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Photo of Michael Gazzaniga The Man With Two Brains

Through his study of patients with hemispheric disconnection, also called "split brains," Michael Gazzaniga has discovered that the two sides of the brain interpret and organize information differently. Following the premiere of the Scientific American Frontiers Special Pieces of Mind, Michael answered viewers' questions as part of an Ask the Scientists panel. Here are viewers' questions and his answers:

QHow does being left-handed affect the processes of the two sides of the brain? Is there any correlation to left-handedness and memory?

Left-handed people have a different half brain dominance pattern. About 40 percent have a dominant left hemisphere and 40 percent have a dominant right hemisphere. The others have mixed dominance. I don't know if there are implications for memory.


QWould you please give me the references for articles you have published on this subject.

Gazzaniga, M.S. (1995). Principles of human brain organization derived from split-brain studies. Neuron, 14, 217-228.

Gazzaniga, MS; Eliassen JC; Nisenson L; Wessuger, CM; and Baynes, K.B. (1996) Collaboration between the hemispheres of a callosotomy patient - emerging right hemisphere speech and the left brain interpreter. BRAIN, 119:1255-1262.

QDoes a disconnected brain hemisphere human show any coordination problems with the left and right side of their body?

Not really.

QHow do your findings compare with studies of the children who have had a hemisphere removed?

The findings are of a different kind. Children who have undergone brain removal at an early age can sometimes develop more or less normally. The remaining cortex takes over the functions once provided by the removed cortex. If the removal occurs later in life, however, this sort of compensation does not occur.

QWhat future experiments are being planned to continue your research?

There are many things to do. Some of the patients have developed right hemisphere speech. That means a half brain that normally does not talk is now talking. Using new brain imaging methods we want to see where that is occurring in the right hemisphere. It has interesting implications for how plastic the brain might be.

QMy students were wondering what you might be able to do with two "separate" brains that you couldn't do with one.

We have done studies that suggest separated hemispheres can process more visual information in a moment's time than can an intact brain. But all in all, it is better to have your brain connected as it normally is.

QThe demonstration in a split-brain person that one side of the brain preferentially responded to faces, the other to vegetables, was used to support the idea that the right side of the brain is primarily adapted for processing faces. However, another possibility is that the experiment that was shown was actually assessing "global" versus "local" processing differences: i.e., the right hemisphere really was adapted to recognizing "large" stimuli, the left side for intricate details. My question: What would happen if you were to show the patient a scene constituted from many small faces, e.g., composing a larger object (perhaps a simple geometric design)? Would the right hemisphere preference for facial processing still hold?

GREAT QUESTION. DON'T KNOW THE ANSWER. You ought to go to graduate school!

QIs it true that while male brains are set up with logic in the left brain and creativity in the right, in female brains the functions are split up more evenly between the two sides?


QMike Gazzaniga knows what half of the brain he would want to have if he had to pick. How does he know and why?

It is the left brain that carries out high level thinking and problem solving. It also is the hemisphere that interprets our actions and feelings. The right brain is very poor or unable at these kinds of tasks.

QIs there anyway to help the main with two brains' left and right hemispheres communicate again?


They do communicate on emotional issues. And each hemisphere pulls attentional resources from a common pool. As for conversing on perceptual or cognitive information, communication does not seem to be possible.

QIf the right brain tells us what faces look like, what happens if we shut our left eye when looking at an Arcimboldo painting?

Remember how the visual system works. Each eye sends information to both half brains and the information is divided up in the following way. If you fixate a point, everything to the left of fixation goes the right brain and everything to right of fixation goes to the left brain. So, what you means to ask is, what happens when you place the Arcimboldo pictures to the left of fixation? In normals, not much since their corpus callosum is still intact.

QHow did you started to work with the man that has two separate brains -- and how did you come up with your ways of testing his skills?

I started working on patients with two brains over 35 years ago at Caltech. Roger Sperry and I were the first two studies these kinds of patients in modern times. The ideas for the experiments come to me on long airplane flights!!


Scientific American Frontiers
Fall 1990 to Spring 2000
Sponsored by GTE Corporation,
now a part of Verizon Communications Inc.