Narrator: Take the remarkable case of Joe, known as JW in the scientific literature. To control crippling epileptic fits, the main connections between the two sides of his brain were severed.

Joe's split brain allows researchers to explore the workings of the left and right hemispheres which are now almost completely separated. His party trick is to draw two different objects simultaneously.

Susan: Amazing.

Mike Gazzaniga: There we go. Beautiful. OK. So Susan, why don't you try it?

Susan: [? no. ?] [? stretching ?] [? that. ?]

Mike Gazzaniga: Huh?

Susan: OK. So as long as Joe promises not to laugh at me.

JOE (jw): I won't laugh.

Susan: Thanks. OK. All right.

Mike Gazzaniga: OK. You ready? Here we go. draw them simultaneously as best you can.

Susan: What is there different?

Narrator: Over 30 year's work with split-brainers has convinced Mike Gazzaniga that asymmetry is the key to understanding our mental abilities. To him, intelligence comes only from the left.

Susan: Oh, gracious.

Mike Gazzaniga: Take a split-brain patient and you measure their preoperative IQ and problem solving skills and what have you. And then, you split the brain, where you've now disconnected the two hemispheres.

And you go back and measure the left hemisphere's problem solving, nonverbal IQ and all that, it doesn't change a wit. The right hemisphere, on the other hand, is sort of dumb.

Narrator: But when he tested Joe's ability to recognize visual patterns, he was surprised how poorly the left performed. These are called illusory contours. The white disks create the illusion of a black shape which either curves inwards or bulges out.

Both sides of the brain can tell the difference. But add a line inside the disk, and the illusion of a shape is harder to see. Unexpectedly, Joe's left hemisphere is totally stumped.

Mike Gazzaniga: All of a sudden, the right hemisphere could do this test, still judge whether the rectangles where were thin or fat. But the left hemisphere just fell apart. Could not do the tests.

Narrator: It turned out that a similar test has been tried once before on mice. Astonishingly, the human's giant left hemisphere failed to distinguish two patterns that the tiny mouse brain could tell apart. Does this failure shed new light on the evolution of advanced human abilities?

Mike Gazzaniga: Maybe the left hemisphere begins to mutate in order to develop language. And as the language requires more cortex and more cortex, perceptual processes that used to be in the left hemisphere sort of gets squeezed out.

Narrator: Gazzaniga's theory is that as we evolved, our left hemisphere acquired more and more advanced cognitive functions, while our right changed very little. And he believes that this half is just an evolutionary relic.

Mike Gazzaniga: In fact, if you look at the great skills we have that the chimp doesn't have, there's reason to believe that they're largely in the left hemisphere, that they're an outgrowth of some changes that must have occurred through mutations to the left cortex.