The most obvious difference between the human brain and those of our nearest relatively is simple size: Our brain is different? But how can that account for the vast behavioral difference? Perhaps the difference in size has, indirectly, led to differences in wiring pattern. That's what Harvard's Randy L. Buckner and Fenna M. Krienen have argued:
Our association cortices are crucial for the kinds of thought that we humans excel at. Among other tasks, association cortices are crucial for making decisions, retrieving memories and reflecting on ourselves.Association cortices are also unusual for their wiring. They are not connected in the relatively simple, bucket-brigade pattern found in other mammal brains. Instead, they link to one another with wild abandon. A map of association cortices looks less like an assembly line and more like the Internet, with each region linked to others near and far.Dr. Buckner and Dr. Krienen argue that this change occurred because of the way brains develop. In the human brain, some neurons still receive chemical signals that cause them to form a bucket brigade from the sensory cortices to the motor cortices. But because of the brain’s size, some neurons are too far from the signals to follow their commands. “They may have broken off and formed a new circuit,” Dr. Buckner said.
Left hemisphere association areas are reminiscent of the general layout of right hemisphere cortex, where long-range connections are the rule rather than the exception. I would guess, just based on symmetry, that right hemisphere equivalents will be more tightly connected locally, in keeping with left hemisphere generalization. That way each hemisphere is able to subsume some functions of its evil twin.
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