. It derived much of its force from two observations: first, animals with hippocampal damage tend to be hyperactive; second, animals with hippocampal damage often have difficulty learning to inhibit responses that they have previously been taught. Jeffrey Gray developed this line of thought into a full-fledged theory of the role of the hippocampus in anxiety. The inhibition theory is not, however, very popular at present.

The second important line of thought relates the hippocampus to memory. Although it has precursors, this idea derived its main force from a very well-known report by Scoville and MilnerScoville and Milner, 1957 of the results of surgical destruction of the hippocampus (in an attempt to relieve epileptic seizures), in a patient known as H.M. The unexpected outcome was severe amnesia: H.M. was unable to consciously remember events that occurred after his surgery or for several years before it. This case occasioned such enormous interest that H.M. is now said to be the most intensively studied medical case in history. In the ensuing years, other patients with similar levels of hippocampal damage and amnesia (caused by accident or disease) have been studied as well, and literally thousands of experiments have studied the physiology of neural plasticity in the hippocampus. There is now almost universal agreement that the hippocampus plays some sort of important role in memory; however, the precise nature of this role remains widely debatedSquire, 1992Eichenbaum and Cohen, 1993.