Your Two Brains

Most brain functions are localized within specific areas of the cortex. Some functions are bilateral (active on both sides), while many are unilateral. For example, the ability to speak is a unilateral function, located just behind your left temple in Wernicke’s area. Motion and sensation are also unilateral, with the left hemisphere controlling the right side of the body and vice versa. Vision, hearing, and the sense of smell are similarly unilateral, with each sensory field processed on just one side. To date, we have identified few abilities that are completely unilateral. This is not to be confused with the popular notion of “left-brained vs. right-brained,” which is largely a misconception; rather, it reflects the localization of function, not ability.

This specialization means that individuals with Split-Brain continuously process two distinct sets of sensory input. Consequently, they also generate two separate motor outputs, granting them some unique capabilities. With practice, Split-Brain individuals can simultaneously draw different images with each hand. If that doesn’t impress you, give it a try—it’s nearly impossible for those of us with intact communication between brain hemispheres. Each side of their body can perform different, intricate motor tasks because each hemisphere can send independent messages. Our brains synchronize both sides, but theirs cannot.

Due to this lack of communication, peculiar phenomena can arise within the somatosensory system. We’ll examine two categories of these anomalies: touch and smell. Because of this split, odors entering the right nostril cannot be communicated to the left side of the brain. If a scent originates on the right, the individual cannot articulate its name, as that information cannot reach the vocal center on the left. They may recognize the scent, but it requires the molecules to enter the left nostril before they can verbalize its name. Conversely, with touch, the situation is reversed. If an object is held in the left hand, without visual confirmation, the person would be unable to name it. Sensory fibers from the left cross to the right side of the brain, where they lack access to the vocal centers. Again, recognition is possible, but verbalization is not.

The repercussions of this split extend beyond speech capabilities. Many aspects of normal functioning would also be altered or lost. One of the most notable cases of split-brain is Joe, the one-named individual (to protect his identity) featured in an Alan Alda Scientific American special from 2002. He underwent corpus callosum removal to manage epileptic seizures, resulting in split-brain syndrome. Overall, he leads a largely typical life and has been a key subject in much of the split-brain research conducted in the U.S. in recent decades. In the special, the physician utilizes Joe’s unique insights into the unilateralization of the mind to demonstrate a special ability. The video below provides a clearer illustration than words can convey, but I will summarize it as follows.

Joe's unique perception of art due to his split-brain condition.

When Joe is shown works by Giuseppe Archimboldo, a Renaissance painter renowned for creating images of fruits and vegetables arranged to resemble human faces, he perceives platters of fruit and vegetables when the images are presented to the left side of his brain. However, when the same images are shown to the right side, he sees faces. The unilateral nature of facial recognition means he can only identify the face if it appears in the correct hemisphere of his brain. When both hemispheres view the image, he can recognize both the fruit and the face, just like anyone else.

Despite the challenges, living without a corpus callosum is surprisingly manageable. There seem to be relatively few adverse effects from this brain portion's absence, although it does alter one’s perception of the world. Consequently, it is a common procedure for treating certain types of epileptic seizures. Removing the corpus callosum prevents seizure signals from traversing between the hemispheres, thereby reducing their intensity and enhancing the patient’s quality of life. If you were to lose your corpus callosum today, you would certainly notice the difference, as would those around you. However, over time, you would adapt and continue your life with two distinctly functioning brains.