我們平時經常聽到這樣的言論：什么樣的訓練可以開發左腦，什么樣的人右腦發達。但是你想過這樣一個問題嗎，為什么我們的大腦要分成左右兩部分呢？

    Why does the brain have two hemispheres?

    “In both humans and other mammals, as the body became more complex and reacted to environmental stressors, the brain developed in response to that,” said Dr. Philip E. Stieg, chief of neurosurgery (神經外科) at NewYork-Presbyterian(長老會的) Hospital/Weill Cornell. “We do not know what specific stimulus changed or caused a species that has a single brain lobe to evolve to have two hemispheres. It was probably a series of stimuli.”

    All animals that show complex responses have two hemispheres, Dr. Stieg said. A worm, for example, reacts to simple sensory input with a simple set of motor responses, he said. But the human brain deals with not just complex sensory input, but more diverse and complex motor responses mixed with an array of emotional and cognitive interplays.

    The result, he said, is that parts of the different hemispheres of the cerebral cortex(大腦皮層), the top part of the brain, developed specialties. For just a few examples, he said, “the dominant side of the temporal lobes (the left in 97 percent of us) have speech and visual pathways; the parietal lobes, straight up from the ear, are intermixed on the dominant side, with the left handling speech; and the sensory function of one side of the body is handled by the other side, so that if the brain is bruised on the right side of the parietal lobe, and I hit you on the left side of the body, you might not pay attention to it.”

    Patients can have half their brain cut out to treat severe seizures and still function, but not at the same cognitive level, Dr. Stieg said.