The cerebral cortex is the outermost layer of the brain in complex animals such as mammals, including humans. Despite a thickness of less than 0.2 inch (5 mm), it is responsible for most higher brain functions, including language, memory, and consciousness. The cerebral cortex is folded in on itself in complex grooves to aid and quicken brain activity. In brains preserved for study, the cortex is gray, inspiring the phrase “gray matter,” often used casually to refer to any part of the brain.
If the cerebral cortex were to be removed and unfolded, it would cover several yards or meters. The grooves that condense the cortex are called sulci, and the ridges formed by this process are called gyri. This structure allows more mental processing power in a smaller space and also allows neurons involved in similar brain functions to communicate information more quickly. A brain with more grooves and ridges is smarter; that is, it can retain more information and process it faster.
The cerebral cortex in humans has six layers, each dealing with different mental or physical functions. The parts of the brain dealing with sensory input have thinner cortical layers, while those dealing with motor functions are thicker. Each cortical area has a different brain function. Functions of the cortex include problem solving, emotional response, complex motor control, language, memory, speech, and processing of complex sensory data such as vision and sound.
These functions are not evenly distributed. The human brain is divided into two hemispheres, right and left, and each hemisphere has its own specialized functions. The right hemisphere, for example, processes complex visual data and focused attention. The left hemisphere processes language, and each hemisphere deals with different emotions. Left- or right-handedness indicates that the opposite brain hemisphere is “dominant” in that individual.
So-called gray matter is actually grayish-brown in living tissue, because of blood flow to the brain. The gray areas are nerve cells, or neurons, which occur throughout the brain and nervous system. It is their concentration in the cerebral cortex that gives the brain its familiar color.
Beneath the cortex is “white matter,” which, again, looks different depending on whether the brain is living or preserved. White matter is made of cellular structures, called axons, which relay information from the neurons to other parts of the brain and the body. Deeper regions of the brain control basic, involuntary functions such as heartbeat, respiration and digestion.