Cortical Organization

When neuroscientists use the term cortex, they usually refer to the neocortex of mammals. This neocortex is part of the outer shell or bark of the forebrain that was thought to be a new structure that evolved with mammals. Now it is widely accepted that the neocortex of mammals is the same structure as the dorsal cortex of reptiles. However, the dorsal cortex is much smaller and simpler in laminar organization than the neocortex is. Thus, the neocortex is new, in the sense that all mammals have a more complexly laminated and functionally subdivided cortex than do reptiles. Although some use the term isocortex for neocortex, the term neocortex is almost universally used.

When we look at the human brain, most of what we see is the neocortex (color insert, Figure 38). It occupies the outer surface of the two cerebral hemispheres of the forebrain, just behind the olfactory bulb, above the piriform or olfactory cortex, and in front of the cerebellum and brainstem (color insert, Figure 38). All mammals have a neocortex. However, the structure is highly varied in size, structural differentiation, subdivisions of functional significance, and cellular and regional specializations across mammalian species. This variability is one of the reasons why the neocortex is such an important part of the brain; the neocortex has specialized features for the various needs of different species. Differences in cortical size (Figure 1) and organization largely account for the considerable differences in the abilities and behaviors of mice and men, or even monkeys and apes. But, as this entry describes, basic features of the neocortex are also shared by most present-day mammals because they were retained from an early mammalian ancestor.

Figure 38 Cortical Organization (1)—The Cerebral Cortex of the Human Brain

Note: The four lobes of the cortex are indicated, as well as parts of primary motor and sensory areas. Because areas extend into the central fissure, only parts of the primary motor cortex (M1 or area 4) and primary somatosensory cortex (area 3b) are visible. The primary auditory cortex is deep in the lateral fissure, and most of the primary visual cortex (V1 or area 17) is hidden in the calcarine fissure on the medial wall of the hemispheres. The human brain has many other cortical areas. See the Cortical Organization entry for additional information (pp. 327–333).