tral gyrus, the areas devoted to the hand and the mouth are disproportionately large. Their size reflects the elaborate brain circuitry that makes possible the precision grip of the human hand, the fine motor and sensory signals required for striking up a violin arpeggio or sharpening a tool, and the coordination of the lips, tongue, and vocal apparatus to produce the highly arbitrary and significant sounds of human language.

Close observations of animals and humans after injury to particular sites of the brain indicate that many areas of the cortex control quite specific functions. Additional findings have come from stimulating sites on the cortex with a small electrical charge in experimental procedures or during surgery; the result might be an action in some part of the body (if the motor cortex is involved) or (for a sensory function) a pattern of electrical discharges in other parts of the cortex. Careful exploration has established, for example, that the auditory area in the temporal lobe is made up of smaller regions, each attuned to different sound frequencies.

But for much of the cortex, no such direct functions have been found, and for a time these areas were known as “silent” cortex. It is now clear that “association” cortex is a better name for them because they fill the crucial role of making sense of received stimuli, piecing together the signals from various sensory pathways and making the synthesis available as felt experience. For instance, if there is to be not merely perception but conscious understanding of sounds, the auditory association area (just behind the auditory area proper) must be active. In the hemisphere that houses speech and other verbal abilities—the left hemisphere, for most people—the auditory association area blends into the receptive language area (which also receives signals from the visual association area, thereby providing a neural basis for reading as well as for the comprehension of speech in most languages).

A large portion of the association cortex is found in the frontal lobes, which have expanded most rapidly over the past 20,000 or so generations (about 500,000 years) of human evolution. Medical imaging shows increased activity in the association cortex after other areas of the brain have received electrical stimulation and also before the initiation of movement. On present evidence, it is in the association cortex that we locate

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