Neocortex

The neocortex (Latin for "new bark" or "new rind") is a part of the brain of mammals. It is the top layer of the cerebral hemispheres, 2-4 mm thick, 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). It is involved in higher functions such as sensory perception, generation of motor commands, spatial reasoning, conscious thought, and in humans, language. Other names for the neocortex include neopallium ("new mantle") and isocortex ("equal rind").

Anatomy
The neocortex consists of grey matter surrounding the deeper white matter of the cerebrum. While the neocortex is smooth in rats and some other small mammals, it has deep grooves (sulci) and wrinkles (gyri) in primates and several other mammals. These folds increase the surface area of the neocortex considerably. The female neocortex contains approximately 19 billion neurons while the male neocortex has 23 billion. It is unknown what effect, if any, results from this difference.

The structure of the cortex is relatively uniform (whence the names "iso" and "homotypical"): it consists of six horizontal layers segregated by cell type, neuronal input, or cell density. The neurons are arranged in 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 half million of these columns, each of which contains approximately 60,000 neurons.

The neocortex is divided into several lobes, and different regions of the neocortex have different functions. (See Cerebral cortex and Cerebrum.)

Evolution
With respect to evolution, the neocortex is the newest part of the cerebral cortex (hence the name "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 neopallium.

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.