The projection areas take up less than one-fourth of the human cortex. The remaining areas were traditionally called “association areas” and were implicated in higher men-tal functions like thinking, remembering, and speaking. The idea behind the “associa-tion” label was that these areas provided the links—the associations—making these higher functions possible.
More recent research confirms that these regions are involved in higher mental functions but has made it clear that we need to subdivide the association areas. We also now know that some sites within the cortex’s association areas function as additional projection areas, over and above those we’ve just described.
For example, in front of the primary motor projection area, nonprimary motor areas appear critical for initiating and coordinating complex, skilled movements. On the sensory side, each sensory modality may have dozens of secondary projection areas located in the temporal and parietal lobes. What’s more, each of these areas shows its own mapping and is involved in the processing of different aspects of sensation. Indeed, the cortex seems to have at least 25 nonprimary projection areas for vision, each specialized for a different visual quality such as form, color, or movement (Gazzaniga et al., 2002).