Organization of the brain

The brain is organized into a number of distinct areas, which serve different types of functions, with the cerebral cor- tex playing the major role in higher cog- nitive functions.

The central nervous system consists of the brain and the spinal cord. The major function of the spinal cord is to carry neural messages from the brain to the muscles, and sensory messages from the body to the brain.The lower parts of the brain are evolutionarily more primitive. The higher portions are well developed only in the higher species.
Correspondingly, it appears that the lower portions of the brain are respon- sible for more basic functions. The medulla controls breathing, swallowing, digestion, and heartbeat. The hypothalamus regulates the expression of basic drives. The cerebellum plays an important role in motor coordination and vol- untary movement. The thalamus serves as a relay station for motor and sensory information from lower areas to the cortex. Although the cerebellum and thala- mus serve these basic functions, they also have evolved to play an important role in higher human cognition,
The cerebral cortex, or neocortex, is the most recently evolved portion of the brain. Although it is quite small and primitive in many mammals, it accounts for a large fraction of the human brain. In the human, the cerebral cortex can be thought of as a rather thin neural sheet with a surface area of about 2,500 cm2. To fit this neural sheet into the skull, it has to be highly convoluted. The large amount of folding and wrinkling of the cortex is one of the striking physical dif- ferences between the human brain and the brains of lower mammals. A bulge of the cortex is called a gyrus, and a crease passing between gyri is called a sulcus.
The neocortex is divided into left and right hemispheres. One of the in- teresting curiosities of anatomy is that the right part of the body tends to be connected to the left hemisphere and the left part of the body to the right hemi- sphere. Thus, the left hemisphere controls motor function and sensation in the right hand. The right ear is most strongly connected to the left hemisphere. The neural receptors in either eye that receive input from the left part of the visual world are connected to the right hemisphere.
Brodmann (1909/1960) identified 52 distinct regions of the human cortex based on differences in the cell types in various regions. Many of these regions proved to have functional differences as well. The corti- cal regions are typically organized into four lobes: frontal, parietal, occipital, and temporal.
Major folds, or sulci, on the cortex separate the areas. The occipital lobe con- tains the primary visual areas. The parietal lobe handles some perceptual functions, including spa- tial processing and representation of the body. It is also involved in control of attention. The temporal lobe receives input from the occipital area and is involved in ob- ject recognition. It also has the primary auditory areas and Wernicke’s area, which is involved in language processing. The frontal lobe has two major functions: The back portion of the frontal lobe is involved primarily with motor functions. The front portion, called the prefrontal cor- tex, is thought to control higher level processes, such as planning. The frontal portion of the brain is disproportionately larger in primates than in most mammals and, among primates, humans are distinguished by having disproportionately larger anterior por- tions of the prefrontal cortex (Area 10 in Color Plate 1.1—Semendeferi, Armstrong, Schleicher, Zilles, & Van Hoesen, 2001).
The neocortex is not the only region that plays a significant role in higher level cognition. There are many important circuits that go from the cortex to subcortical structures and back again. A particularly significant area for memory proves to be the limbic system, which is at the border between the cortex and the lower struc- tures. The limbic system contains a structure called the hippocampus (located inside the temporal lobes), which appears to be critical to human memory. It is not possible to show the hippocampus in a cross section like because it is a structure that occurs in the right and left halves of the brain between the surface and the center.  Dam- age to the hippocampus and to other nearby structures produces severe amnesia.
Another important collection of subcorti- cal structures is the basal ganglia. The critical connections of the basal ganglia The basal ganglia are involved both in basic motor control and in the control of complex cognition. These structures receive pro- jections from almost all areas of the cortex and have projections to the frontal cortex. Disorders such as Parkinson’s disease and Huntington’s disease result from damage to the basal ganglia. Although people suffering from these diseases have dramatic motor control deficits character- ized by tremors and rigidity, they also have diffi- culties in cognitive tasks. The cerebellum, which has a major role in motor control, also seems to play a role in higher order cognition. Many cog- nitive deficits have been observed in patients with damage to the cerebellum.
Localization of functions
The left and right hemispheres of the cerebral cortex appear to be somewhat specialized for different types of processing. In general, the left hemisphere seems to be associated with linguistic and analytic processing, whereas the right hemisphere is associated with per- ceptual and spatial processing. The left and right hemispheres are connected by a broad band of fibers called the corpus callosum. The corpus callosum has been surgically severed in some patients to prevent epileptic seizures. Such patients are referred to as split- brain patients. The operation is typically suc- cessful, and patients seem to function fairly well. Much of the evidence for the differences between the hemispheres comes from re- search with these patients. In one experiment, the word key was flashed on the left side of a screen the patient was viewing. Because it was on the left side of the screen, it would be received by the right, nonlanguage hemisphere. When asked what was presented on the screen, the patient was not able to say because the language-dominant hemisphere did not know. However, his left hand (but not the right) was able to pick out a key from a set of objects hidden from view.

Studies of split-brain patients have enabled psychologists to identify the separate functions of the right and left hemispheres. The research has shown a linguistic advantage for the left hemisphere. For instance, commands might be presented to these patients in the right ear (and hence to the left hemisphere) or in the left ear (and hence to the right hemisphere). The right hemisphere can comprehend only the simplest linguistic commands, whereas the left hemi- sphere displays full comprehension. A different result is obtained when the ability of the right hand (hence the left hemisphere) to perform manual tasks is compared with that of the left hand (hence the right hemisphere). In this situa- tion, the right hemisphere clearly outperforms the left hemisphere.
Research with other patients who have had damage to specific brain re- gions indicates that there are areas in the left cortex, called Broca’s area and Wernicke’s area that seem critical for speech, because dam- age to them results in aphasia, the severe impairment of speech. These may not be the only neural areas involved in speech, but they certainly are impor- tant. Different language deficits appear depending on whether the damage is to Broca’s area or Wernicke’s area. People with Broca’s aphasia (i.e., damage to Broca’s area) speak in short, ungrammatical sentences. For instance, when one patient was asked whether he drives home on weekends, he replied:
Why, yes . . . Thursday, er, er, er, no, er, Friday . . . Bar-ba-ra . . . wife . . . and, oh, car . . . drive . . . purnpike . . . you know . . . rest and . . . teevee. (Gardner, 1975, p. 61)
In contrast, patients with Wernicke’s aphasia speak in fairly grammatical sen- tences that are almost devoid of meaning. Such patients have difficulty with their vocabulary and generate “empty” speech. The following is the answer given by one such patient to the question “What brings you to the hospital?”
Boy, I’m sweating, I’m awful nervous, you know, once in a while I get caught up, I can’t mention the tarripoi, a month ago, quite a little, I’ve done a lot well. I impose a lot, while, on the other hand, you know what I mean, I have to run around, look it over, trebbin and all that sort of stuff. (Gardner, 1975, p. 68)
■ Different specific areas of the brain support different cognitive functions.

Topographic Organization
In many areas of the cortex, information processing is structured spatially in what is called a topographic organization. For instance, in the visual area at the back of the cortex, adjacent areas represent information from adjacent areas of the visual field.
  The body is distorted, with certain areas receiving a considerable overrepresentation. It turns out that the overrepresented areas correspond to those that are more sensitive. Thus, for instance, we can make more subtle discriminations among tactile stimuli on the hands and face than we can on the back or thigh. Also, there is an overrepresentation in the visual cortex of the visual field at the center of our vision, where we have the greatest visual acuity.
It is thought that topographic maps exist so that neurons processing similar regions can interact with one another (Crick & Asanuma, 1986). Although there are fiber tracks that connect different regions of the brain, the majority of the connections among neurons are to nearby neurons. This emphasis on local con- nections is driven to minimize both the communication time between neurons and the amount of neural tissue that must be devoted to connecting them. The extreme of localization is the cortical minicolumn (Buxhoeveden & Casanova,
2002)—tiny vertical columns of about 100 neurons that have a very restricted mission. For instance, cortical columns in the primary visual cortex are special- ized to process information about one orientation, from one location, in one eye.
Neurons in a minicolumn do not represent a precise location with pin- point accuracy but rather a range of nearby locations. This relates to another aspect of neural information processing called coarse coding, which refers to the fact that single neurons seem to respond to a range of events. For in- stance, when the neural activity from a single neuron in the somatosensory cortex is recorded, we can see that the neuron does not respond only when a single point of the body is stimulated, but rather when any point on a large patch of the body is stimulated. How, then, can we know exactly what point has been touched? That information is recorded quite accurately, but not in the response of any particular cell. Instead, different cells will respond to dif- ferent overlapping regions of the body, and any point will evoke a different set of cells. Thus, the location of a point is reflected by the pattern of activation, which reinforces the idea that neural information tends to be represented in patterns of activation.
■ Adjacent cells in the cortex tend to process sensory stimuli from ad- jacent areas of the body.