The neocortex, also called the neopallium and isocortex, is a part of
the mammalian brain. In the human brain, it is the largest part of the cerebral
cortex which covers the two cerebral hemispheres, with the allocortex making
up the rest. The neocortex is made up of six layers, labelled from the outer in,
I to VI. In humans, the neocortex is involved in higher functions such as
sensory perception, generation of motor commands, spatial reasoning, conscious
thought and language. There are two types of cortex in the neocortex – the
true isocortex and the proisocortex. Anatomy
The neocortex is the most developed of the cerebral tissues. The neocortex
consists of the grey matter, or neuronal cell bodies and unmyelinated fibers,
surrounding the deeper white matter in the cerebrum. There are two types of
cortex in the neocortex, the proisocortex and the true isocortex. The
pro-isocortex is a transitional area between the true isocortex, and the
periallocortex. It is found in the cingulate cortex, in Brodmann's areas
24, 25, 30 and 32, the insula and the parahippocampal gyrus.
The neocortex is smooth in rodents and other small mammals, whereas in primates
and other larger mammals it has deep grooves and ridges. These folds allow
the surface area of the neocortex to increase far beyond what could otherwise
be fit in the same size skull. All human brains have the same overall pattern of
main gyri and sulci, although they differ in detail from one person to
another. The mechanism by which the gyri form during embryogenesis is not
entirely clear, and there are several competing hypotheses that explain
gyrification. Axonal tension, cortical buckling, or differences in cellular
proliferation rates in different areas of the cortex during embryonic
development may contribute to the formation of gyri.
The neocortex contains two primary types of neurons, excitatory pyramidal neurons
and inhibitory interneurons. The structure of the neocortex is relatively
uniform, consisting of six horizontal layers segregated principally by cell
type and neuronal connections. However, there are many exceptions to this
uniformity; for example, the motor cortex lacks layer IV. There is some
canonical circuitry within the cortex; for example, pyramidal neurons in the
upper layers II and III project their axons to other areas of neocortex, while
those in the deeper layers V and VI project primarily out of the cortex,
e.g. to the thalamus, brainstem, and spinal cord. Neurons in layer IV receive
all of the synaptic connections from outside the cortex, and themselves make
short-range, local connections to other cortical layers. Thus, layer IV receives
all incoming sensory information and distributes it to the other layers for
further processing. The neurons of the neocortex are also
arranged in vertical structures called cortical columns. These are patches of
the neocortex with a diameter of about 0.5 mm. Each column typically responds
to a sensory stimulus representing a certain body part or region of sound or
vision. These columns are similar, and can be thought of as the basic repeating
functional units of the neocortex. In humans, the neocortex consists of about
a half-million of these columns, each of which contains approximately 70,000
neurons. Function
The neocortex is derived embryonically from the dorsal telencephalon, which is
the rostral part of the forebrain. The neocortex is divided, into regions
demarcated by the cranial sutures in the skull above, into frontal, parietal,
occipital, and temporal lobes, which perform different functions. For
example, the occipital lobe contains the primary visual cortex, and the temporal
lobe contains the primary auditory cortex. Further subdivisions or areas of
neocortex are responsible for more specific cognitive processes. In humans,
the frontal lobe contains areas devoted to abilities that are enhanced in or
unique to our species, such as complex language processing localized to the
ventrolateral prefrontal cortex. In humans and other primates, social and
emotional processing is localized to the orbitofrontal cortex.
The neocortex has also been shown to play an influential role in sleep,
memory and learning processes. Semantic memories appear to be stored in the
neocortex, specifically the anterolateral temporal lobe of the
neocortex. It is also involved in instrumental conditioning; responsible
for transmitting sensory information and information about plans for movement to
the basal ganglia. The firing rate of neurons in the neocortex also has an
effect on slow-wave sleep. When the neurons are at rest and are
hyperpolarizing, a period of inhibition occurs during a slow oscillation, called
the down state. When the neurons of the neocortex are in the excitatory
depolarizing phase and are firing briefly at a high rate, a period of
excitation occurs during a slow oscillation, called the up state.
There is still much to learn about the roles the neocortex has in the
neurological processes exemplified in human behavior. To further understand
the vital role the neocortex plays in human cognition, IBM’s computational
model of the human brain was created that simulated the electrochemistry of
the neocortex. The super computer, the Blue Brain Project, was created to
improve understanding of the processes of perception, learning and memory and
gain further knowledge about mental health disorders.
Clinical Significance Lesions that develop in
neurodegenerative disorders, such as Alzheimer's disease, interrupt the
transfer of information from the sensory neocortex to the prefrontal neocortex.
This disruption of sensory information contributes to the progressive symptoms
seen in neurodegenerative disorders such as changes in personality, decline in
cognitive abilities, and dementia. Damage to the neocortex of the
anterolateral temporal lobe results in semantic dementia, which is the loss of
memory of factual information. These symptoms can also be replicated by
transcranial magnetic stimulation of this area. If damage is sustained to
this area, patients do not develop anterograde amnesia and are able to
recall episodic information. Evolution
The neocortex is the newest part of the cerebral cortex to evolve; the other
part of the cerebral cortex is the allocortex. The cellular organization of
the allocortex is different from the six-layered neocortex. In humans, 90% of
the cerebral cortex and 76% of the entire brain is neocortex.
For a species to develop a larger neocortex, the brain must too evolve in
size so that it’s large enough to support the region. Body size, basal
metabolic rate and life history are factors affecting brain evolution and
the coevolution of neocortex size and group size. The neocortex increased in
size in response to pressures for greater cooperation and competition in
early ancestors. With the size increase, there was greater voluntary inhibitory
control of social behaviors resulting in increased social harmony.
The six-layer cortex appears to be a distinguishing feature of mammals; it
has been found in the brains of all mammals, but not in any other animals.
There is some debate, however, as to the cross-species nomenclature for
neocortex. In avians, for instance, there are clear examples of cognitive
processes that are thought to be neocortical in nature, despite the lack
of the distinctive six-layer neocortical structure. In a similar manner,
reptiles, such as turtles, have primary sensory cortices. A consistent,
alternative name has yet to be agreed upon.
Neocortex ratio The neocortex ratio of a species is the
ratio of the size of the neocortex to the rest of the brain. A high neocortex
ratio is thought to correlate with a number of social variables such as group
size and the complexity of social mating behaviors. Humans have a large neocortex
as a percentage of total brain matter when compared with other mammals. For
example, there is only a 30:1 ratio of neocortical gray matter to the size of
the medulla in the brainstem of chimpanzees, while the ratio is 60:1 in
humans. See also
List of regions in the human brain Blue Brain, a project to produce a
computer simulation of a neocortical column and eventually a whole neocortex
Memory-prediction framework, a theory of the neocortex function by Jeff Hawkins
and related software models Model of the neocortex by the Brain
Engineering Laboratory at Dartmouth College
Comparative Neuroscience at Wikiversity Claustrum
External links Proisocortex, BrainInfo.
References