Overview
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Much of the expansion comes from the part of the brain called the cerebral cortex, especially the frontal lobes, which are associated with executive functions such as self-control, planning, reasoning, and abstract thought. The human cerebral cortex is a thick layer of neural tissue that covers most of the brain. This layer is folded in a way that increases the amount of surface that can fit into the volume available. The pattern of folds is similar across individuals, although there are many small variations. The cortex is divided into four "lobes," called the frontal lobe, parietal lobe, temporal lobe, and occipital lobe. Within each lobe are numerous cortical areas, each associated with a particular function such as vision, motor control, language, etc. The left and right sides of the cortex are broadly similar in shape. In most people, the left hemisphere is "dominant" for language, with the right hemisphere playing only a minor role. There are other functions, such as spatiotemporal reasoning, for which the right hemisphere is usually dominant. Despite being protected by the thick bones of the skull, suspended in cerebrospinal fluid, and isolated from the bloodstream by the blood–brain barrier, the human brain is susceptible to many types of damage and disease. The most common forms of physical damage are closed head injuries, including blows to the head, strokes, or poisoning by a variety of chemicals that can act as neurotoxins. Infection of the brain, though serious, is rare due to the biological barriers, which protect it. The human brain is also susceptible to degenerative disorders, such as Parkinson's disease, multiple sclerosis, and Alzheimer's disease. A number of psychiatric conditions, such as schizophrenia and depression, are thought to be associated with brain disfunctions, although the nature of such brain anomalies are not well understood.