From Wikipedia, the free encyclopedia
In animals, the brain is the control center of the
central nervous system, responsible for
behavior. In mammals, the brain is located in the head,
protected by the
skull
and close to the primary sensory apparatus of
vision,
hearing,
equilibrioception (balance), sense of
taste,
and
olfaction (smell).
While all
vertebrates have a brain, most
invertebrates have either a centralized brain or collections of
individual
ganglia. Some animals such as
cnidarians and
echinoderms do not have a centralized brain, and instead have a
decentralized
nervous system, while animals such as
sponges
lack both a brain and nervous system entirely.
Brains can be extremely complex. For example, the
human brain contains roughly 100 billion
neurons,
linked with up to 10,000 connections each.
[edit]
History
-
Early views on the function of the brain regarded it as little
more than cranial stuffing. In
Ancient Egypt, from the late
Middle Kingdom onwards, in preparation for
mummification, the brain was regularly removed, for it was the
heart
that was assumed to be the seat of intelligence. According to
Herodotus, during the first step of mummification, "The most
perfect practice is to extract as much of the brain as possible with
an iron hook, and what the hook cannot reach is mixed with drugs."
Over the next five-thousand years, this view came to be reversed;
the brain is now known to be seat of intelligence, although
idiomatic
variations of the former remain, as in "memorizing something by
heart".[1]
The first thoughts on the field of
psychology came from
ancient philosophers, such as
Aristotle. As thinkers became more in tune with
biomedical research over time, as was the case with
medieval psychologists such as
Alhazen and
Avicenna for example, the concepts of
experimental psychology and
clinical psychology began emerging. From that point, different
branches of psychology emerged with different individuals creating
new ideas, with modern
psychologists such as
Freud and
Jung contributing to the field.
[edit]
Mind and brain
The
mind-body problem is one of the central problems in the history
of
philosophy. The brain is the physical and biological
matter
contained within the
skull,
responsible for electrochemical neuronal processes. The mind,
in contrast, consists in mental attributes, such as
beliefs,
desires,
perceptions, and so on. There are scientifically demonstrable
correlations between mental events and neuronal events; the
philosophical question is whether these phenomena are identical, at
least partially distinct, or related in some unknown way.
Philosophical positions on the mind-body problem fall into two
main categories. The first category is
dualism, according to which the mind exists independently of
the brain. Dualist theories are further divided into
substance dualism and
property dualism.
René Descartes is perhaps the most prominent substance dualist,
while property dualism is more popular among contemporary dualists
like
David Chalmers. Dualism requires admitting non-physical
substances or properties into
ontology, which is in apparent conflict with the
scientific world view. The second category is
materialism, according to which mental phenomena are
identical to neuronal phenomena. A third category of view,
idealism, claims that only mental substances and phenomena
exist. This view, most prominently held by 18th century Irish
philosopher
Bishop George Berkeley, has few contemporary adherents.
[edit]
Comparative anatomy
Three groups of animals have notably complex brains: the
arthropods (insects,
crustaceans,
arachnids, and others), the
cephalopods (octopuses,
squids,
and similar
mollusks), and the
craniates (vertebrates
and
hagfish).[2]
The brain of arthropods and cephalopods arises from twin parallel
nerve cords that extend through the body of the animal. Arthropods
have a central brain with three divisions and large optical lobes
behind each
eye for visual processing.[2]
The brain of craniates develops from the
anterior section of a single dorsal
nerve cord, which later becomes the
spinal cord.[3]
In craniates, the brain is protected by the
bones
of the
skull.
Mammals have a six-layered
neocortex (or homotypic cortex, neopallium), in addition to
having some parts of the brain that are allocortex.[3]
In mammals, increasing convolutions of the brain are characteristic
of animals with more advanced brains. These convolutions provide a
larger surface area for a greater number of neurons while keeping
the volume of the brain compact enough to fit inside the skull. The
folding allows more grey matter to fit into a smaller volume. The
folds are called
sulci, while the spaces between the folds are called
gyri.
In birds,
the part of the brain that functionally corresponds to the neocortex
is called
nidopallium and derives from a different part of the brain. Some
birds (like
corvids and
parrots),
are thought by some to have high intelligence, but even in these,
the brain region that forms the mammalian neocortex is in fact
almost entirely absent.
All vertebrates have a similar general
histology of the brain, but may have differing structural
anatomy. Apart from the gross
embryological divisions of the brain, the location of specific
gyri and sulci, primary sensory regions, and other structures
differs between species.
[edit]
Insects
In insects, the brain has four parts, the
optic lobes, the
protocerebrum, the
deutocerebrum, and the
tritocerebrum. The optic lobes are behind each eye and process
visual stimuli.[2]
The protocerebrum contains the
mushroom bodies, which respond to
smell, and the central body complex. In some
species such as
bees, the
mushroom body receives input from the visual pathway as well. The
deutocerebrum includes the
antennal lobes, which are similar to the mammalian
olfactory bulb, and the mechanosensory
neuropils which receive information from
touch receptors on the head and
antennae. The antennal lobes of
flies and
moths
are quite complex.
[edit]
Cephalopods
In cephalopods, the brain has two regions: the supraesophageal
mass and the subesophageal mass,[2]
separated by the
esophagus. The supra- and subesophageal masses are connected to
each other on either side of the esophagus by the basal lobes and
the dorsal magnocellular lobes.[2]
The large optic lobes are sometimes not considered to be part of the
brain, as they are anatomically separate and are joined to the brain
by the optic stalks. However, the optic lobes perform much visual
processing, and so functionally are part of the brain.
[edit]
Mammals and other vertebrates
The
telencephalon (cerebrum) is the largest region of the mammalian
brain. This is the structure that is most easily visible in brain
specimens, and is what most people associate with the "brain". In
humans and several other animals, the fissures (sulci) and
convolutions (gyri) give the brain a wrinkled appearance. In
non-mammalian vertebrates with no cerebrum, the
metencephalon is the highest center in the brain. Because humans
walk upright, there is a flexure, or bend, in the brain between the
brain stem and the cerebrum. Other vertebrates do not have this
flexure. Generally, comparing the locations of certain brain
structures between humans and other vertebrates often reveals a
number of differences.
Behind (or in humans, below) the cerebrum is the cerebellum. The
cerebellum is known to be involved in the control of movement,[3]
and is connected by thick white matter fibers (cerebellar peduncles)
to the pons.[4]
The cerebrum has two
cerebral hemispheres. The
cerebellum also has hemispheres. The telencephalic hemispheres
are connected by the
corpus callosum, another large white matter tract. An outgrowth
of the telencephalon called the
olfactory bulb is a major structure in many animals, but in
humans and other primates it is relatively small.
Vertebrate nervous systems are distinguished by
bilaterally symmetrical
encephalization. Encephalization refers to the tendency for more
complex organisms to gain larger brains through evolutionary time.
Larger vertebrates develop a complex, layered and interconnected
neuronal circuitry. In modern species most closely related to the
first vertebrates, brains are covered with gray matter that has a
three-layer structure (allocortex). Their brains also contain deep
brain nuclei and fiber tracts forming the white matter. Most regions
of the human cerebral cortex have six layers of neurons (neocortex).[4]
[edit]
Vertebrate brain regions
(See related article at
List of regions in the human brain)
Diagram depicting the main subdivisions of the
embryonic vertebrate brain. These regions will later
differentiate into forebrain, midbrain and hindbrain
structures.
According to the hierarchy based on embryonic and evolutionary
development,
chordate brains are composed of the three regions that later
develop into five total divisions:
The brain can also be classified according to function, including
divisions such as:
In recent years it was realized that certain
birds
have developed high intelligence entirely
convergently from
mammals such as humans. Hence, the functional areas of the avian
brain have been redefined by the
Avian Brain Nomenclature Consortium. See also
Bird intelligence.
[edit]
Humans
Human brain with color coded lobes
-
Main article:
Human brain
The structure of the
human
brain differs from that of other animals in several important ways.
These differences allow for many abilities over and above those of
other animals, such as advanced cognitive skills. Human
encephalization is especially pronounced in the
neocortex, the most complex part of the
cerebral cortex. The proportion of the human brain that is
devoted to the neocortex—especially to the
prefrontal cortex—is larger than in all other
mammals
(indeed larger than in all animals, although only in mammals has the
neocortex evolved to fulfill this kind of function).
Humans have unique neural capacities, but much of their brain
structure is similar to that of other mammals. Basic systems that
alert the nervous system to stimulus, that sense events in the
environment, and monitor the condition of the body are similar to
those of even non-mammalian vertebrates. The neural circuitry
underlying human consciousness includes both the advanced neocortex
and prototypical structures of the
brain stem. The human brain also has a massive number of
synaptic connections allowing for a great deal of
parallel processing.
[edit]
Neurobiology
The brain is composed of two broad classes of cells,
neurons
and
glia, both of which contain several different cell types which
perform different functions. Interconnected neurons form
neural networks (or
neural ensembles). These networks are similar to man-made
electrical circuits in that they contain circuit elements
(neurons) connected by biological wires (nerve fibers). These do not
form simple one-to-one electrical circuits like many man-made
circuits, however. Typically neurons connect to at least a thousand
other neurons.[5]
These highly specialized circuits make up systems which are the
basis of
perception, different types of action, and higher cognitive
function.
[edit]
Structure
Structure of a typical neuron
|
Neuron |
|
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Neurons are the cells that convey information to other cells;
these constitute the essential class of brain cells.
In addition to neurons, the brain contains
glial cells in a roughly 10:1 proportion to neurons. Glial cells
("glia" is Greek for “glue”) form a support system for neurons. They
create the insulating myelin, provide structure to the neuronal
network, manage waste, and clean up neurotransmitters. Most types of
glia in the brain are present in the entire
nervous system. Exceptions include the
oligodendrocytes which myelinate neural
axons
(a role performed by
Schwann cells in the peripheral nervous system). The myelin in
the oligodendrocytes insulates the axons of some neurons.
White matter in the brain is myelinated neurons, while
gray matter contains mostly cell
soma,
dendrites, and unmyelinated portions of axons and glia. The
space between neurons is filled with dendrites as well as
unmyelinated segments of axons; this area is referred to as the
neuropil.
In mammals, the brain is surrounded by
connective tissues called the
meninges, a system of
membranes that separate the skull from the brain. This
three-layered covering is composed of (from the outside in) the
dura mater,
arachnoid mater, and
pia mater. The arachnoid and pia are physically connected and
thus often considered as a single layer, the pia-arachnoid. Below
the arachnoid is the subarachnoid space which contains
cerebrospinal fluid, a substance that protects the nervous
system.
Blood vessels enter the central nervous system through the
perivascular space above the pia mater. The cells in the blood
vessel walls are joined tightly, forming the
blood-brain barrier which protects the brain from
toxins
that might enter through the blood.
The brain is bathed in
cerebrospinal fluid (CSF), which circulates between layers of
the meninges and through cavities in the brain called
ventricles. It is important both chemically for
metabolism and mechanically for shock-prevention. For example,
the human brain weighs about 1-1.5 kg or about 2-3
lb. The
mass and
density of the brain are such that it will begin to collapse
under its own weight if unsupported by the CSF. The CSF allows the
brain to float, easing the physical
stress caused by the brain’s mass.
[edit]
Function
Vertebrate brains receive signals through nerves arriving from
the sensors of the organism. These signals are then processed
throughout the central nervous system; reactions are formulated
based upon reflex and learned experiences. A similarly extensive
nerve network delivers signals from a brain to control important
muscles throughout the body. Anatomically, the majority of afferent
and efferent nerves (with the exception of the
cranial nerves) are connected to the spinal cord, which then
transfers the signals to and from the brain.
Sensory input is processed by the brain to recognize danger, find
food, identify potential mates, and perform more sophisticated
functions.
Visual, touch, and
auditory sensory pathways of vertebrates are routed to specific
nuclei of the
thalamus and then to regions of the cerebral cortex that are
specific to each
sensory system, the
visual system, the
auditory system, and the
somatosensory system. Olfactory pathways are routed to the
olfactory bulb, then to various parts of the
olfactory system.
Taste
is routed through the brainstem and then to other portions of the
gustatory system.
To control movement the brain has several parallel systems of
muscle control. The motor system controls voluntary muscle movement,
aided by the
motor cortex,
cerebellum, and the
basal ganglia. The system eventually projects to the spinal cord
and then out to the muscle effectors. Nuclei in the brain stem
control many involuntary muscle functions such as heart rate and
breathing. In addition, many automatic acts (simple reflexes,
locomotion) can be controlled by the spinal cord alone.
Brains also produce a portion of the body's
hormones that can influence organs and glands elsewhere in a
body—conversely, brains also react to hormones produced elsewhere in
the body. In mammals, the hormones that regulate hormone production
throughout the body are produced in the brain by the structure
called the
pituitary gland.
Evidence strongly suggests that developed brains derive
consciousness from the complex interactions between the numerous
systems within the brain. Cognitive processing in mammals occurs in
the cerebral cortex but relies on midbrain and
limbic functions as well. Among "younger" (in an evolutionary
sense) vertebrates, advanced processing involves progressively
rostral (forward) regions of the brain.
Hormones, incoming sensory information, and cognitive processing
performed by the brain determine the brain state. Stimulus from any
source can trigger a general arousal process that focuses cortical
operations to processing of the new information. This focusing of
cognition is known as
attention. Cognitive priorities are constantly shifted by a
variety of factors such as hunger, fatigue, belief, unfamiliar
information, or threat. The simplest dichotomy related to the
processing of threats is the
fight-or-flight response mediated by the
amygdala and other limbic structures.
[edit]
Neurotransmitter systems
-
Neurons expressing certain types of neurotransmitters sometimes
form distinct systems, where activation of the system causes effects
in large volumes of the brain, called volume transmission.
The major neurotransmitter systems are the
noradrenaline (norepinephrine) system, the
dopamine system, the
serotonin system and the
cholinergic system.
Drugs targeting the neurotransmitter of such systems affects the
whole system, which explains the mode of action of many drugs;
Diseases may affect specific neurotransmitter systems. For
example,
Parkinson's disease is at least in part related to failure of
dopaminergic cells in
deep-brain nuclei, for example the
substantia nigra. Treatments potentiating the effect of dopamine
precursors have been proposed and effected, with moderate success.
A brief comparison of the major neurotransmitter systems follows:
[edit]
Origin
Since even unicellular organisms can have, at least,
photosensitive
eyespots and react to tactile stimuli, it is hypothesized that
sensory organs developed before the brain did.[7]
The brain is an information-processing organ and its evolution is
dependent on the presence of information accessed into sensory
organs, sensory input, and the need to process this information and
transmit it.
[edit]
Pathology
Clinically,
death
is defined as an absence of brain activity as measured by
EEG. Injuries to the brain tend to affect large areas of the
organ, sometimes causing major deficits in intelligence, memory, and
movement. Head trauma caused, for example, by vehicle or industrial
accidents, is a leading cause of death in youth and middle age. In
many cases, more damage is caused by resultant
edema
than by the impact itself.
Stroke,
caused by the blockage or rupturing of blood vessels in the brain,
is another major cause of death from brain damage.
Other problems in the brain can be more accurately classified as
diseases rather than injuries.
Neurodegenerative diseases, such as
Alzheimer's disease,
Parkinson's disease,
motor neurone disease, and
Huntington's disease are caused by the gradual death of
individual neurons, leading to decrements in movement control,
memory, and cognition. Currently only the symptoms of these diseases
can be treated.
Mental illnesses, such as
clinical depression,
schizophrenia,
bipolar disorder, and
post-traumatic stress disorder are brain disorders that impact
personality and, typically, other aspects of mental and somatic
function. These disorders may be treated by
psychiatric therapy,
pharmaceutical intervention, or through a combination of
treatments; therapeutic effectiveness varies significantly among
individuals.
Some infectious diseases affecting the brain are caused by
viruses
and
bacteria. Infection of the
meninges, the membrane that covers the brain, can lead to
meningitis.
Bovine spongiform encephalopathy (also known as mad cow
disease), is deadly in
cattle
and humans and is linked to
prions.
Kuru is a similar prion-borne degenerative brain disease
affecting humans. Both are linked to the ingestion of neural tissue,
and may explain the tendency in some species to avoid
cannibalism. Viral or bacterial causes have been reported in
multiple sclerosis and
Parkinson's disease, and are established causes of
encephalopathy, and
encephalomyelitis.
Many brain disorders are
congenital.
Tay-Sachs disease,
Fragile X syndrome, and
Down syndrome are all linked to
genetic
and
chromosomal errors. Many other syndromes, such as the intrinsic
circadian rhythm disorders, are suspected to be congenital as
well. Malfunctions in the embryonic
development of the brain can be caused by genetic factors,
drug use,
nutritional deficiencies, and
infectious diseases during
pregnancy.
Certain brain disorders are treated by brain
neurosurgeons while others are treated by neurologists and
psychiatrists.
[edit]
Study of the brain
[edit]
Fields of study
Neuroscience seeks to understand the nervous system, including
the brain, from a biological and
computational perspective.
Psychology seeks to understand behavior and the brain.
Neurology refers to the
medical applications of neuroscience. The brain is also one of
the most important organs studied in
psychiatry, the branch of medicine which exists to study,
prevent, and treat
mental disorders.[8][9][10]
Cognitive science seeks to unify neuroscience and psychology
with other fields that concern themselves with the brain, such as
computer science (artificial
intelligence and similar fields) and
philosophy.
[edit]
Methods of observation
-
Main article:
neuroimaging
Each method for observing activity in the brain has its
advantages and drawbacks.
[edit]
Electrophysiology
Electrophysiology allows scientists to record the electrical
activity of individual neurons or groups of neurons.
By placing electrodes on the scalp one can record the summed
electrical activity of the cortex in a technique known as
electroencephalography (EEG). EEG measures the mass changes in
electrical current from the cerebral cortex, but can only detect
changes over large areas of the brain with very little sub-cortical
activity.
Apart from measuring the electric field around the skull it is
possible to measure the magnetic field directly in a technique known
as
magnetoencephalography (MEG). This technique has the same
temporal resolution as EEG but much better spatial resolution,
although admittedly not as good as fMRI. The main advantage over
fMRI is a direct relationship between neural activation and
measurement.
[edit]
fMRI and PET
A scan of the brain using fMRI
Functional magnetic resonance imaging (fMRI) measures changes in
blood flow in the brain, but the activity of neurons is not
directly measured, nor can it be distinguished whether this activity
is inhibitory or excitatory. fMRI is a noninvasive, indirect method
for measuring neural activity that is based on BOLD; Blood
Oxygen Level Dependent changes. The changes in
blood flow that occur in capillary beds in specific regions of the
brain are thought to represent various neuronal activities (metabolism
of synaptic reuptake). Similarly, a
positron emission tomography (PET), is able to monitor
glucose and
oxygen
metabolism as well as neurotransmitter activity in different areas
within the brain which can be correlated to the level of activity in
that region.
[edit]
Behavioral
Behavioral tests can measure symptoms of disease and mental
performance, but can only provide indirect measurements of brain
function and may not be practical in all animals. In humans however,
a neurological exam can be done to determine the location of any
trauma,
lesion, or
tumor
within the brain, brain stem, or spinal cord.
[edit]
Anatomical
Autopsy analysis of the brain allows for the study of anatomy
and
protein expression patterns, but is only possible after the
human or animal is dead.
Magnetic resonance imaging (MRI) can be used to study the
anatomy of a living creature and is widely used in both research and
medicine.
[edit]
Other studies
Computer scientists have produced simulated "artificial
neural networks" loosely based on the structure of neuron
connections in the brain. Some
artificial intelligence research seeks to replicate brain
function—although not necessarily brain mechanisms—but as yet has
been met with limited success.
Creating
algorithms to mimic a biological brain is very difficult because
the brain is not a static arrangement of circuits, but a network of
vastly interconnected neurons that are constantly changing their
connectivity and sensitivity. More recent work in both neuroscience
and artificial intelligence models the brain using the
mathematical tools of
chaos theory and
dynamical systems. Current research has also focused on
recreating the neural structure of the brain with the aim of
producing human-like cognition and artificial intelligence.
[edit]
Brain energy consumption
PET Image of the human brain showing energy
consumption
Although the brain represents only 2% of the body weight, it
receives 15% of the cardiac output, 20% of total body oxygen
consumption, and 25% of total body glucose utilization. The energy
consumption for the brain to simply survive is 0.1 Calories per
minute, while this value can be as high as 1.5 Calories per minute
(100W) during crossword puzzle-solving.[11]
The demands of the brain limit its size in many species.
Molossid bats and the
Vespertilionid
Nyctalus spp. have brains that have been reduced from the
ancestral form to invest in wing-size for the sake of
maneuverability. This contrasts with
fruit bats, which require more advanced neural structures and do
not pursue their prey.[12]
The brain most utilizes glucose for energy, but certain areas can
use fatty acids. Although supply of glucose to the brain is
generally plentiful, as the brain focuses on a specific task, it
uses up the glucose in that particular area and makes the task
harder to do. Studies have shown that glucose stores are available
to a particular area of the brain for approximately 20 minutes.
Deprivation of glucose to the brain, as can happen in hypoglycemia,
can result in loss of consciousness. About half of the brain's
energy is used up in cell-to-cell signalling, which represents about
10% of the body's entire energy supply.
[edit]
As food
Like most other internal organs, the brain can serve as
nourishment. For example, in the
Southern United States canned
pork
brain in
gravy can be purchased for consumption as food. This form of
brain is often fried with
scrambled eggs to produce the famous "Eggs
n' Brains".[13]
The brain of animals also features in
French cuisine such as in the dish tête de veau, or
head of calf. Although it sometimes consists only of the outer
meat of the skull and
jaw, the
full meal includes the brain,
tongue,
and
glands.
A serving of Norwegian smalahove
Similar delicacies from around the world include
Mexican
tacos
de sesos made with cattle brain as well as
squirrel brain in the US South.[14]
The Anyang tribe of
Cameroon practiced a tradition in which a new
tribal chief would consume the brain of a hunted
gorilla while another senior member of the
tribe
would eat the heart.[15]
Indonesian cuisine specialty in
Minangkabau cuisine also served beef brain in a gravy coconut
milk named
gulai otak (beef brain curry). Roasted or fried goat brain is
eaten in south India and some parts of north India. Norwegian
cuisine includes
smalahove where a singed lamb's head, including the brain,
tongue and eye, serves two people.
Consuming the brain and other nerve tissue of animals is not
without risks. The first problem is that the makeup of the brain is
60% fat due to large quantities of
myelin
(which itself is 70% fat) insulating the axons of neurons.[16]
As an example, a 140 g can of "pork brains in milk gravy", a single
serving, contains 3500 milligrams of
cholesterol, 1170% of our recommended daily intake.[17]
Brain consumption can result in contracting fatal
transmissible spongiform encephalopathies such as Variant
Creutzfeldt-Jakob disease and other
prion
diseases in humans and
mad cow disease in cattle.[18]
Another prion disease called
kuru has been traced to a funerary ritual among the
Fore people of
Papua New Guinea in which those close to the dead would eat the
brain of the deceased to create a sense of
immortality.[19]
Some
archaeological evidence suggests that the mourning rituals of
European
Neanderthals also involved the consumption of the brain.[20]
Because of the risk of being infected by prions one should always
wear gloves when handling brains.
It is also well-known in the hunting community that the brain of
wild animals should not be consumed, due to the risk of
chronic wasting disease. The brain is still useful to hunters,
in that most animals have enough brain matter for use in the
tanning of their own
External links
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[show]
Brain:
telencephalon (cerebrum,
cerebral cortex,
cerebral hemispheres) |
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Frontal lobe |
Precentral gyrus ( Primary
motor cortex,
4)
Superior frontal gyrus/Frontal
eye fields (6,
8,
9),
Middle frontal gyrus (46),
Inferior frontal gyrus/Broca's
area (44-Pars
opercularis,
45-Pars
triangularis)
Orbitofrontal cortex (10,
11,
12,
47)
Prefrontal cortex,
Premotor cortex
Precentral sulcus -
Superior frontal sulcus -
Inferior frontal sulcus -
Olfactory sulcus
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Parietal lobe |
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Occipital lobe |
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Temporal lobe |
Primary auditory cortex ( 41,
42),
Superior temporal gyrus ( 38,
22/ Wernicke's
area),
Middle temporal gyrus ( 21),
Inferior temporal gyrus ( 20)
Fusiform gyrus ( 37)
Medial temporal lobe ( Amygdala,
Parahippocampal gyrus ( 27,
28,
34,
35,
36)
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Cingulate cortex/gyrus |
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Interlobar sulci/fissures |
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White matter tracts |
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Other |
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Some categorizations are
approximations, and some
Brodmann areas span gyri. |
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