Neocortex

The neocortex (Latin for "new bark" or "new rind"), also called the neopallium ("new mantel") and isocortex ("equal rind"), is a part of the brain of mammals. It is the outer layer of the cerebral hemispheres, and made up of six layers, labelled I to VI (with VI being the innermost and I being the outermost). The neocortex is part of the cerebral cortex (along with the archicortex and paleocortex, which are cortical parts of the limbic system). In humans, it is involved in higher functions such as sensory perception, generation of motor commands, spatial reasoning, conscious thought and language.

Anatomy
The neocortex consists of the grey matter, or neuronal cell bodies and unmyelinated fibers, surrounding the deeper white matter (myelinated axons) in the cerebrum. The neocortex is smooth in rodents and other small mammals, whereas in primates and other larger mammals it has deep grooves (sulci) and wrinkles (gyri). These folds increase the surface area of the neocortex considerably without taking up too much more volume. This has allowed primates and especially humans to evolve new functional areas of neocortex that are responsible for enhanced cognitive skills such as working memory, speech, and language.

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): it consists 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 neocortical 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. In humans, the neocortex consists of about a half-million of these columns, each of which contains approximately 60,000 neurons. However, because of inhomogeneity in columnar borders outside the visual cortex, further studies are needed before accepting the columnar cortex theory as fact.

The neocortex is divided 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 (Broca's area). In humans and other primates, social and emotional processing is localized to the orbitofrontal cortex. (See Cerebral cortex and Cerebrum.)

Evolution
The neocortex is the newest part of the cerebral cortex to evolve (hence the prefix "neo"); the other parts of the cerebral cortex are the paleocortex and archicortex, collectively known as the allocortex. The cellular organization of the allocortex is different from the six-layer structure mentioned above. In humans, 90% of the cerebral cortex is neocortex.

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. (See Dunbar's number)