The neocortex (Latin for “new bark” or “new rind”), also called the neopallium (“new mantle”) and isocortex (“equal rind”), is a part of the mammalian brain. In humans 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.
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 (myelinated axons) 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 (part of the allocortex). It is found in the cingulate cortex (part of the limbic system), 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 (sulci) and ridges (gyri). 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 (~80% of neocortical neurons) and inhibitory interneurons (~20%). The structure of the neocortex is relatively uniform (hence the alternative names “iso-” and “homotypic” cortex), 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 (mostly from thalamus), 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 (and a depth of 2 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. The neocortex consists of about a half-million of these columns, each of which contains approximately 70,000 neurons.
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. 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 (Broca’s area). 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.
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 (semantic memories). 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.
The neocortex is the newest part of the cerebral cortex to evolve (prefix neo meaning new); the other part of the cerebral cortex is the allocortex. The cellular organization of the allocortex is different from the six-layered neocortex. 90% of the cerebral cortex 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.
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.