When you add two halves, do you get a whole?

December 6th, 2022

Written by: Barnes Jannuzi

If you cut your brain in half, right down the middle, some bizarre things happen that you likely would not predict. But before we get into the amazing things to be learned, we need to address the elephant in the room. Why would anyone, under any circumstances, cut their brain in half?! If the idea of damaging your brain in any way sounds like a bad idea to you, that is probably a good thing. However, as surprising as it may be, surgically separating the brain in half is an effective, if outdated, treatment for patients with certain severe forms of the disease epilepsy. 

The brains of patients with epilepsy can be a battleground. Neural activity can spread like wildfire across the brains, often leading to terrible seizures that negatively impact their quality of life. This toxic level of activity travels down neural highways in your brain called white matter tracts. The largest of these highways is called the corpus callosum (figure 1A), and it has the task of connecting the left half (or “hemisphere”) of your brain to the right half. Severing of the corpus callosum destroys that connection, resulting in a split-brain. Cutting this connection effectively blocks that toxic level of activity from rapidly jumping from one hemisphere to another. This treatment substantially reduced the severity and frequency seizures. However, this sledge hammer solution also stops non-toxic activity as well. But how exactly does disruption of this communication impact brain function? Through their participation in research studies, several Individuals with a split-brain have given us incredible insight into how the two sides of the brain communicate.If you had a split-brain, you would not be able to read aloud the word “car” when they are shown just to the left of where you are looking, but have no trouble when it is shown to the right1. But that’s not all, you suddenly gain the ability to draw a different shape with each hand at the same time! The reason for this is specialization. You would not guess it by looking at the gelatinous pink mass that is your brain, but the left and right hemispheres of your brain are not symmetrical, and each has specialties that allow your brain to function.

You may have heard talk about being “right-brained” or “left-brained” as a way of saying “creative” or “logical” respectively. This is a myth. There is no evidence for a dominant side of the brain for individuals, nor has it been shown that more creative people have larger right brains. However, certain functions have been shown to be primarily localized to one half of your brain. Notably, for most of the population, language production is primarily carried out by the left hemisphere. Equally amazing to hemisphere specialization is that the hemispheres work together to coordinate everything your brain does into a single… you. By quickly and constantly communicating down white matter tracts, your brain is unified into a working whole. When the corpus callosum structure is severed that whole is broken, and by understanding what changes about the function of the brain when it breaks, we can understand what those connections were doing in the first place. 

When talking to someone with a split-brain you probably would not realize anything was strange at first. They could carry on a conversation, tell you about what they were thinking that day, crack some jokes, and talk about a job they successfully work. You could only notice anything different  when these individuals voluntarily participate in research studies that expose differences in how their brains function. For example, when subjects are asked to read words that are presented just to the right of where they are looking, they are able to do so without issue. However, when they are asked to do the same for words displayed on the left, they are unable to speak aloud what their brain is seeing1

To understand why the location of the word impacts whether or not a split brian patient can name it, you must consider two things.: 1) As we discussed above, language production is primarily done by the left brain. 2) Opposite of what you might expect, your left brain controls your right side, and your right brain controls your left side (figure 1A). In the word naming task, your left visual field (i.e., things shown to the left of where you are looking) is processed by your right brain, and your right visual field is processed by your left brain (figure 1B5). Normally, when you read a word in your left visual field, it makes its way to your left brain language centers by crossing the corpus callosum, which allows you to read that word aloud. However, when the left and right halves of your brain can no longer communicate, anything your right brain sees is locked in that half and can never be understood by the language producing parts of your left brain. Therefore, split brain patients can only read aloud words that are seen and read by their left brains!

If that is not amazing enough, things get even cooler when these patients have a pen to draw with. Before this next section, I urge you to check out this video from minute 2:52– 4:37 to see a patient with a split brain participating in a research study by Dr. Michael Gazzaniga. Patients with a split-brain are able to draw a picture with their left hand of the word they saw in their left visual field (and processed by their right brain). This may seem unexpected as this is the same word that they were unable to verbally vocalize moments before. What’s more is that after looking at the picture they just drew, they can verbally name it! (figure 2) Breaking this down step by step: 1) “phone” is shown briefly just to the left of where a patient is looking. 2) the right brain receives that information, but cannot name the object as that is a specialty of the left brain. 3) The right brain controls the left hand to draw a picture of a phone. 4) when looking at the drawing with the right visual field, the left brain can see and process the picture. 5) the language producing left brain identifies a word for the drawing and allows the patient to say “phone” aloud. At this moment, the paper and pen are functioning as the corpus callosum. They are the connection between the hemispheres of the brain. 

The above example clearly illustrates the value of coordination between the halves of the brain, but these remarkable patients also illustrate the potential cost of that communication, namely interference. When the two sides of your brain are both trying to do something very similar but not quite the same, they can cause interference with each other. To get an intuition of this for yourself you will need two pencils/pens and a piece of paper. Now, try to draw two squares like in (figure 3A) , using one hand for each shape and drawing them at the exact same time.

Now do this again with a circle and square (figure 3B). You likely found that 3B was much more difficult, or resulted in a square that resembled a circle, and a circle that resembled a square. This is because in 3B the motor signals your left and right brains were sending to your left and right hands were mismatched. This led to interference and caused your imprecise actions. It is also the reason it’s difficult to pat your head and rub your stomach at the same time. In minute 1:19-2:50 of the earlier video, we can first see a non-split brain individual attempt this task followed by an individual with a split-brained. The split-brain patient suffers no interference between hemispheres and thus has a much easier time completing the drawing.

Specialization of brain hemispheres, and connection between them is crucial for normal functioning. When we separate the halves of our brain, we lose the ability to do fundamental processes like reading words we see aloud. However, we also get a sense for the interference that can be caused by those connections. I will leave you with some parting questions to let your brain halves chew on: under the assumption that you, your consciousness, is rooted in your brain, who are you when your brain is split in two? Are you both? Are there now two conscious beings in one body working together… but not together3? Or is the way we think about consciousness now too limiting to confront this example4? These are just a few of the exciting questions that scientists are asking right now in an attempt to better understand our brains and ourselves.


  1. Gazzaniga, Michael S. “The Split Brain in Man.” Scientific American, vol. 217, no. 2, 1967, pp. 24–29. JSTOR, http://www.jstor.org/stable/24926082. Accessed 5 Dec. 2022.
  2. Sperry, R. (1982). Some Effects of Disconnecting the Cerebral Hemispheres. Science, 217(4566), 1223–1226. http://www.jstor.org/stable/1689416
  3. de Haan EHF, Corballis PM, Hillyard SA, Marzi CA, Seth A, Lamme VAF, Volz L, Fabri M, Schechter E, Bayne T, Corballis M, Pinto Y. Split-Brain: What We Know Now and Why This is Important for Understanding Consciousness. Neuropsychol Rev. 2020 Jun;30(2):224-233. doi: 10.1007/s11065-020-09439-3. Epub 2020 May 12. PMID: 32399946; PMCID: PMC7305066.
  4. Pinto Y, de Haan EHF, Lamme VAF. The Split-Brain Phenomenon Revisited: A Single Conscious Agent with Split Perception. Trends Cogn Sci. 2017 Nov;21(11):835-851. doi: 10.1016/j.tics.2017.09.003. Epub 2017 Sep 25. PMID: 28958646.
  5. “File:1420 Optical Fields.jpg.” Wikimedia Commons, the free media repository. 26 Sep 2020, 17:25 UTC. 5 Dec 2022, 15:05 <https://commons.wikimedia.org/w/index.php?title=File:1420_Optical_Fields.jpg&oldid=472316433>.

Cover image by Sylvester Sabo on Unsplash

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