Hippocampal formation
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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 pro ...
of the
brain A brain is an organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. It is located in the head, usually close to the sensory organs for senses such as vision. It is the most complex organ in a v ...
. It forms a c-shaped bulge on the floor of the temporal horn of the
lateral ventricle The lateral ventricles are the two largest ventricular system, ventricles of the brain and contain cerebrospinal fluid (CSF). Each cerebral hemisphere contains a lateral ventricle, known as the left or right ventricle, respectively. Each lateral ...
. There is no consensus concerning which brain regions are encompassed by the term, with some authors defining it as the
dentate gyrus The dentate gyrus (DG) is part of the hippocampal formation in the temporal lobe of the brain, which also includes the hippocampus and the subiculum. The dentate gyrus is part of the hippocampal trisynaptic circuit and is thought to contribute ...
, the
hippocampus proper The hippocampus proper refers to the actual structure of the hippocampus which is made up of three regions or subfields. The subfields CA1, CA2, and CA3 use the initials of cornu Ammonis, an earlier name of the hippocampus. Structure There are ...
and the
subiculum The subiculum (Latin for "support") is the most inferior component of the hippocampal formation. It lies between the entorhinal cortex and the CA1 subfield of the hippocampus proper. The subicular complex comprises a set of related structures in ...
; and others including also the
presubiculum Area 27 of Brodmann-1909 is a cytoarchitecturally defined cortical area that is a rostral part of the parahippocampal gyrus. It is commonly regarded as a synonym of presubiculum. The dorsal part of the presubiculum is more commonly known as the ...
,
parasubiculum In the rodent, the parasubiculum is a retrohippocampal isocortical structure, and a major component of the subicular complex. It receives numerous subcortical and cortical inputs, and sends major projections to the superficial layers of the entor ...
, and
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 the ...
. 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 grey matter, neural structure of the vertebrate forebrain involved in olfaction, the sense of odor, smell. It sends olfactory information to be further processed in the amygdala, the orbitof ...
varies with the size of the parahippocampal gyrus, the hippocampal formation was thought to be part of the olfactory system. In 1937
Papez James Wenceslas Papez (;Livingston, Kenneth E. '. U.S. National Library of Medicine, 1981 1883–1958) was an American neuroanatomist, most famous for his 1937 description of the Papez circuit, a neuron, neural pathway in the brain thought to ...
theorized that a 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 Amygdaloid, derived from the ancient Greek for almond, may refer to: * The amygdala in the brain * Any shape resembling an almond nut * Amygdule Amygdules or amygdales () form when the vesicles (pores from gas bubbles in lava) of a volcanic ...
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, it ...
began to recede. In an influential 1947 review,
Alf Brodal Alf Brodal (25 January 1910 – 29 February 1988) was a Norwegian professor of anatomy. Personal life He was born in Kristiania as a son of the doctor of engineering Peter Brodal (1872–1935) and his wife Helene Kathrine Obenauer (1879–1934). ...
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 or fibre (from la, fibra, links=no) 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 incorporate ...
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 Vladimir Mikhailovich Bekhterev ( rus, Влади́мир Миха́йлович Бе́хтерев, p=ˈbʲextʲɪrʲɪf; January 20, 1857 – December 24, 1927) was a Russian neurologist and the father of objective psychology. He is best know ...
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, Scoville and 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 Electroencephalography (EEG) is a method to record an electrogram of the spontaneous electrical activity of the brain. The biosignals detected by EEG have been shown to represent the postsynaptic potentials of pyramidal neurons in the neocortex ...
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 human brain, the anterior cingulate cortex (ACC) is the frontal part of the cingulate cortex that resembles a "collar" surrounding the frontal part of the corpus callosum. It consists of Brodmann areas 24, 32, and 33. It is involved ...
) in the control of attention. In 1971, John O'Keefe and his student Jonathan Dostrovsky discovered
place cells A place cell is a kind of pyramidal neuron in the hippocampus that becomes active when an animal enters a particular place in its environment, which is known as the place field. Place cells are thought to act collectively as a cognitive repres ...
, 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 Lynn Nadel (born November 12, 1942) is an American psychologist who is the Regents' Professor of psychology at the University of Arizona. Nadel specializes in memory, and has investigated the role of the hippocampus in memory formation. Together ...
, 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''. In addition to place cells, three further classes of spatial cell have since been identified in the hippocampal formation:
head direction cells Head direction (HD) cells are neurons found in a number of brain regions that increase their firing rates above baseline levels only when the animal's head points in a specific direction. They have been reported in rats, monkeys, mice, chinchillas a ...
,
grid cells A grid cell is a type of neuron within the entorhinal cortex that fires at regular intervals as an animal navigates an open area, allowing it to understand its position in space by storing and integrating information about location, distance, and ...
, and
boundary cell Boundary cells (also known as border cells or boundary vector cells) are neurons found in the hippocampal formation that respond to the presence of an environmental boundary at a particular distance and direction from an animal. The existence of cel ...
s. 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)