我们平时经常听到这样的言论：什么样的训练可以开发左脑，什么样的人右脑发达。但是你想过这样一个问题吗，为什么我们的大脑要分成左右两部分呢？

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 (神经外科) at NewYork-Presbyterian(长老会的) 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(大脑皮层), 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.