>> The Divided Brain

	Interview with Michael Gazzaniga, Dartmouth College From Studying The Mind, VHS © 2003, W. W. Norton

How did you come to understand that consciousness is divided?

I had devised a back-projection screen built from an old picture frame, and then hung it up in the middle of this room by a rope over a steam pipe. We then projected pictures onto it, which were presented to the patient's left or right visual field, and the patient would name them. So, for instance, we'd project a simple object like an apple, presented in the right visual field (going into the left cerebral hemisphere), and the subject would say, just like you or I would, “apple.” And then we’d present it to the other side of fixation and there was silence. And then through tests we could then watch as the patient's left hand found the matching stimulus, and if that part of the test was done out of view the answer would be in the patient's left hand, and yet he would have no idea that he had actually completed the entire task. So it was pretty exciting stuff.

We were so amazed that the silent disconnected right hemisphere could carry out all of these match to sample functions and visual tactile retrieval tests—and yet have that be outside of the realm of consciousness of the left side—that we probably over-spoke when we talked about the co-conscious properties of the the left brain system and the right brain system. We talked as though we had split consciousness. And now, after forty years of studies of all kinds, we find that really it’s the left hemisphere that is the hypothetical, pattern-seeking, hypothesis-driven hemisphere. It's the one you want to know–youknow, it’s like you and me. And the right hemisphere has all kinds of specialized skills in the perceptual domain, perhaps, and some potential control, and maybe some subtle emotional cueing, but it’s sort of an intellectual thug, and not somebody you really want to be with. And so this notion that the left brain consciousness and the right brain consciousness are equal just isn’t true. They are both conscious only at their level of function. And so it’s actually that realization over the years that suggests that consciousness is on a continuum throughout the animal kingdom.

What might account for the evolution of divided consciousness in the human brain?

There are probably a lot of functions that are bilaterally symmetrical in animal brains. Random mutations that occurred—cortical areas in the left hemisphere, for instance—may have been re-crafted to instantiate language, and then maybe perceptual skills that were once there were dissipated. They disappeared because there was remodeling of the cortex for this other function. But there was really no cost to the whole organism because you had this connection cable, the corpus callosum, and while only one side of the brain had this specialized capacity, that was enough--you didn’t need two of these systems. So there was a re-crafting of the cortex that allowed for more specialization to occur to support the human condition, but there was no cost to the organism for re-modeling part of the cortex because that part ofbrain was connected via the corpus callosum to other side of the brain, and it did not experience remodeling in that particular domain.