The gray “bark,” or surface layer of the cerebral hemispheres, containing the nerve cells involved in the higher mental processes. It is part of the telencephalon, or forebrain, and lies in folds near the inner surface of the skull.Structurally speaking, the cerebral cortex looks like a large shelled walnut, covered with ridges known as gyri, and crevices called sulci or fissures (Fig. 11). One of these crevices, the longitudinal fissure, divides the brain into two symmetrical halves—the cerebral hemispheres—which are mirror images of each other in both structure and function. A second major groove, the fissure of Rolando or central sulcus, runs across the top and down the sides of each hemisphere (Fig. 12). In front of this fissure lies the frontal lobe, often described as the expressive part of the brain, since it contains the motor senses which control action and movement. Behind it lie the three lobes which comprise the receptive areas of the brain because they contain most of the centers for incoming sensory impulses. The third major crevice, known as the lateral CEREBRAL CORTEX.Localization of functions in the four lobes of the left hemisphere.sulcus or fissure of Sylvius, serves as a dividing line between two of these lobes: the parietal lobe lies above this fissure, the temporal lobe lies below it, while the occipital lobe is situated at the rear of the brain.The cortex contains an estimated ten billion cell bodies from which extend a mass of fibers of enormous complexity. These fibers fall into three categories. The commissural fibers connect the cortex of one hemisphere to the cortex of the other; the association fibers are found within each cortex, connecting one part to another; and the projection fibers carry the impulses to and from the cortex. Fibers carrying impulses upward are called afferent or corticopetal fibers and originate for the most part in the thalamus; those carrying impulses downward are called efferent or corticofugal fibers, and these end in the lower parts of the brain (the basal ganglia, thalamus, midbrain, hindbrain) or the spinal cord.Many methods are used in exploring the “wiring diagram” of the cortex. Since the fibers can only be roughly traced by their natural white color, anatomists and physiologists—usually working with animals—resort to (1) staining them with various dyes; (2) cutting them and noting their degeneration and change of color; (3) studying the specific effects of injury or disease, such as tumor or gunshot wounds, in different regions; (4) extirpating specific fibers by surgery or electrolytic lesions and noting the effects on the behavior; (5) recording brain waves (EEG) from different areas during different activities; (6) inserting electrodes to determine which fibers or sets of fibers are active under different conditions; (7) stimulating different areas electrically or chemically in order to activate different parts of the body or different functions such as memory; and, most recently, (8) direct implantation of CEREBRAL CORTEX.A diagram of the upper half of the left hemisphere, opened up along the Fissure of Rolando to indicate the approximate location of sensory and motor functions.electrodes to study the deeper functions of the brain.Although the cerebral cortex is by no means fully “mapped” as yet, these techniques have succeeded in locating a great many functions. They are usually grouped into the following major areas:First, the motor area, immediately in front of the fissure of Rolando, and next to it the premotor area. These frontal lobe areas control muscular movements throughout the body, but the specific locations are in reverse order: the right hemisphere controls the left half of the body and vice versa, while fibers at the top of the area control the toes and legs, and fibers at the bottom control the tongue and mouth movements. The motor area makes these movements possible while the pre- motor area is believed to make them function smoothly.Second, the somesthetic area, which receives sensory experiences of movement, temperature, touch, and pain from different areas of the body—again in reverse order.Third, the visual area, located in the parts of each occipital lobe known as the striate areas. Reversal occurs here, too, but it is somewhat more complicated: the right striate area controls the right half of each retina, and the left controls the left half. This means that the right enables us to see the left half and left enables us to see the right half of the visual field. The fibers cross at a point below the cortex known as the optic chiasm. More specific correspondences between parts of the retina and parts of the visual areas have also been established.Fourth, the auditory area, located on the surface of the temporal lobes. The general organization of these areas is unlike that of the visual areas in one major respect: both ears are totally represented on both sides, so that the loss of one temporal lobe has little effect on hearing, while loss of one striate area causes blindness in half of each eye. Some specific localizations have been determined—for example, different regions are sensitive to high and low tones.Fifth, the speech area. Several regions appear to be involved in speech and language. This is not surprising because these functions include a wide variety of activities: speaking, writing, and understanding both oral and written language. There is considerable experimental evidence against the early theory of Broca (1861) that the control of the tongue, jaws, and vocal organs is centered in the left hemisphere in right-handed people and the right hemisphere in left-handed people (Humphrey and Zangwill, 1952). Studies of brain- injured people indicate that the sensory or receptive type of aphasia, in which the individual cannot recognize words or names, involves the temporal and perhaps the occipital lobes; while motor or expressive asphasia, in which the patient cannot speak or write even though he understands, is centered in the frontal lobes. Neurologists are beginning to localize these areas more precisely—for instance, there seems to be an area for speech in the lateral frontal lobe and one for writing and drawing farther up in the frontal lobe. Moreover, direct stimulation of an area in front of the fissure of Rolando produces long vowel-like sounds in conscious patients undergoing brain surgery, and counting is interrupted by the stimulation of certain areas on either side of this fissure (Penfield and Rasmussen, 1950).Sixth, the association areas. The five general regions just described are all designated primary projection areas since they “project” sensory or motor fibers to parts of the body outside the brain. Another group of areas, association areas or “intrinsic systems,” as Pribram calls them, (1960) contain a huge number of fibers that apparently integrate the different parts of the brain and the different aspects of behavior. These areas are practically nonexistent in the lower animals, such as the rabbit, and are far less extensive in apes than in men. They are believed to be responsible for the psychological abilities which are most distinctively human: learning, perception, memory, and thinking.There are association areas near the primary cortical areas for each of the senses. The visual association area (or prestriate) is in the occipital lobe, the parietal association area occupies most of the parietal lobe, and the auditory association area is found in the temporal lobe. Each of these is concerned with complex sensory discriminations such as those involved in distinguishing between a circle and an ellipse, noting the difference between musical chords, judging heat or cold, or getting the “feel” of a golf swing.Investigators have also begun to localize certain memory areas. In 1958, Penfield applied a small electrical current directly to the temporal lobe of a surgical patient. When one point was stimulated, the patient recalled a long- past experience, and when another point was stimulated, he heard a piece of music. Studies of apraxia cases have indicated that there are memory areas for sequential tasks, such as dressing one’s self or driving a car, in the frontal lobe. Other investigations indicate that lesions in the parietal lobe produce agnosia-—that is, the patient cannot remember what a fork or an automobile is used for. Finally, relatively large areas in the frontal lobe, called the prefrontal areas or frontal association areas, are concerned with more general intellectual processes. Destruction of these areas drastically reduces the individual’s ability to concentrate, solve problems, take responsibility, and plan ahead. When two investigators, Freeman and Watts, noted that these patients often became relaxed and unworried, they began performing prefrontal lobotomies on deeply disturbed mental patients. See PSYCHOSURGERY, AGNOSIA, APHASIA, APRAXIA, MEMORY STORAGE, LASHLEY, FRANZ.The cerebral cortex is the culmination of an evolutionary process that has spanned billions of years. This process began with the first nervous system, or “nerve net,” in primitive organisms like the jellyfish, and progressed through three general stages. The first was ganglionic organization, in which nerve cells gathered into clusters or ganglia which controlled different segments of the body. In the second stage, encephaliza- tion, a major ganglion developed at the head end of the organism. In the third and final stage, corticalization took place—that is, a single brain center gradually took over control of all the major mental and physical functions of the organism. In the early vertebrates, the entire nervous system was devoted to routine activities such as breathing, digestion, moving, and mating; but as evolution went on, control centers for all sensory and motor functions “moved upstairs” to the forebrain.Finally, the forebrain itself enlarged to a point where it could accommodate additional functions which are most characteristically human: thinking, learning, and language. This development can be graphically expressed by comparing the weight of the brain with the weight of the spinal cord in different species. The alligator’s forebrain, which is only rudimentary, weighs just about the same as its spinal cord. The more highly developed brain of the chimpanzee is fifteen times as heavy as its spinal cord, and man’s brain is fifty-five times the weight of his spinal cord.In general, then, the cerebral cortex is not only the major center for sensory discrimination and motor functions, but the “control tower” for the elaborate associative processes that distinguish man from animal. Without it we would be unable to solve scientific problems, create works of art, organize a complex society, or preserve and extend our knowledge of the world in which we live.