Brain Anatomy

The key idea in the study of neuropsychology is functional localization. To understand this concept, it helps to look back in history about one hundred years, to a time when phrenologists roamed the land. Phrenology was the "science" of studying the bumps on a person's head and, by those bumps, predicting the skills, proclivities, and personality of that person--a practice not unlike palm reading. Phrenologists believed that the brain was divided into distinct parts, each of which controlled some aspect of a person's intellect or personality. Furthermore, they believed that if a person had an excess or a dearth of some trait, the anatomical area representing that trait would be appropriately large or small. Phrenologists developed a detailed map of the brain, with portions labeled "greed," "sloth," "trustworthiness," and so forth, and used this map to determine what a particular person's bumps signified.

We now know that the phrenologists were half right. Their main idea was sound: the brain is divided into distinct parts, each of which controls some aspect of a person's intellect or personality. However, the size of these parts has no discernable impact on personality, and you cannot feel them as bumps on the skull, so their "science" has no real worth. Only their idea of functional localization remains: that each function of the brain is localized into one or more physical areas. Neurons tend to cluster according to their function, so that we can say, "this area in the front of the brain helps us plan," and "this area in the center of the brain directs our movements." The areas are not isolated, however; the brain relies on communication among its parts, and almost all functions that it carries out are multi-step, requiring processing efforts by many areas of the brain. The anatomy of the brain is complex, but it consists basically of a few large parts divided functionally into many increasingly smaller sections. The large parts are the hemispheres and the lobes.

The brain is divided into two distinct hemispheres: the left and the right. Each side is essentially a mirror image of the other. They two halves are physically separate entities, communicating only through a thick bundle of nerves called the corpus collosum. Despite their apparent similarity, however, the two halves show some definite functional differences. One basic difference is that the two hemispheres each control the opposite, or contralateral, side of the body. For example, the right somatic sensory cortex receives input from the left side of the body, and the left motor cortex sends signals to the right side of the body. Beyond this basic discrepancy, there are differences in the overall functions carried out by the hemispheres. The most well-known examples are the predominance of language functions in the left hemisphere and of spatial functions in the right.

For simplicity of organization, and somewhat on the basis of functionality, the cortex of the brain is divided into four lobes along the lines of the major lumps (gyri) and creases (sulci) in the brain. The four lobes, pictured below, are the frontal, temporal, parietal, and occipital. Each lobe does not carry out a single function, but rather contains smaller structures that have their own jobs to do. In some cases, as in the case of the frontal lobe, we will not discriminate between these structures, but rather will pretend that the lobe is a unified whole; in others, as in the case of the temporal lobe, we will only look at the smaller structures and their separate functions. Overall, we will study the function of each major structure in the brain as it applies to neuropsychology.