The hippocampal formation is a compound structure in the

medial temporal lobe The temporal lobe is one of the four major lobes of the cerebral cortex in the brain of mammals. The temporal lobe is located beneath the lateral fissure on both cerebral hemispheres of the mammalian brain. The temporal lobe is involved in pr ...

of the

brain The brain is an organ (biology), organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. It consists of nervous tissue and is typically located in the head (cephalization), usually near organs for ...

. It forms a c-shaped bulge on the floor of the inferior horn of the

lateral ventricle The lateral ventricles are the two largest ventricular system, ventricles of the brain and contain cerebrospinal fluid. Each cerebral hemisphere contains a lateral ventricle, known as the left or right lateral ventricle, respectively. Each later ...

. Typically, the hippocampal formation is said to included the dentate gyrus, the

hippocampus The hippocampus (: hippocampi; via Latin from Ancient Greek, Greek , 'seahorse'), also hippocampus proper, is a major component of the brain of humans and many other vertebrates. In the human brain the hippocampus, the dentate gyrus, and the ...

, and the subiculum. The presubiculum, parasubiculum, and the

entorhinal cortex The entorhinal cortex (EC) is an area of the brain's allocortex, located in the medial temporal lobe, whose functions include being a widespread network hub for memory, navigation, and the perception of time.Integrating time from experience in t ...

may also be included. The hippocampal formation is thought to play a role in memory, spatial navigation and control of attention. The neural layout and pathways within the hippocampal formation are very similar in all mammals.

History and function

During the nineteenth and early twentieth centuries, based largely on the observation that, between species, the size of the

olfactory bulb The olfactory bulb (Latin: ''bulbus olfactorius'') is a neural structure of the vertebrate forebrain involved in olfaction, the sense of smell. It sends olfactory information to be further processed in the amygdala, the orbitofrontal cortex (OF ...

varies with the size of the

parahippocampal gyrus The parahippocampal gyrus (or hippocampal gyrus') is a grey matter cortical region, a gyrus of the brain that surrounds the hippocampus and is part of the limbic system. The region plays an important role in memory encoding and retrieval. It ha ...

, the hippocampal formation was thought to be part of the olfactory system. In 1937, Papez theorized that a circuit (the Papez circuit) including the hippocampal formation constitutes the neural substrate of emotional behavior, and Klüver and Bucy reported that, in monkeys, surgical removal of the hippocampal formation and the amygdaloid complex has a profound effect on emotional responses. As a consequence of these publications, the idea that the hippocampal formation is entirely dedicated to

olfaction The sense of smell, or olfaction, is the special sense through which smells (or odors) are perceived. The sense of smell has many functions, including detecting desirable foods, hazards, and pheromones, and plays a role in taste. In humans, ...

began to recede. Medial surface of cerebral cortex - entorhinal cortex

In an influential 1947 review, Alf Brodal pointed out that mammal species thought to have no sense of smell nevertheless have fully intact hippocampal formations, that removal of the hippocampal formation did not affect the ability of dogs to perform tasks dependent on olfaction, and that no

fibers Fiber (spelled fibre in British English; from ) is a natural or artificial substance that is significantly longer than it is wide. Fibers are often used in the manufacture of other materials. The strongest engineering materials often inco ...

were actually known that carry information directly from the olfactory bulb to any part of the hippocampal formation. Though massive direct input from the olfactory bulb to the entorhinal cortex has subsequently been found, the current view is that the hippocampal formation is not an integral part of the olfactory system. In 1900, the Russian neurologist Vladimir Bekhterev described two patients with a significant memory deficit who, on autopsy, were found to have softening of hippocampal and adjacent cortical tissue; and, in 1957, William Beecher Scoville and Brenda Milner reported memory loss in a series of patients subsequent to their removal of the patients' medial temporal lobes. Thanks to these observations and a great deal of subsequent research, it is now broadly accepted that the hippocampal formation plays a role in some aspects of memory. EEG evidence from 1938 to the present, stimulation evidence from the 1950s, and modern imaging techniques together suggest a role for some part of the hippocampal formation (in concert with the anterior cingulate cortex) in the control of attention. In 1971, John O'Keefe and his student Jonathan Dostrovsky discovered place cells: neurons in the rat hippocampus whose activity relates to the animal's location within its environment. Despite skepticism from other investigators, O'Keefe and his co-workers, including Lynn Nadel, continued to investigate this question, in a line of work that eventually led to their very influential 1978 book ''The Hippocampus as a Cognitive Map''. The discovery of place cells, together with the discovery of grid cells by May-Britt Moser and

Edvard Moser Edvard Ingjald Moser () is a Norwegian psychologist and neuroscientist, who is a professor at the Norwegian University of Science and Technology (NTNU) in Trondheim. He shared the Nobel Prize in Physiology or Medicine in 2014 with long-term col ...

, and the mapping of the function of the hippocampal formation in spatial awareness, led to the joint award of the

Nobel Prize in Physiology or Medicine The Nobel Prize in Physiology or Medicine () is awarded yearly by the Nobel Assembly at the Karolinska Institute for outstanding discoveries in physiology or medicine. The Nobel Prize is not a single prize, but five separate prizes that, acco ...

in 2014. In addition to place cells and grid cells, two further classes of spatial cell have since been identified in the hippocampal formation: head direction cells and boundary cells. As with the memory theory, there is now almost universal agreement that the hippocampal formation plays an important role in spatial coding, but the details are widely debated.

References

External links

Image
University of California Davis Brain Atlas {{Olfactory system Hippocampus (brain)