Neocortex

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

Anatomy
The neopallium consists of grey matter surrounding the deeper white matter of the cerebrum. While the neopallium 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 serve to increase the area of the neopallium considerably. In humans it accounts for about 80% of the brain's weight.

The structure of the neopallium 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 millions of these columns, and each column has up to 100,000 neurons per square millimeter.

The neocortex is divided into several lobes, and different regions of the neocortex have different functions. See cerebral cortex and cerebrum for the details.

Evolution
Evolutionarily, the neopallium is the newest part of the cerebral cortex (whence 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-layered structure mentioned above. In humans, 90% of the cerebral cortex is neopallium.

The six-layer neopallium 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.