Materialists believe that our cognitive experience is solely the product of the material elements of the central nervous system, primarily the brain. Dualists argue that in addition to the material "stuff" of the brain we also have a mind that plays an important role in our thought life.
An article at MindMatters features a transcript of an interview with another neurosurgeon, Michael Egnor, who explains the three findings that caused Penfield to abandon materialism.
Egnor notes that during the course of his surgeries on epileptic patients Penfield could stimulate the brains of the patients, and since the brain doesn't experience pain, the patients were conscious and responsive during the operation.
Wilder Penfield c. 1958
When Penfield would stimulate different areas of the brain he could get patients to move a limb, or retrieve a memory, but he could never induce a patient to have a thought about an abstract concept like justice or mercy. He was never able to stimulate a patient's reason or intellect.
Here's Egnor:
All the stimulations were concrete things: Move your arm or feel a tingling or even a concrete memory, like you remember your grandmother’s face or something. But there was never any abstract thought stimulated.
And Penfield said hey, if the brain is the source of abstract thought, once in a while, putting an electrical current on some part of the cortex, I ought to get an abstract thought. He never, ever did. So he said that the obvious explanation for that is that abstract thought doesn’t come from the brain.
Penfield also found by studying hundreds of thousands of cases of epileptic seizures that the person never had a seizure that had intellectual content or involved abstract reasoning.
When people have seizures, sometimes they have a generalized seizure. Sometimes they just fall on the ground and go unconscious. Or sometimes they’ll have what’s called a focal seizure where they’ll have a twitching of a finger or a twitching of a limb or they’ll have tingling feeling, the same kind of things that he got when he stimulated the surface of the brain.
But nobody ever had a calculus seizure. Nobody ever had a seizure where they couldn’t stop doing arithmetic. Or couldn’t stop doing logic.
And he said, why is that? If arithmetic and logic and all that abstract thought come from the brain, every once in a while you ought to get a seizure that makes it happen. So he asked rhetorically, why are there no intellectual seizures? His answer was, because the intellect doesn’t come from the brain.
Penfield's third bit of evidence was this:
He would ask people to move their arm during the surgery. So he’d be playing around with their brain. And he’d say. “Whenever you want to, move your right arm.” The person would move their arm.
And, once in a while, he’d stimulate the part of the brain that made the arm move.And they moved their arm also when he did that. And then he would ask them, “I want you to tell me when I’m making your arm move and when you’re moving your arm without me making you do it. Tell me if you can tell the difference.” And the patients could always tell the difference.
The patients always knew that when he stimulated their arm, it was him doing it, not them. And when they stimulated their arm, they were doing it, not him. So Penfield said, he couldn’t stimulate the will. He could never trick the patients into thinking it was them doing it. He said, the patients always retained a correct sense of agency. They always knew if they did it or if he did it.
So he said the will was not something he could stimulate, meaning it was not material.
Egnor remarks that these three lines of evidence were dispositive for Penfield. He believed he could not sustain his belief that the brain was the only thing involved in mentation:
So he had three lines of evidence: His inability to stimulate intellectual thought, the inability of seizures to cause intellectual thought, and his inability to stimulate the will. … So he concluded that the intellect and the will are not from the brain.
This is not proof, of course, that materialism is wrong, but it's certainly interesting evidence against it.