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Why Splitting The Brain In Two Was A Key Milestone In Modern Neuroscience

And, there are cats.

Laura Simmons - Editor and Staff Writer

Laura Simmons

Laura Simmons - Editor and Staff Writer

Laura Simmons

Editor and Staff Writer

Laura is an editor and staff writer at IFLScience. She obtained her Master's in Experimental Neuroscience from Imperial College London.

Editor and Staff Writer

left and right brain hemispheres on blue background
Being "left-brained" or "right-brained" is a myth, but what happens when the two halves are no longer connected? Image credit: Wirestock Creators/

You’ve probably heard of the idea of being "left-brained" or "right-brained". Despite the lack of scientific evidence to back it up, this myth continues to be popular; being "left-brained" is supposed to indicate strong logical and mathematical skills, while the so-called "right-brained" are meant to be more creative and intuitive types. In reality, research has so far failed to find any reason to suspect that either hemisphere alone has a dominant effect over someone’s personality. 

However, it is true that the hemispheres are specialized, and that certain tasks are largely controlled by brain areas on one side or the other. Many of the discoveries in this field were made by a series of pioneering experiments in the mid-20th century.


Neuroscientist Roger Sperry first became interested in the roles of the two brain hemispheres when he noticed something unexpected. Some years prior, neurosurgeon William van Wagenen had originated a surgical procedure for treatment-resistant epilepsy, which involved literally splitting the brain in half. The surgeon would cut through the corpus callosum, a bundle of millions of nerve fibers that connects the left and right hemispheres together. 

Sounds pretty serious, right? You would think that severing all these connections would have a significant effect on the patient. Maybe that would be worth it, in the case of severe refractory epilepsy, but still.

However, the thing that so surprised Sperry, entering into this research area over a decade later, was that patients who had had a corpus callosotomy, dividing their two hemispheres, did not seem to be too badly affected. Puzzled, he sought to find out what actually happens when the two hemispheres are prevented from talking to each other.

Roger Sperry
Roger Sperry, photographed for the 1986 yearbook of the California Institute of Technology. Image credit: Associated Students of the California Institute of Technology

The first experiments Sperry performed used animals that are not a common sight in today’s labs, but which played an im-purr-tant historical role in many fields of neuroscience research. That’s right – Sperry’s odyssey began with split-brained cats. 


In a frankly astounding feat of feline wrangling, the cats were trained to recognize two different wooden blocks, marked with a circle or a cross, while each of their eyes were covered in turn. When the cats were viewing the blocks with one eye, they learned that there would be food under the circle block; if they saw the blocks through the other eye, the food would be under the cross block. The cats were trained to push away the correct block with their paws to get at the food – an accomplishment most cat owners could only dream of.

This was all well and good, but when the cats’ eyes were both uncovered, they seemed unable to remember which block contained the treats, selecting both equally. Sperry suspected that, with the hemispheres unable to communicate, the cats were effectively trying to use two separate brains at the same time.

He then moved on to similar experiments in monkeys. This time, Sperry used a clever set-up of mirrors and projectors to allow the monkeys to keep both eyes open while looking at completely separate views through each eye. Instead of blocks, there were two different buttons for the monkeys to press, one of which would release a snack. 

Although the monkeys were effectively being expected to learn two tasks – with each eye seeing a different correct button – they mastered it in the time it would normally take to learn just one. This confirmed for Sperry that each hemisphere, when separated from the other, could become its own mini brain.


Finally, the experiments progressed to human subjects. To be clear, these were not people who fancied volunteering for a spot of random brain surgery; they were all people who had already undergone a corpus callosotomy, mostly due to epilepsy, as we mentioned earlier.

Sperry performed a series of experiments on his human volunteers over the next few years. One of the most important findings was to do with language.

A quick side-note here: one of the many ways in which the wiring of the human brain can seem counterintuitive is that the optic nerves leading from the backs of our eyes actually cross over. So, the left eye feeds into the right hemisphere, while the right eye feeds into the left hemisphere.

In one test, Sperry noted that the participants could only process and repeat a word they had been shown when it was viewed through their right eye. This suggested that the regions that allow humans to recognize and articulate language must be located in the left hemisphere.


To further this theory, the participants were asked to place their left hands into a box containing tools that they could not see. A word describing one of the tools was then flashed in front of their left eyes. When asked, the participants could select the correct tool from the box; but, the kicker is that they had no idea why they had selected that object, nor were they able to actually say the word that they had been shown.

This finding demonstrated that, while speech seemed to be left-lateralized, the right hemisphere did retain some rudimentary ability to recognize or read words.

We now know that Broca’s area – the region of the brain responsible for speech – does in fact sit in the left hemisphere of the brain for almost everyone (although, possibly not if you’re left-handed).

Swedish stamp from the 1980s commemorating Roger Sperry's Nobel Prize
Sperry's achievements were commemorated in Sweden with this stamp. Image credit: rook76/

Sperry’s results were momentous enough to win him the Nobel Prize in Physiology or Medicine in 1981 – although ironically, he did have to split (get it?) the prize with another pair of researchers. 


As the Nobel Assembly said at the time, “[Sperry] has provided us with an insight into the inner world of the brain, which hitherto had been almost completely hidden from us. With his discoveries of the specialization of both cerebral hemispheres he has given us an entirely new dimension in our comprehension of the higher functions of the brain.”

Not bad for a man who started out with an astute observation, a clever theory – and a cat.


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