You know what’s weird? It’s possible to be a happy, independent and intelligent person with only half a brain. It’s not always the case, but that it’s possible at all is still pretty dazzling.
I recently wrote an article for the Post-Gazette on the relative success of hemispherectomy for treating intractable childhood seizures. It involved some of the most satisfying reporting I’ve ever done, interviewing Suzann, a mother whose son had lost part of his brain in an accident. She was generous with her time, honest in conversation and let me interrupt her at all hours of the day with intrusive questions. Thankfully, she also loved telling her son’s story because his life had been so radically changed — for the better — by a hemispherectomy.
But there was one profoundly stupid question that I kept asking myself. Like a good reporter, I found a scientist and asked her to answer my stupid question:
“How can someone have half their brain removed and then wake up the same person?”
The scientist I asked was Marlene Behrmann, a cognitive neuroscientist at Carnegie Mellon who studies visual perception using fancy-pants brain imaging. Her lab has just started some studies on how visual processing adapts in hemispherectomy patients. Surprisingly little is known about the subject, despite the fact the surgery has become relatively routine in large pediatric epilepsy surgery units since the 1980s.
I figured the answer to my question would go something like: Our memories and consciousness don’t exist in a particular place in the brain, they exist in a network, like distributed computer systems or something. And like, processing happens in both hemispheres at once. And like, reasons and stuff. Or something. Did I mention I know nothing about neuroscience?
Marlene’s answer was not what I was expecting.
“I don’t know that they are the same person,” she said.
I thought about Evan, Suzann’s brain-injured son, who had woken up after his hemispherectomy with eyes so swollen he couldn’t open them. “I can’t see you Mama,” he had complained to Suzann. She was terrified that the half-field of vision that remained in his one functioning eye might have been disrupted by the surgery. She sounded joyful when she told the story of Evan’s third day after surgery. “Hi Mama!” he said, on waking up. “Can you see me Evan?” she had asked frantically. “I can see you Mama,” he replied.
Surely this Evan was the same person as before the surgery? He still had the same memories.
“It’s a really difficult question to answer,” Marlene had said and then apologized for her reluctance to speculate.
“What cognitive skills make a person a person? And when they change, what of those do you still need to have to be the same person? What if somebody has lost all of their memories? Does that mean they’re a different person?” she said. “I don’t know.”
I didn’t know either. I left the interview with even more stupid questions than I had before. How did I know Evan was the same person? What did I even mean by the same person? I suppose I was thinking of continuity of consciousness, an unbroken sense of self. Presumably Evan felt like he was the same person when he woke up. But what did that feeling mean? Would I notice if I woke up tomorrow with different memories? Would I notice if I woke up with a different personality?
Then I remembered something I had read during my research. It was in an article about all the things scientists had learned from people whose brain hemispheres had been surgically disconnected from each other to prevent the spread of seizures. Both hemispheres remained intact and functioning, but the two halves could no longer talk to each other.
The isolation of each hemisphere had some spooky effects. For example, a patient who read a word presented in their right field of view might have been able to say the word aloud, but not if it were presented in their left field of view. That’s because the right visual field is processed by the left hemisphere, which is usually dominant in verbal processing. But the person might have been able to draw what was presented to their left visual field (right hemisphere).
Despite the independence of the two halves of their brain, the patients didn’t feel like two people in one:
…patients never reported feeling anything less than whole. As Gazzaniga wrote many times: the hemispheres didn’t miss each other. Gazzaniga developed what he calls the interpreter theory to explain why people — including split-brain patients — have a unified sense of self and mental life3. It grew out of tasks in which he asked a split-brain person to explain in words, which uses the left hemisphere, an action that had been directed to and carried out only by the right one. “The left hemisphere made up a post hoc answer that fit the situation.” In one of Gazzaniga’s favourite examples, he flashed the word ‘smile’ to a patient’s right hemisphere and the word ‘face’ to the left hemisphere, and asked the patient to draw what he’d seen. “His right hand drew a smiling face,” Gazzaniga recalled. “’Why did you do that?’ I asked. He said, ‘What do you want, a sad face? Who wants a sad face around?’.” The left-brain interpreter, Gazzaniga says, is what everyone uses to seek explanations for events, triage the barrage of incoming information and construct narratives that help to make sense of the world.
This idea of a rationalizing, storytelling “interpreter” that helps maintain our sense of self makes me think that my original question — how Evan remained the same person after having so much of himself removed — is unanswerable. Evan will always be himself because the human experience of the world is jury rigged from whatever sensations are available and whatever stories our brains can construct.