The neocortex is a part of the cerebral cortex, which is the outermost layer of the brain. It is involved in higher-order brain functions such as sensory perception, cognition, generation of motor commands, spatial reasoning, and language. The neocortex is further subdivided into the true isocortex and the proisocortex. It is made up of six layers, labeled from the outermost inwards, I to VI. The term "neo" means new, and the neocortex is so named because its appearance is thought to be relatively new in vertebrate evolution. In humans, 90% of the cerebral cortex is the neocortex. The neocortex is often described as being arranged in vertical structures called cortical columns, patches of neocortex with a diameter of roughly 0.5 mm (and a depth of 2 mm, i.e., spanning all six layers). The cellular organization of the neocortex is different from the allocortex, which is the other part of the cerebral cortex. The allocortex is phylogenetically older and has a different cellular organization.