The hippocampus is a major component of the brains of humans and other mammals. It belongs to the limbic system and plays important roles in long term memory and spatial navigation. Like the cerebral cortex, with which it is closely associated, it is a paired structure, with mirror-image halves in the left and right sides of the brain. In humans and other primates, the hippocampus is located inside the medial temporal lobe, beneath the cortical surface. Its curved shape reminded early anatomists of the horns of a ram (Cornu Ammonis), or 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 medial temporal lobe epilepsy. People with extensive hippocampal damage may experience amnesia – the inability to form or retain new memories.

In rodents, the hippocampus has been studied extensively as part of the brain system responsible for spatial memory and navigation. Many neurons in the rat and mouse hippocampus respond as place cells: that is, they fire bursts of action potentials when the animal passes through a specific part of its environment. Hippocampal place cells interact extensively with head direction cells, whose activity acts as an inertial compass, and with grid cells in the neighboring entorhinal cortex.

Because of its densely-packed layers of neurons, the hippocampus has frequently been used as a model system for studying neurophysiology. The form of neural plasticity known as long term potentiation (LTP) was first discovered to occur in the hippocampus and has often been studied in this structure. LTP is widely believed to be one of the main neural mechanisms by which memory is stored in the brain.

Functions

Historically, the earliest widely held hypothesis was that the hippocampus is involved in olfaction. This idea was largely motivated by a belief, later shown to be false, that the hippocampus receives direct input from the olfactory bulb. There continues to be some interest in hippocampal olfactory responses, particularly the role of the hippocampus in memory for odors, but few people believe today that olfaction is a 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 "slam on the brakes!") was very popular up to the 1960s. It derived much of its justification from two observations: first, that animals with hippocampal damage tend to be hyperactive; second, that 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 currently the least popular of the three.