Association cortex is the cerebral cortex outside the primary areas (Figure 1). It is essential for mental functions that are more complex than detecting basic dimensions of sensory stimulation, for which primary sensory areas appear to be necessary. In humans the association areas are by far the most developed part of the cerebral cortex, and the brain in general. These areas are necessary for perceptual activities, like recognizing objects (toasters, horses, trees, words, etc), rather than simple contours, edges or sensory qualities like color or pitch.
Association areas take up an increasingly larger percentage of the cerebral cortex as brain size increases among different species. Figure 2-1e illustrates the increase in relative size of association areas as the brain gets bigger. Association cortex is shown as the pink area outside the primary cortical areas. It is much larger and takes up a much greater percentage of cortex on the human's cerebral hemisphere than on the rat's. The surface area of the human cerebral cortex (and monkey's, dog's, and horse's as well) is further enlarged by the sulci and gyri shown by the curving lines on the cerebral hemispheres.
Figure 1-2e. Primary and
association cortical areas in human and rat. The pink area, which shows the association cortex, is
much larger and also takes up a much larger percentage of cortex in the human than in the rat cerebral
hemisphere.
The increasing size of association cortex correlates with the complexity of behavior and inferred mental functions that different species show.
Each sensory system has its own association areas on the cerebral cortex. Recall from asgn2d that the sensory systems (vision, hearing, etc.) each have their own primary area on the cortex, which gets the most direct connections from its sense. Each primary sensory area sends information to its own cortical association areas, which are next to their primary areas, as shown in Figure 2-2e. (The motor system is organized in the same way, but in the reverse direction: from motor association areas to the primary motor area to the motor systems in the brain stem and spinal cord.)
Figure
2-2e. Primary sensory and motor areas and sensory and motor association areas. Arrows show the
direction of information flow from primary areas to sensory association areas. The information flows in
the opposite direction for the motor system.
The processing that occurs in the sensory association areas is the basis of complex mental processes associated with each sense. Each sensory association area appears necessary for perception of objects and events in its sensory modality. The information that each sensory association area gets from its primary area is about simple contours, boundaries, and sensory qualities like color or pitch. Sensory association areas combine this kind of information to represent complex objects. For example, the visual association area on the lower part of the temporal lobe plays a primary role in your ability to recognize faces, dogs, cars, trees, etc., whereas the primary visual cortex is required for detecting basic features of the visual world: edges, light and dark, location, etc.
The activity of nerve cells in visual association cortex also shows that these areas are involved in a higher level of processing. For example, nerve cells in (a part of) the visual association area respond to visual stimuli that have some kind of complex pattern or structure. They usually respond only when the eye looks at complex patterns, such as images of objects, abstract forms, hands, faces, or even specific faces (K. Tanaka et al., 1991). This means that when such cells respond, the brain has information telling that the specific stimulus object that triggers the active cells is getting to the sense organ.
