Narrator: In the backwoods of New Hampshire, Joe, known famously as the case of JW, has inspired some thought-provoking ideas about where the illusion of free will comes from.

20 years ago, Joe decided to submit himself to a drastic operation to control his epilepsy.

Joe: Yeah, one day you'd have two or three different seizures. And when you do that long enough and you're willing to go through anything. So what the heck? If they want to crack your head open and have an operation, what have you got to lose? If everything's going wrong anyway.

So I might as well go for it. So I just went for it anyway. And I thought, well, I'd try it and see. And it turned out it worked good.

Narrator: The operation was to split his brain in two.

Doctor 1: Right here, yeah.

Doctor 2: [inaudible]

Narrator: Having exposed the top of the head, the surgeon works his way into the cleft between the two hemispheres, revealing the white bundle of nerve fibers connecting left and right sides of the brain.

Doctor 1: OK, Dave. I'm going to start to divide the corpus callosum.

Narrator: Tearing apart these 15 million fibers in the corpus callosum prevents epileptic seizures from spreading from one side to the other. But it also prevents almost all the information from the senses traveling across.

The operation has made Joe a valuable research subject for neuroscientists. After years of studying split brainers, Professor Gazzaniga believes he may have found the source of the illusion of conscious free will.

Professor Gazzaniga: Good. OK, you ready?

Narrator: He started by examining the linguistic abilities of the left and right sides of the brain.

Joe: Storm.

Narrator: Words on the right of the screen go to his left hemisphere. But he calls them out easily.

Joe: Gun.

Professor Gazzaniga: Good.

Narrator: But when words are flashed to the other hemisphere—

Joe: Didn't see.

Narrator: —He says he didn't see anything. But remarkably, he then draws a picture of the word. Joe draws the telephone his right hemisphere saw, but strangely, he can't tell what it is.

Joe: Shoe? I don't know what it is. I can't tell. It's not very good, whatever it is. Can't exactly tell what it is. It looks like a shoe, I guess.

Professor Gazzaniga: What else?

Joe: Coffee, [inaudible], I don't know. I can't tell what it is.

Narrator: Because he thinks that speech comes from the left hemisphere, Mike Gazzaniga believes that the left must also be dominant in generating consciousness.

Professor Gazzaniga: If you think about the consciousness differences between the left and the right separated hemisphere, the left hemisphere is an interesting cognitive machine. It has all these problem solving capacities— talk and language and speech as you saw. And the right hemisphere, basically, isn't a very interesting entity. You would not want to have a date with a right hemisphere.

Joe: I've been pretty much the same person like I've always been. Just chopped up a little different, that's all. As far as the operation goes, I don't think it really affected me too bad. Just helped. But I don't think as far as making it worse, making me worse or anything, it didn't do anything.

And as far as two brains, I've only got one brain. It's just not quite the same design as everybody else's.

Narrator: The theory goes, if the conscious feeling of who you are came from both hemispheres, then Joe would feel changed by his operation. Since he doesn't, Mike Gazzaniga's bold conclusion is that Joe's inner voice must come from just one side of his brain. And since our inner thoughts are all in words, they must come from the linguistic left.

Professor Gazzaniga: The inner voice in the left hemisphere— it's got to be huge, robust. It's Pavarotti-like. And this right hemisphere probably has a chirp. A little bird sound.

Because the devices that allow for the inner voice to really expand and express itself are mostly located in the left hemisphere.

Narrator: Experiments on Joe have led Mike Gazzaniga to believe that the left hemisphere may also provide an explanation— the sensation of free will.

Joe: Sundial.

Professor Gazzaniga: What did you see?

Joe: Dial.

Professor Gazzaniga: You saw a dial?

Narrator: In this test, Joe is flashed two words simultaneously.

Professor Gazzaniga: Draw what you see, OK? Here we go.

Narrator: Shown "hour" and "glass," he draws an hourglass. With his left hemisphere, Joe names it immediately.

Professor Gazzaniga: What's that?

Joe: Hourglass.

Professor Gazzaniga: Good.

Joe: Time ticking by.

Narrator: But Joe's left only processed the word glass. So he goes on to invent a reason why he drew a timepiece.

Professor Gazzaniga: Did you see it? What did you see?

Joe: I saw a glass.

Professor Gazzaniga: Why did you draw that?

Joe: I don't know. Probably still thinking about the clock when that—

Narrator: It's as if Joe has been fooled by his own left hemisphere.

Professor Gazzaniga: And so he basically makes up a story to explain the behavior that he's carried out. And he comes to believe that that's the reason why he did a particular act. And you see those experiments, that kind of phenomenon, time and time again in these split studies.

But the importance of it is not that it is unique to split brain patients at all. It is a metaphor for what you and I do. Is we're constantly trying to figure out and put a spin on what's coming out of our body.

I'm not conscious about the fact, that how that last sentence just came out. It just came out, because it's being— some guys are down there with framing hammers, putting this stuff together. And bingo, out it comes. And it's more or less orderly.

So just think about the fact that the major management of your body walking through space, your responding to auditory, visual, tactile— all that's being done for you. It's just absolutely being done for you. And you're not paying any attention to it at all. You don't even know about it.

But it does a wonderful job. And it just turns out that more and more of your cognitive acts are the same.