Hippocampus
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/* Evolution */
{{Otheruses}}
{{Infobox Brain|
Name = Hippocampus |
Latin = |
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Image = hippocampus.png |
Caption = The hippocampus is located in the [[temporal lobe|medial temporal lobe]] of the [[brain]]. (In this illustration of the inferior surface (underside) of the brain, the [[frontal lobe]] of the brain is at the top, while the [[occipital lobe]] is at the bottom.) |
Image2 = hippocampus-mri.jpg |
Caption2 = The location of one of the hippocampi is indicated by the crosshairs |
Width = 200 |
IsPartOf = |
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BrainInfoType = hier |
BrainInfoNumber = 164 |
MeshName = Hippocampus |
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DorlandsPre = h_12 |
DorlandsSuf = 12422843 |
}}
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 (fish)|seahorse]] ([[Greek language|Greek]]: ''hippos'' = ''horse'', ''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 almost nobody now believes that the primary function of the hippocampus is olfactory.
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<ref>[[#refGray2000|Gray and McNaughton, 2000]]</ref>. The inhibition theory is not, however, very popular at present.
The second important line of thought relates the hippocampus to memory. Although it had precursors, this idea derived its main force from a very well-known report by Scoville and Milner<ref name="Scoville">[[#refScoville1957|Scoville and Milner, 1957]]</ref> 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 debated<ref name="Squire">[[#refSquire1992|Squire, 1992]]</ref><ref name="Eichenbaum">[[#refEichenbaum1993|Eichenbaum and Cohen, 1993]]</ref>.
The third important line of thought relates the hippocampus to space. The spatial theory was originally championed by O'Keefe and Nadel, who were influenced by [[Edward C. Tolman|E. C. Tolman's]] theories about "cognitive maps" in humans and animals. O'Keefe and his student Dostrovsky discovered, in 1971, neurons in the rat hippocampus that appeared to them to show activity that encoded the rat's location within its environment. O'Keefe and his co-workers, especially Lynn Nadel, continued to investigate this question, in a line of work that eventually led to their very influential 1978 book called, "The hippocampus as a cognitive map"<ref name="O'Keefe">[[#refOKeefe1978|O'Keefe and Nadel, 1978]]</ref>. As with the memory theory, there is now almost universal agreement that spatial coding somehow plays an important role in hippocampal function, but the details are widely debated.
=== Role in general memory ===
{{main|Amnesia}}
[[psychology|Psychologists]] and [[neuroscience|neuroscientists]] generally agree that the hippocampus has an important role in the formation of new [[memory|memories]] about experienced events (''[[episodic memory|episodic]]'' or ''[[autobiographical memory]]'')<ref name="Eichenbaum" /><ref name="SquireSchacter">[[#refSquire2002| Squire and Schacter, 2002]]</ref>. Some researchers prefer to consider the hippocampus as part of a larger [[medial temporal lobe]] memory system responsible for general ''[[declarative memory]]'' (memories that can be explicitly verbalized — these would include, for example, [[semantic memory|memory for facts]] in addition to episodic memory)<ref name="Squire" />.
Some evidence supports the idea that, although these forms of memory often last a lifetime, the hippocampus ceases to play a crucial role in the retention of the memory after a period of ''[[memory consolidation|consolidation]]''<ref>[[#refSquireSchacter2002|Squire and Schacter, 2002, Ch. 1]]</ref>. Damage to the hippocampus usually results in profound difficulties in forming new memories (''[[anterograde amnesia]]''), and normally also affects access to memories prior to the damage (''[[retrograde amnesia]]''). Although the retrograde effect normally extends some years prior to the brain damage, in some cases older memories remain - this sparing of older memories leads to the idea that consolidation over time involves the transfer of memories out of the hippocampus to other parts of the brain. However, experimentation has difficulties in testing the sparing of older memories; and, in some cases of retrograde amnesia, the sparing appears to affect memories formed decades before the damage to the hippocampus occurred, so its role in maintaining these older memories remains uncertain.
Damage to the hippocampus does not affect some aspects of memory, such as the ability to learn new skills (playing a musical instrument, for example), suggesting that such abilities depend on a different type of memory (''[[procedural memory]]'') and different brain regions. And there is evidence to suggest that [[HM (patient)|patient H. M.]] (who had his medial temporal lobes removed bilaterally as a treatment for epilepsy<ref name="Scoville" />) can form new [[semantic memory|semantic memories]].<ref>[[#refOKane2004|O'Kane et al., 2004]]</ref>
=== Role in spatial memory and navigation ===
{{main|Place cell}}
[[Image:Triangle-place-cells.png|thumb|300px|right|Spatial firing patterns of 7 place cells recorded from the same location in the dorsal CA1 layer of a rat.The rat ran several hundred laps clockwise around an elevated triangular track, stopping in the middle of each arm to eat a small portion of food reward. Black dots indicate positions of the rat's head; colored dots indicate action potentials from one of the cells, using a different color for each cell.<ref>[[#refSkaggs1996|Skaggs et al., 1996]]</ref>]]
Evidence suggests the hippocampus is used in storing and processing spatial information. Studies in rats have shown that [[neurons]] in the hippocampus have spatial firing fields. These cells are called ''[[place cell]]s''. Some cells fire when the animal finds itself in a particular location, regardless of direction of travel, while most are at least partially sensitive to head direction and direction of travel. In rats, some cells, termed ''context-dependent cells'', may alter their firing depending on the animal's past (''retrospective'') or expected future (''prospective''). Different cells fire at different locations, so that, by looking at the firing of the cells alone, it becomes possible to tell where the animal is. Place cells have now been seen in humans involved in finding their way around in a [[virtual reality]] town. The findings resulted from research with individuals with electrodes implanted in their brains as a diagnostic part of surgical treatment for serious epilepsy.<ref name="Ekstrom ">[[#refEkstrom2003|Ekstrom et al., 2003]]</ref>
The discovery of place cells led to the idea that the hippocampus might act as a ''cognitive map'' — a neural representation of the layout of the environment. Recent evidence has cast doubt on this perspective, indicating that the hippocampus might be crucial for more fundamental processes within navigation.{{Fact|date=May 2008}} Regardless, studies with animals have shown that an intact hippocampus is required for simple spatial memory tasks (for instance, finding the way back to a hidden goal).
Without a fully functional hippocampus, humans may not successfully remember where they have been and how to get where they are going. Researchers believe that the hippocampus plays a particularly important role in finding shortcuts and new routes between familiar places. Some people exhibit more skill at this sort of navigation than do others, and brain imaging shows that these individuals have more active hippocampi when navigating.
[[London]]'s taxi drivers must learn a large number of places — and know the most direct routes between them (they have to pass a strict test, ''[[The Knowledge]]'', before being licensed to drive the famous black cabs). A study at University College London by Maguire, et al (2000)<ref name="Maguire ">[[#refMaguireFrith2000|Maguire et al., 2000]]</ref> showed that part of the hippocampus is larger in taxi drivers than in the general public, and that more experienced drivers have bigger hippocampi. Whether having a bigger hippocampus helps an individual to become a cab driver or finding shortcuts for a living makes an individual's hippocampus grow is yet to be elucidated. However, in that study Maguire, et al examined the correlation between size of the grey matter and length of time that had been spent as a taxi driver, and found that the longer an individual had spent as a taxi driver, the larger the volume of the right hippocampus. It was found that the total volume of the hipocampus remained constant, from the control group vs. taxi drivers. That is to say that the posterior portion of a taxi driver's hippocampus is indeed increased, but at the expense of the anterior portion. There have been no known detrimental effects reported from this disparity in hippocampal proportions. <ref name="Maguire "/> This finding suggested to the authors that the hippocampus increases in size with use over time.<ref name="Maguire "/>
== History ==
The anatomist [[Giulio Cesare Aranzi]] (circa 1564) first used the term '''hippocampus''' to describe the cerebral organ because of its visual resemblance to a seahorse. This organ was initially connected with the sense of smell, rather than with its known function in memory acquisition. The Russian [[Vladimir Bekhterev]] noted the role of the hippocampus in memory around 1900, based on observations of a patient with profound memory disturbances. However, for many years, the conventional view of the hippocampus was that, like the rest of the [[limbic system]], it was responsible for emotion.
The importance of the hippocampus in memory was brought to the attention of researchers by patient [[HM (patient)|HM]]. HM suffered from a number of anterograde and temporally-graded retrograde memory impairments (such impairments are the subject of the movie ''[[Memento (film)|Memento]]'') following the bilateral removal of various medial-temporal lobe structures (including bilateral ablation of his hippocampi) to relieve frequent [[epileptic seizure]]s. Of particular importance is that HM was still able to learn procedural tasks (which are associated with the [[striatum]]) and had an above-average IQ. HM demonstrated a striking single-dissociation between intelligence and declarative memory. The relative size of the hippocampal formation in relation with the total volume of the brain is often conserved in most of the mammalian species. Nevertheless, it has been found that these areas are relatively [[Wiktionary:Hypotrophy|hypotrophic]] in [[cetaceans]].
Recently work has been done on creating an artificial version of the CA3 region of the hippocampus using [[VLSI]] integrated circuits.<ref>"[http://www.newscientist.com/article.ns?id=dn3488 World's first brain prosthesis revealed]", by [[Duncan Graham-Rowe]], [[New Scientist]], 12 March, 2003</ref><ref>"Restoring Lost Cognitive Function--Hippocampal-Cortical Neural Prostheses", by [[Theodore Berger]] et al., [[IEEE Engineering in Medicine and Biology Magazine]] 24, 5, pp. 30-44, Sept.-Oct. 2005.</ref>
==Anatomy==
{{main|Hippocampus anatomy}}
[[Image:Gray739-emphasizing-hippocampus.png|thumb|300px|right|Human hippocampus.]]
[[Image:Brainmaps-macaque-hippocampus.jpg|thumb|300px|right|Nissl-stained coronal section of the brain of a macaque monkey, showing hippocampus (circled). Source: brainmaps.org]]
Anatomically, the hippocampus is an elaboration of the edge of the
cortex. It can be distinguished as a zone where the cortex narrows into a single layer of very
densely packed neurons, which curls into a tight S shape. The structures that line the edge of the cortex make up the so-called [[limbic system]] (Latin ''limbus'' =
''border''): these include the hippocampus, cingulate cortex, olfactory
cortex, and amygdala. [[Paul D. MacLean|Paul MacLean]] once suggested, as
part of his [[triune brain]] theory, that the limbic structures comprise
the neural basis of emotion. Most neuroscientists no longer believe
that the concept of a unified "limbic system" is valid, though.
The hippocampus, as a whole,
ends up looks something like a curved tube, which has been
analogized variously to a seahorse, or a ram's horn (''Cornu Ammonis''), or a banana.
This general layout holds across the full range of mammalian species,
from hedgehog to human, although the details vary. In the rat, the
two hippocampi look astonishingly like a pair of bananas, joined at
the stem. In human or monkey brains, the portion of the hippocampus
down at the bottom, near the base of the temporal lobe, is much
broader than the part at the top. One of the consequences of this complex
geometry is that cross-sections through the hippocampus can show a variety of shapes, depending on the angle and location of
the cut.
The strongest connections of the hippocampus are with the entorhinal cortex (EC), which lies next to it in the temporal lobe. The superficial layers of the EC provide the most numerous inputs to the hippocampus, and the deep layers of the EC receive the most numerous outputs. The EC, in turn, is strongly, and reciprocally, connected with many other parts of the cortex. The hippocampus also receives a very important projection from the medial septal area. Destruction of the septal area abolishes the hippocampal theta rhythm, and severely impairs certain types of memory. (So-called "date rape" drugs are thought to exert their amnestic effects at least partly by antagonizing the cholinergic projection from the medial septum to the hippocampus.)
== Physiology ==
[[Image:Rat-hippocampal-activity-modes.png|thumb|350px|right|Examples of rat hippocampal EEG and CA1 neural activity in the theta (awake/behaving) and LIA (slow-wave sleep) modes. Each plot show 20 seconds of data, with a hippocampal EEG trace at the top, spike rasters from 40 simultaneously recorded CA1 pyramidal cells in the middle (each raster line represents a different cell), and a plot of running speed at the bottom. The top plot represents a time period during which the rat was actively searching for scattered food pellets. For the bottom plot, the rat was asleep.]]
The hippocampus shows two major "modes" of activity, each associated
with a distinct pattern of EEG waves and neural population activity.
These modes are named after the EEG patterns associated with them:
''theta'' and ''large irregular activity'' (LIA). Here are some of
their main characteristics in the rat, the animal that has been most
extensively studied:<ref>[[#refBuzsaki2006|Buzsaki, 2006]]</ref>
The '''theta''' mode appears during states of active, alert behavior
(especially locomotion), and also during REM (dreaming) sleep. In the
theta mode, the EEG is dominated by large regular waves with a
frequency range of 6-9 Hz, and the main groups of hippocampal neurons
(pyramidal cells and granule cells) show sparse population activity,
which means that in any short time interval, the great majority of
cells are silent, while the small remaining fraction fire at
relatively high rates, up to 50 spikes in one second for the most
active of them. An active cell typically stays active for from
half a second to a few seconds. As the rat behaves, the active cells
fall silent and new cells become active, but the overall percentage of
active cells remains more or less constant. In many situations, cell
activity is determined largely by the spatial location of the animal,
but other behavioral variables also clearly influence it.
The '''LIA''' mode appears during slow-wave (non-dreaming) sleep, and also
during states of waking immobility, such as resting or eating. In the
LIA mode, the EEG is dominated by sharp waves, which are
randomly-timed large deflections of the EEG signal lasting for 200-300
msec. These sharp waves also determine the population neural activity
patterns. Between them, pyramidal cells and granule cells are very
quiet (but not silent). During a sharp wave, as many as 5-10% of the
population may emit action potentials during a period of 50 msec; many
of these cells emit not one but a burst of spikes.
These two hippocapampal activity modes can be seen in primates as well
as rats, with the important exception that it has been difficult to
see robust theta rhythmicity in the primate hippocampus. There are,
however, qualitatively similar sharp waves, and similar
state-dependent changes in neural population
activity.<ref>[[#refSkaggs2007|Skaggs et al., 2007]]</ref>.
=== The theta rhythm ===
Because of its densely packed neural layers, the hippocampus generates
some of the largest EEG signals of any brain structure. In some
situations the EEG is dominated by regular waves, often continuing for
many seconds. This EEG pattern is known as the [[theta wave|theta rhythm]]. It was
one of the earliest EEG phenomena to be discovered: the first
description came from Jung and Kornmuller, in 1938. It was not until
1954, however, with the publication by Green and Arduini of a long and
thorough study of theta rhythm in rabbits, cats, and monkeys, that
interest really took off.<ref>[[#refGreen1954|Green and Arduini, 1954]]</ref>
Perhaps largely because they related the
theta rhythm to arousal, which was the hot topic of the day, their
paper provoked a flood of followup studies, resulting in the
publication of literally hundreds of studies of the physiology and
pharmacology of theta during the 1950s and 1960s. In spite of this
rather daunting body of work, many questions remained unanswered,
especially the question of function. Even at present this most
critical of questions has not yet been convincingly answered.
Theta rhythmicity is very obvious in rabbits and rodents, and also
clearly present in cats and dogs. Whether theta can be seen in
primates is a vexing question. Green and Arduini reported only very
short bursts of rather irregular rhythmicity in monkeys, and most
later studies have seen little more. However, variations in
methodology have made it difficult to draw strong conclusions.<ref>[[#refCantero2003|Cantero et al., 2003]]</ref>
In rats (the animals that have been by far the most extensively
studied), theta is seen mainly in two conditions: first, when an
animal is walking or in some other way actively interacting with its
surroundings; second, during REM sleep.<ref>[[#refVanderwolf1969|Vanderwolf, 1969]]</ref>
The frequency increases as a
function of running speed, starting at about 6.5 Hz on the low end,
and increasing to about 9 Hz on the high end, although higher
frequencies are sometimes seen for dramatic movements such as jumps
across wide gaps. In other, larger, species of animals, theta
frequencies are generally a bit lower. The behavioral dependency also
seems to vary by species: in cats and rabbits, theta is often
observed during states of motionless alertness. This has been
reported for rats as well, but only when they are severely frightened.<ref>[[#refSainsbury1987|Sainsbury et al., 1987]]</ref>
Theta is not just confined to the hippocampus. In rats, it can be
observed in many parts of the brain, including nearly all that
interact strongly with the hippocampus. The pacemaker for the rhythm
is thought to lie within the medial septal area: this area projects
to all of the regions that show theta rhythmicity, and destruction of
it eliminates theta throughout the brain. (There may be one
exception, a small area in the hypothalamus called the supramamillary
nucleus, which seems to be capable of sustaining theta independently
of the septum in some situations.<ref>[[#refKirk1991|Kirk and McNaughton, 1991]]</ref>)
The function of theta, presuming it has one, has not yet been
convincingly explained, although numerous theories have been proposed.<ref>[[#refBuzsaki2006|Buzsáki, 2006]]</ref>
The most popular trend has been to relate it to learning and memory.
It is well established that lesions of the medial septum---the central
node of the theta system---cause severe disruptions of memory.
However, the medium septum is more than just the controller of theta,
it is also the main source of cholinergic projections to the
hippocampus. It has not been established that septal lesions exert
their effects specifically by eliminating theta.
=== Sharp waves ===
During sleep, or during waking states when an animal is resting or
otherwise not engaged with its surroundings, the hippocampal EEG shows
a pattern of irregular slow waves, somewhat larger in amplitude than
theta waves. This pattern is occasionally interrupted by large surges
called '''sharp waves'''. These events are associated with bursts of
spike activity, lasting 50-100 msec, in pyramidal cells of CA3 and
CA1. They are also associated with short-lasting high-frequency EEG
oscillations called "ripples". Ripples, with frequencies in the range
150-200 Hz in rats, can usually be detected only by electrodes located
either inside, or very close to, the CA1 cell body layer. In
contrast, electrodes located anywhere inside the hippocampus, or even
in neighboring brain structures, will often pick up sharp waves as
large slow EEG deflections, lasting 200-400 msec.
In rats, sharp waves are most robust during sleep, when they occur at
an average rate around 1 per second, but in a very irregular temporal
pattern. Sharp waves also occur during inactive waking states, but
they are less frequent then and usually smaller. Sharp waves have
also been observed in the human temporal lobe and monkey hippocampus.
In monkeys, sharp waves are quite robust, but do not occur nearly as
frequently as in rats.
One of the most interesting aspects of sharp waves is that they appear
to be associated with memory. Wilson and McNaughton 1994, and
numerous later studies, reported that when hippocampal place cells
have overlapping spatial firing fields (and therefore often fire in
near-simultaneity), they tend to show correlated activity during sleep
following the behavioral session. This enhancement of correlation,
commonly known as '''reactivation''', has been found to be confined
mainly to sharp waves. It has been proposed that sharp waves are, in
fact, reactivations of neural activity patterns that were memorized
during behavior, driven by strengthening of synaptic connections
within the hippocampus. This idea forms a key component of the
"two-stage memory" theory, advocated by Buzsaki and others, which
proposes that memories are stored within the hippocampus during
behavior, and then later transferred to the neocortex during sleep:
sharp waves are suggested to drive Hebbian synaptic changes in the
neocortical targets of hippocampal output pathways.
==Evolution==
The hippocampus has a generally similar appearance across the range of
mammal species, from basal ones such as the hedgehog to the most
"advanced" ones such as humans<ref>[[#refWest1990|West, 1990]]</ref>.
The hippocampal-size-to-body-size ratio broadly increases, being about
twice as large for primates as for the hedgehog. It does not,
however, increase at anywhere close to the rate of the neocortex-to-body-size ratio.
Thus, the hippocampus takes up a much larger volume of the cortical
mantle in rodents than in primates.
There is also a general relationship between the size of the
hippocampus and spatial memory: when comparisons are made between
similar species, ones that have a greater capacity for spatial memory
tend to have larger hippocampal volumes.<ref name="Jacobs2003">[[#refJacobs2003|Jacobs,
2003]]</ref>. This relationship also extends to sex differences: in
species where males and females show strong differences in spatial
memory ability, they also tend to show corresponding differences in
hippocampal volume<ref>[[#refJacobs1990|Jacobs et al., 1990]]</ref>
Non-mammalian species do not have a brain structure that looks like
the mammalian hippocampus, but they have one that is considered
[[Homology (biology)|homologous]] to it. The hippocampus, as pointed out above,
is essentially the medial edge of the cortex. Only mammals have a
fully developed cortex, but the structure it evolved from, called the
[[pallium_(anatomy)|pallium]], is present in all vertebrates, even the most primitive
ones such as the lamprey or hagfish<ref>[[#refAboitiz2003|Aboitiz et
al., 2003]]</ref>. The pallium is usually divided into three
zones: medial, lateral, and dorsal. The medial pallium forms the
precursor of the hippocampus. It does not resemble the hippocampus
visually, because the layers are not warped into an S shape or
enwrapped by the dentate gyrus, but the homology is indicated by
strong chemical and functional affinities. There is now evidence that
these hippocampal-like stuctures are involved in spatial cognition in
birds, reptiles, and fish.<ref>[[#refRodriguez2002|Rodríguez et al., 2002]]</ref>
In birds, the correspondence is sufficiently well established that
most anatomists refer to the medial pallial zone as the "avian
hippocampus".<ref>[[#refColombo2000|Colombo and Broadbent,
2000]]</ref> Numerous species of birds have strong spatial skills,
particularly those that cache food. There is evidence that
food-caching birds have a larger hippocampus than other types of
birds, and that damage to the hippocampus causes impairments in
spatial memory.<ref>[[#refShettleworth2003|Shettleworth,
2003]]</ref>.
The story for fish is more complex. In [[teleost]] fish (which make
up the great majority of existing species), the forebrain is weirdly
distorted in comparison to other types of vertebrates. Most
neuroanatomists believe that the teleost forebrain is essentially
everted, like a sock turned inside-out, so that structures that lie in
the interior, next to the ventricles, for most vertebrates, are found
on the outside in teleost fish, and vice
versa.<ref>[[#refNieuwenhuys1982|Nieuwenhuys, 1982]]</ref> One of the
consequences of this is that the medial pallium ("hippocampal" zone)
of a typical vertebrate is thought to correspond to the lateral
pallium of a typical fish. Several types of fish (particularly
goldfish) have been shown experimentally to have strong spatial memory
abilities, even forming "cognitive maps" of the areas they
inhabit.<ref name="Jacobs2003" /> There is evidence
that damage to the lateral pallium impairs spatial
memory.<ref>[[#refPortavella2002|Portavella et al., 2002]]</ref>
<ref>[[#refVargas2006|Vargas et al., 2006]]</ref>
(Long-distance navigation, such as homing by salmon, seems to rely on
different mechanisms, however.)
Thus, the role of the hippocampal region in navigation appears to begin
far back in vertebrate evolution, predating splits that occurred
hundreds of millions of years ago.<ref>[[#refBroglio2005|Broglio et
al., 2005]]</ref> It is not yet known whether the medial pallium plays
a similar role in even more primitive vertebrates, such as sharks and
rays, or even lampreys and hagfish. Some types of insects, and
molluscs such as the octopus, also have strong spatial learning and
navigation abilities, but these appear to work differently from the
mammalian spatial system, so there is as yet no good reason to think
that they have a common evolutionary origin; nor is there sufficient
similarity in brain structure to enable anything resembling a
"hippocampus" to be identified in these species.
==Notes==
{{Reflist|2}}
==References==
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<references /></div>
==Additional images==
<gallery>
Image:Gray682.png|Superficial dissection of brain-stem. Lateral view.
Image:Gray717.png|Coronal section of brain immediately in front of pons.
Image:Gray739.png|Posterior and inferior cornua of left lateral ventricle exposed from the side.
Image:Gray740.png|Inferior and posterior cornua, viewed from above.
Image:Gray748.png|The fornix and [[corpus callosum]] from below.
Image:Human brain frontal (coronal) section description 2.JPG|Human brain frontal (coronal) section
Image:Human brain right dissected lateral view description.JPG|Human brain right dissected lateral view
Image:Hippocampus (brain).jpg|Diagram of the human hippocampus
Image:Dopamineseratonin.gif|Dopamine and serotonin pathways
Image:HippocampalRegions.jpg|Hippocampal areas in a Nissl-stained coronal section of the rat brain. DG: [[Dentate gyrus]].]]
</gallery>
==See also ==
* [[List of regions in the human brain]]
==External links==
* {{BrainMaps|hippocampus}}
* [http://www.bris.ac.uk/Depts/Synaptic/info/pathway/figs/hippocampus.gif Diagram of network]
* [http://www.hippo2006.psn.ru International Symposium “HIPPOCAMPUS AND MEMORY”]
* [http://www.stanford.edu/group/maciverlab/hippocampal.html Diagram of a Hippocampal Brain Slice]
* [http://www.temporal-lobe.com Temporal-lobe.com An interactive diagram of the rat parahippocampal-hippocampal region]
{{Limbic system}}
{{Papez circuit}}
[[Category:Limbic system]]
[[Category:Cerebrum]]
[[da:Hippocampus]]
[[de:Hippocampus]]
[[es:Hipocampo (anatomía)]]
[[fr:Hippocampe (cerveau)]]
[[id:Hippocampus]]
[[is:Dreki (heilastöð)]]
[[it:Ippocampo (anatomia)]]
[[he:היפוקמפוס]]
[[hu:Hippokampusz]]
[[nl:Hippocampus (hersenen)]]
[[ja:海馬 (脳)]]
[[no:Hippocampus]]
[[pl:Hipokamp]]
[[pt:Hipocampo]]
[[ru:Гиппокамп (часть мозга)]]
[[fi:Hippokampus]]
[[sv:Hippocampus]]
[[vi:Hippocampus]]
[[zh:海马体]]