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The uniqueness of the human brain derives from the higher mental functions, such as reasoning, memory, language, speech, calculations, praxis, recognition of objects (gnosis), and the asymmetrical organization of these functions in the cerebral hemispheres. Cerebral dominance, or the specialization of one cerebral hemisphere in specific functions, relates to the unique cytoarchitectural organization of the brain.
The cerebral cortex is a large, complex, convoluted structure and is the site of higher mental functions. The cortex makes up an area of about 2.5 ft 2 and contains as many as 20 billion neurons, arranged in a wrinkled assembly of gyri that allows a vast surface area of cortex to be enfolded into the small space within the human skull. The information stored in the cerebral cortex rivals that of major libraries and computer systems. As many as 70% of neurons in the human central nervous system reside in the association cortex, the site of higher functions. The cortex is composed of vertical modules, usually containing six layers. The precise anatomy varies with the function of the area. For example, the motor cortex contains large pyramidal cells, whereas sensory cortex lacks pyramidal cells but has many more granular cells. The visual cortex has a visible white matter bundle called the line of Gennari, visible to the naked eye. Areas of the cortex were numbered by Brodmann, a system still in wide use today.
The motor cortex principally involves the precentral gyrus, the supplementary motor area in the medial frontal lobe, and the prefrontal cortex. The prefrontal area is involved in the planning and initiation of movement.
Sensory cortices include the postcentral gyrus for somatosensory function, the auditory cortex (Heschl gyrus) in the superior temporal lobes, the visual cortex in the occipital lobes, and olfactory (basal frontal) and probably gustatory cortices.
Beyond this basic anatomy, the brain is divided into four primary lobes: frontal, parietal, temporal, and occipital. Each lobe has a relatively constant cortical anatomy and is divided by sulci or fissures into discrete gyri.
Functional Localization
Clinical data obtained from imaging studies such as magnetic resonance imaging (MRI), positron emission tomography (PET), and from patients undergoing surgical ablation of brain structures for epilepsy or brain tumor have helped reveal the functional anatomy of the brain in terms of the four major lobes and their cortical gyri. Within these gyri are modules dedicated to specific functions. The modules are organized not as individual centers but as parts of interconnecting networks in disparate regions of the brain. The cortical modules also connect with subcortical centers, such as the basal ganglia and other deep brain structures.
In general, the cerebral cortex is divided into primary motor areas, primary sensory areas (for vision, hearing, touch, smell, and possibly taste), and association areas. The association cortex is divided into unimodal association cortices, such as the visual and auditory association cortices, and heteromodal association cortices. The heteromodal cortical areas include the parietal cortex, a sensory association area for interaction among the senses, and the frontal cortex, which supports the executive function for the entire brain. Executive function refers to the processes that determine which of the many incoming sensory stimuli should receive attention and in what priority and what responses should be activated and in which order. These heteromodal association cortices represent the areas of largest expansion of the brain from apes to humans, and they are critically important in enabling the extraordinary cognitive and behavioral capabilities that human beings have attained.
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