The human brain is the center of the human nervous system and is a highly complex organ. It has the same general structure as the brains of other mammals, but is over five times as large as the "average brain" of a mammal with the same body size. Most of the expansion comes from the cerebral cortex, a convoluted layer of neural tissue that covers the surface of the forebrain. Especially expanded are the frontal lobes, which are involved in executive functions such as self-control, planning, reasoning, and abstract thought. The portion of the brain devoted to vision is also greatly enlarged in humans.