I love saying Corpus Callosum out-loud. It sounds like the name of a gladiator arena or a gigantic cyclops. But it actually means “tough body” and it’s the bundle of brain fibres that connects the two hemispheres of your brain. If you chopped open your skull and stuck a finger between the two hemispheres of your brain, you could easily push all of the way down until you hit the corpus callosum. But that would be gross so don’t try that at home.

The corpus callosum is the bridge that facilitates communication between the two hemispheres. So when you see a written word in your left field of vision (if you’re right-handed), the trolls on the right side of your brain (which processes your left field of vision), quickly run that word down to the corpus callosum to transfer it over to the left side of your brain where you do language processing.

This back and forth is going on all of the time across the 225 million axons (give or take a couple million) bundled into the corpus callosum. If we didn’t have it, we’d only be able to consciously read, for instance, with the right field of our vision. These male birds, for instance, because they lack the corpus callosum, can only check out girl birds (or male birds if they’re so inclined) with one eye: “One-eyed Wooing”. But, on the positive side, we might be able to stay up indefinitely. A component of what enables dolphins to put one side of their brain to sleep while they other watches for sharks might be because they tend to have a narrow or even non-existent corpus callosum: “How Dolphins Stay Awake for Two Weeks Straight

I said “consciously read” back there because your brain knows way more than you know. So just because something goes in one hemisphere and doesn’t make it across the corpus callosum to the part of your brain that normally processes it, doesn’t mean you don’t “know” it. But this is going to fry whatever you thought you knew about “knowing”:

When neuroscientist Michael Gazzinaga was a grad student, he was involved in studies on patients who had received a corpus calloscopy—when the corpus callosum is severed to help prevent seizures. Researchers would show a picture to the right hemisphere of the brain only, and then ask the patient what he saw. But because the picture of the bicycle never made it to the left side of the brain where the word for bicycle was, the patient would say that he saw: “Nothing”. However, the researchers found that the patient could use his left hand (controlled by the right hemisphere) to draw the bicycle. Half of his brain knew what he’d seen and could use half of his body to express that. Learn more about that research in this fascinating New York Times article on Dr. Gazzinaga: “Decoding the Brain’s Cacophony”.

Your corpus callosum is doing a lot of work to help prop up the illusion that you have a unified self up in that brain of yours. Let’s hope it keeps up the good work.

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