(Note: This is part of a series of posts I’m writing in conjunction with my new book, The Tale of the Dueling Neurosurgeons.)
The previous two posts in this triad explored, first, how experiments on cats provided the first real insight into the brain’s vision centers and, second, how experiments on kittens proved that animal brains have “critical windows” during infancy, during which they must experience certain sensations or else their brains won’t get wired properly. Today, I’ll continue this felixitous theme with another cat-neuroscience story, about the left and right hemispheres.
Neuroscientist Roger Sperry surely had one of the most satisfying graduate-school experiences ever: both while pursuing his Ph.D. in the early 1950s and while serving as a postdoc, he ended up demolishing his advisor’s life work with a few devastatingly simple experiments. (You can read about the full details here—the schadenfreude is delightful.)
After settling down as a professor at Cal Tech, Sperry decided to investigate the corpus callosum, the main bundle of nerve fibers connecting the left and right hemisphere. Most scientists at the time suspected that the corpus callosum played a role in interhemispheric communication. But this was just a guess, and certain evidence did argue against it: X-rays revealed that some people were born without a corpus callosum, and they seemed just fine. So Sperry decided to test the theory of interhemispheric communication in cats.
Specifically, he opened up the skulls of a few cats and severed their corpus callosums, dividing their brains in two. (Aside from the pain of cutting open the scalp, the surgery didn’t hurt the cats, since the brain itself cannot feel pain.) After the cats recovered, Sperry then taught them to navigate a maze while wearing an eye patch. As expected, after several attempts, these “split-brain” cats could negotiate the twists and turns without trouble.
But when Sperry switched the patch to the other eye and put the cat back into the maze, something funny happened: it started getting lost again. Why? First, in addition to cutting their corpus callosums, Sperry had doctored the cats’ optics nerves, so that one eye provided information to only one half of the brain. (In keeping with the brain’s general cross-wiring, he wired the left eye to the right hemisphere and vice-versa.) So the eye patch, by blocking sight in one eye, also restricted information to one half of each cat’s brain.
Second, when the information did arrive in that half of the brain, it remained stuck there, in a silo, because of the severed corpus callosum. As a result, even though each split-brain cat could learn the maze with half its brain, when Sperry switched the patch—forcing the other half of the brain to navigate—the cat had no idea where to go suddenly. Crucially, this didn’t happen to full-brain control cats. They could navigate just as well with either eye, no matter which eye they’d used to learn the maze, because their intact corpus callosums could share any information between both hemispheres.
Overall, this was a big enough deal on its own—real proof that the purpose of the corpus callosum was to share information between the left and right hemispheres. But it led to even greater insight into the human brain.
To see why, we have to take a quick detour into epilepsy. No one knows quite the reason, but surgically severing the corpus callosum can reduce the rate and intensity of seizures. So in the early 1960s, a few patients with severe epilepsy had their corpus callosums cut, turning them into split-brain people.
Follow-up tests showed that the patients did remarkably well afterward: the surgery provided the first real relief many had known in decades, and it did so with no discernible side effects. Still, a few recent disasters with aggressive neurosurgeries—the amnesiac H.M. being the best example—had left the surgeons involved wary. They wanted to make double-sure there weren’t any lurking side effects. So they called in Sperry, a world expert on the corpus callosum, to test the patients. Neuroscience (especially pop neuroscience) was never the same.
I won’t belabor the experimental details, but Sperry placed the patients in front of a screen and then flashed pictures to the far left side or the far right side of it. In this way, much like with the cats’ eye patches, he could channel information into either the left or the right hemisphere alone. And again, because of a lack of a corpus callosum, the information couldn’t escape that hemisphere. In other words, Sperry had effectively isolated each hemisphere, something that wasn’t possible before, since the vast majority of us have an intact corpus callosum.
What Sperry discovered astounded him. For the first time, scientists could tease out the different talents and abilities of the left and right brain. You might have heard before that the left hemisphere is more logical, or that it does a better job of jumping from details to general laws and principles. Meanwhile the right hemisphere recognizes faces better, does a better job at spatial tasks like rotating objects mentally, and handles music and other “arty” things in a superior way. All that work sprang from Sperry and his split-brain cohort.
To be sure, some people today get pretty carried away with “left brain” thinking versus “right brain” thinking, and Sperry didn’t support the more idiotic manifestations of this. But there are real, genuine differences between how the left and right brain view the world. And without Sperry’s work—first on split-brain cats and then on split-brain humans—we would have remained ignorant of this incredible aspect of the human brain.
As a coda to these three blogs posts, I’d like to note that the protagonists of the first two posts, Torsten Wiesel and David Hubel, and the protagonist of this one, Roger Sperry, all shared the Nobel Prize in physiology/medicine in 1981. Frankly, the Nobel Committee seems to have yoked them together arbitrarily, with no unifying theme. But one thing that did unite them was their decision to use cats for their most crucial work. As I noted in the first post, neuroscience has largely shifted toward the use of mice and apes, and with good reason. But some of last centuries’ great insights sprang straight from the brains of our closest felines friends.