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DISCOVERING THE BRAIN
long-term planning, interpretation, and the organization of ideas — perhaps the most recently developed elements of the modern human brain.
Visual functions occupy the occipital lobe, the bulge at the back end of the brain. The primary area for visual perception is almost surrounded by the much larger visual association area. Nearby, extending into the lower part of the temporal lobe, is the association area for visual memory—a specialized area in the cortex. Clearly, this function has been important for an omnivorous foraging primate that probably spent a long evolutionary period ranging among scattered food sources. (For an account of the intricate mechanisms that underlie depth perception and color vision, see Chapter 7 .)
A less specific kind of function has been attributed to the prefrontal cortex, located on the forward-facing part of the frontal lobes. This area is connected by association fibers with all other regions of the cortex and also with the amygdala and the thalamus, which means that it, too, makes up part of the “emotional brain,” the limbic system. Injury to the prefrontal cortex or its underlying white matter results in a curious disability: the patient suffers from a reduced intensity of emotion and can no longer foretell the consequences of things that are said or done. (The injury must be bilateral to produce such an effect; if only one hemisphere is injured, the other can compensate and avert this strange, potentially crippling social deficit.) Among its other functions, the prefrontal cortex is responsible for inhibiting inappropriate behavior, for keeping the mind focused on goals, and for providing continuity in the thought process.
Long-term memory has not yet been found to reside in any exclusive part of the brain, but experimental findings indicate that the temporal lobes contribute to this function. Electrical stimulation of the cerebral cortex in this area gives rise to sensations of déjà vu (“already seen”) and its opposite, jamais vu (“never seen”); it also conjures up images of scenes witnessed or speech heard in the past. That the association areas for vision and hearing and the language areas are all nearby may suggest pathways for the storage and retrieval of memories that include several types of stimuli.
The function of language itself is housed in the left hemisphere (in most cases), in several discrete sites on the cortex.