area A major subdivision of neocortex. Each area performs a specific set of functions. classically, areas have been called the organs of the brain.

column or module Subdivisions of areas that mediate a function or functions that are repeated many times within other modules of the same type within the area. Areas may have two or more types of intermixed modules.

cortical magnification The greater representation of important parts of sensory surfaces in cortical areas.

representation Areas are said to represent a sensory surface, such as the retina, skin, or cochlea, when stimulation in different parts of the sensory surface activates neurons in different parts of the area in a matching or isomorphic pattern. Muscles and movements are also represented in areas of motor cortex. Some areas may have more abstract, higher order representations. In each case, the representation is based on a spatial pattern of neural activity.

Neocortex is the part of the outer shell or ''bark'' of the forebrain that was thought to be new with the evolution of mammals. The cerebral cortex or palli-dum covers the deeper parts of the forebrain or telencephalon, and it is generally divided into three parts: the lateral paleocortex or olfactory cortex, the medial archicortex or hippocampus and subiculum, and the neocortex lying in between. The archicortex and paleocortex are easily recognized in reptiles, and therefore they have terms that reflect the early belief that they are phylogenically old parts of the brain. Whereas all mammals have an obvious neocortex, nothing quite like neocortex exists in reptiles. Hence, early investigators coined the term "neocortex" to refer to this seemingly new part of the brain. Nevertheless, neocortex is not really new with the advent of mammals. Rather, neocortex is homologous with the dorsal cortex of reptiles, a rather small and unimpressively thin sheet of tissue with hardly more than a single row of neurons, a marked contrast to the thick neocortex with several layers of neurons. Because neocortex really did not originate with mammals, some investigators prefer to call the structure isocortex, using an early term that refers to the relatively uniform structure or appearance of neocortex throughout. However, most investigators continue to use the term neocortex, and it is accurate to say that its thick, laminated form is new with mammals. No reptile has a dorsal cortex that looks like neocortex, and no mammal has a neocortex that looks like dorsal cortex. Thus, a simple, stable, and functionally limited structure that has been retained from early to present-day reptiles has been transformed into a more complex, highly variable, and remarkably flexible structure in mammals. No living vertebrates have a cortex that is intermediate between dorsal cortex and neocortex. Thus, mammals are characterized by neocortex as much as by mammary glands. Whereas

Encyclopedia of the Human Brain Volume 3

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neocortex has a somewhat uniform appearance, being easily recognizable as neocortex throughout and across species, its variability in size and organization is what allows mammals to be so different in behavior and abilities. Neocortex allows us to be human and rats to be rats. To understand how this is possible, we need to identify the common features of neocortex, as well as how it varies from mammal to mammal.

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