What if the two halves of your brain were disconnected? — The Split Brain Paradox
What if I told you that each half of your brain has its unique beliefs, skills, and personality traits.
Most people will agree that they trust themselves the most. I would think I can trust my brain more than I can trust someone else’s. After all ‘Seeing is Believing’. But is it really? Simple optical illusions show how easily our brain is fooled. In fact, not only is our brain easily fooled, it also fools us just as well. Our reality is an illusion created by our brain. For example, anatomically speaking there should be a big black spot in the center of our vision because of blind spots in both our eyes. But we see a smooth, continuous visual field instead as our brain fills in the black spot . Also, there’s only a tiny spot in our retina called fovea that captures the most clear image of our visual field. It’s hardly enough to form a full clear picture. To compensate, our eyes dance back and forth in what’s called saccades to capture more information. Even anything we smell, taste, or touch is first sent to our brain, where the information is broken down then our brain feeds it back to our conciousness. Just how disconnected we are not only from the outside world, but also from our own inner world becomes evident in certain situations where the communication between our hemispheres is severed.
In the 1960s, a young neuroscientist named Michael Gazzaniga began a series of experiments with split-brain patients that would change our understanding of the human brain forever. Working in the lab of Roger Sperry, who later won a Nobel Prize for his work, Gazzaniga discovered that the two halves of the brain experience the world quite differently. These experiments on split-brain patients by Dr. Sperry and Dr. Gazzaniga shed insights onto the separate consciousness and personalities of the two hemispheres. One of the fundamental facts of split-brain research is that you can apply all findings to anyone. You can theoretically take any person with a healthy brain and disconnect the hemispheres and all of a sudden you have two consciousnesses.
Dr. Gazzinaga and Dr. Sperry’s experiments were some of the most interesting concepts I learnt about as an undergrad student of Neuroscience. It generated a whole another level of curiosity in young me that I hope to replicate in the readers of this blog. In this blog, I explain some hemispheric specializations such as speech production, facial recognition, sensory and motor processing. I will also explain how visual information is separated into each hemisphere at the optic chiasm according to visual field. I will then go over research done by Dr. Sperry and Dr. Gazzaniga on split brain patients who have had their hemispheric connection severed in order to treat uncontrolled seizures. The results of these experiments were mind blowing and the insights gained from them are invaluable.
You have probably heard before about the whole left-brain vs right-brain specialization or lateralization. Left brain is more analytical and logical. It excels in scientific and math skills. It is also dominant in language such as speech and writing. Right brain on the other hand is more holistic and intuitive. It excels at spatial and non-verbal tasks. It is also great in recognizing objects and faces as well as in timing things. It is dominant in emotions such as humor, empathy and depression. Left brain controls movements of and feels sensations from the right side of body and right brain does with left. This is evident by the thin strip of motor and sensory cortex on both hemispheres each containing a map of opposite half of body. Although it should be noted that details about what object we are touching are sent to contralateral (opposite) hemisphere whereas general touch information (like whether you are touching something or not) is sent ipsilaterally (same side).
Our visual field is split in a similar manner where information from right visual field is processed in left hemisphere and vice versa. But it’s important to note that each eye has both left and right visual field so half the information from an eye goes to same side of the brain while the other half crosses over in optic chiasm and goes to the opposite side. Essentially this boils down to visual field closest to nose crossing over to opposite contralateral hemispheres and peripheral visual fields getting processed in ipsilateral hemispheres. If you think about it, it makes sense that information is split by visual field instead of by eye. Evolutionary speaking, if we were to lose an eye it would be even more detrimental if one hemisphere got absolutely no visual information. You would be wasting all the brain space just because you lost an eye. But by splitting information based on visual field, even if you lose an eye both hemispheres still get some visual information.
The most important hemisphere lateralization is language. Language including speech production and comprehension is localized excusively in the left hemisphere for at least 95% of the people. This includes left-handed people as well. These language areas includes Broca’s area which controls speech production and Wernicke’s area which controls understanding of language. There’s also the Arcuate Fasciculus which connects both areas and processes information from both. These are all located in left hemisphere. What this means is that in patients with disconnected hemispheres if you send information to left hemisphere you can get the patient to talk about it. But if you send the same information to the right hemisphere the patient cannot talk about it as the speech is solely controlled by the left hemisphere.
This lateralization of left and right hemisphere roles was done by Dr. Robert Sperry studying split brain patients. Insights from these experiments won him the Nobel Prize in 1981.
Despite these differences in roles, each hemisphere works together to analyze various inputs and give a single output. For this, it is important for them to communicate and coordinate their actions. And that is the role of corpus callosum. Our brain is divided in gray and white matter. Gray matter comprises cell bodies of neurons and this is where all the data processing happens. White matter is the axons and this is where information relay happens. If you cut open a brain, you will see outer gray cortical areas connected to the inner gray cortical areas with highways of white matter. Corpus Callosum is the largest white matter area. It’s a highway of neuronal fibers connecting both hemispheres. This special part of the brain is unique to placental mammals.It opens up a line of communication between our two hemispheres. You can’t see the corpus callosum without cutting open the brain. But if you sliced it in half dividing the two hemispheres you can see it as it is in the image below:
CORPUS CALLOSOTOMY — Severing of the corpus callosum
Before the time of Dr. Perry and Dr. Gazzaniga, it was already known that certain epileptic patients with uncontrolled seizures got relief if the corpus callosum was disconnected. Seizures are uncontrollable bursts of neuronal firing. In certain types of seizures these uncontrolled neural firing spreads causing other neurons to do the same. This ends up spreading back and forth between the two hemispheres. The way to stop this is to severe the corpus callosum. These extremely effective pioneer surgeries were conducted by Dr. Andrew Akkelaitis. Because the the main goal of the surgery was elimination of seizures any personality changes were not given much attention and went unnoticed for a couple decades. Also these personality changes were very subtle and someone meeting a split brain patient might not even notice something was wrong. It’s only when you put them in certain specific situations and tested them you would see that they have these two separate hemispheres were acting independently of each other.
When Dr. Perry and Dr. Gazzaniga started experimenting on rats and monkeys at Caltech, it became evident that severing this connection did in fact have an effect on behavior and learning abilities. They had many clever ways of disconnecting the hemispheres without actually permanently severing the corpus callosum such as applying potassium soaked pads to one hemisphere or injecting numbing agent in carotid artery to put one hemisphere to sleep. This was extremely helpful as you could repeat the experiments in the animals once the connection was reestablished. Unfortunately or fortunately such experiments woud have been unethical in humans. The only time these reversible tests are done in humans are before a patient undergoes a corpus callosotomy to treat seizures. This is known as a Wada test.
A Wada test is done to evaluate how important each side of the brain is with respect to language and memory functions. This is important information to gain if a patient is going to lose connection between both hemispheres. In the test an x-ray dye is injected in the brain. Next a barbiturate class medication is injected to put one hemisphere to sleep. Generally, the Wada test starts in the morning and the patient is ready to go home by mid-to-late afternoon. When the drug reaches the side of the brain under study, the patient will temporarily lose all strength on the opposite side of the body. Sometimes only one side is tested; other times, one side is tested first and then the other after a short interval.
THE EXPERIMENTS
These are all examples of experiments done by Dr. Gazzaniga and Dr. Sperry in patients with severed corpus callosum. These patients underwent a two-stage resection of the corpus callosum for relief of intractable epilepsy. Complete sectioning of the corpus callosum was confirmed by MRI. Post-surgical MRI also revealed no evidence of other neurological damage. We have these patients to thank for all the insights we have gained in this field.
Keeping track of which hemisphere is processing what information can get confusing so I will try to remind most times but here’s a helpful tool:
-Information from left visual field gets processed in right hemisphere
-Information from right visual field gets processed in left hemisphere
-Touch information from left hand gets processed in right hemisphere
-Touch information from right hand gets processed in left hemisphere
-Left hemisphere has control over language and does the talking
-Right hemisphere can communicate using specific gestures made by left hand
TOUCH
BANANA AND FORK — The patients were blindfolded and given a object to hold in each hand. On the right hand patients were holding a fork. Because of contralateral connection of touch, this information was sent to the left hemisphere. Then patients were asked what they were holding. Patients correctly responded that they were holding a fork using the speech center of the left hemisphere. On the other hand, literally, when a banana was placed in the left hand, the right hemisphere was be unable to express what object they were holding. Note that the right hemisphere knows it’s a banana but it cannot speak as it does not control language production. So, how do we know that the right hemisphere really knows that the left hand is holding a banana? Both fork and banana were placed in front of the patients. They were then asked to draw the object they were holding with their left hand, they drew a banana (since right hemisphere controlling the left hand saw a banana). If the patients were then asked why they drew a banana they would make up some excuse for this behaviour as now you are the asking the speech center which resides in left hemisphere for a reason that only right hemisphere knows. In most cases the reason was that “oh the banana is easier to draw with left hand than fork.” Since our left brain is analytical and logical, everything needs to make sense to it. It doesn’t know why it drew a banana when it actually saw a fork, but there has to be a reason. So, it will make up some excuse that made logical sense to it. That’s just something the left hemisphere seems to be really good at — coming up logical explanations for behavior.
VISUAL
SCREWDRIVER AND KEY- Remember our visual pathway diagram above? In patients with severed corpus callosum but an intact optic chiasm information of visual field still crosses over. So, by carefully displaying an image in a certain part of visual field for a few milliseconds, we can control which hemisphere the information is presented to. You ask the patients to focus on a dot at the center of a screen. This is important because if they move their eyes around information will get sent to both hemispheres. That’s how they do it in normal everyday life and the reason why initially no big changes were noticed in patients undergoing a corpus callosotomy. So, you have them focus in the center and then very briefly display images on the screen just to the right or to the left of the center dot. Anything displayed to the right will get processed in the left hemisphere and vice versa. Then we could ask the patient what he/she saw and gain insights on the information in each hemisphere. Let’s say we display a screwdriver on the right and a key on the left then ask the patients what they saw. The patients will say that they saw a screwdriver because that’s all that the left hemisphere saw. The right hemisphere which saw the key cannot communicate. You could then ask the patients to draw the object with the left hand. The patients will then rightfully draw a key. Again if you asked the patients why they drew a key, the clueless left brain will come up with some excuse for why it drew a key when it actually saw a screwdriver.
WINTER HOUSE AND CHICKEN CLAW — Another example of the left brain making an excuse for behavior is seen in the chicken claw and shovel experiment. In this experiment, a split-brain patient was shown a house in winter time in left visual field and a chicken claw in right visual field. Right after this a series of pictures was placed in front of the patient and he was asked to pick an image with either right or left hand. The images were set up so the choices would be obvious for patients. Two of the images placed in front were a shovel corresponding to the snow in the winter house image and a chicken head corresponding to the chicken claw. The other images placed in front did not relate to the house or the claw in any way. The patient picked the snow shovel with his left hand which is what his right hemisphere made him do. The image processed in the right hemisphere was that of the house in winter so this made sense. With the right hand he picked the chicken head as the left visual field was shown the chicken claw. But when he was asked to give reason for his choices, he said “The chicken claw goes with the chicken head, and you need a shovel to clean out the chicken shed.” Why would he say this when it is obvious the shovel is for plowing the snow in the winter house image? Because his left talking brain was not aware of the right hemisphere being shown this house in winter. So, the left brain had to make up a logical excuse for the patient’s behaviour.
FACES MADE OF FRUITS — One of the tasks right hemisphere excels at is recognizing faces. In one of the experiments by Dr. Gazzaniga, patients were shown paintings of Arcimboldo who painted faces mades of objects such as fruits, flowers, books etc. If asked the left hemisphere what it saw it would respond with object descriptions. But if right hemisphere was asked to point between the objects or faces, it would recognize the paintings as faces and point to face.
Because of experiments by Dr. Sperry and Dr. Gazzaniga we know that corpus callosotomy were not exactly without lifelong side-effects. And so now the severing on corpus callosum is only used as a last resort treatment of life disrupting seizures.
IMPLICATIONS
Interestingly, similar experiments of questioning individual hemispheres has also shown differences in personalities and beliefs of people. In his book, ‘Tales from both sides of the brain’, Dr. Gazzaniga talked about having conversations with the right brain of his split-brain patients using scrabble letters. He began by asking the patient’s left brain what he would do after graduation. The patient replied that he wanted to be a draftsman. But things got interesting when the right brain was asked the same question. The right brain spelled out the words: “automobile racer.” The right brain held a different plan for the future, unbeknownst to the left brain which was dominant. The right brain literally has a mind of its own. According to the science write Rita Carter, “The possible implications of this are mind-boggling. It suggests that we might all be carrying around in our skulls a mute prisoner with a personality, ambition, and self-awareness quite different from the day-to-day entity we believe ourselves to be.” Perhaps there is truth to the oft-heard statement that “inside him, there is someone yearning to be free.” This means that the two hemispheres may even have different beliefs. For examples, the neurologist V.S.Ramachandran describes one split-brain patient who, when asked if he was a believer or not, said he was an atheist, but his right brain declared he was a believer. Apparently, it is possible to have two opposing religious beliefs residing in the same brain. Ramachandran continues: “If that person dies, what happens? Does one hemisphere go to heaven and the other go to hell? I don’t know the answer to that.”
Although most of us have an intact corpus callosum and everything happens in all areas of our brain as the hemispheres share information with each other. There are also examples of few split-brain patients where individual hemispheres started learning tasks of each other. For example, right hemisphere learning to speak.
Do these experiments really prove that there are two different consciousness in our brain. From the experiments where the left brain makes logical explanations for reality, it does seem that the left hemisphere is responsible for our conscious interpretation of the world. It also seems that when separated, the two halves seem to act like two very different people. This seemingly dual consciousness is also evident in split-brain patients with Alien Hand Syndrome. It is an interesting situation in which a person loses control of his or her hand, which starts to act independently. It describes involuntary complex goal-directed activity of one limb. A patient complained of observing her left hand moving without her knowledge while watching television. Her left hand stroked her face and hair as if somebody was controlling it. When one split-brain patient dressed himself, he pulled his pants up with one hand (that side of his brain wanted to get dressed) and down with the other (this side did not). He also reported to have grabbed his wife with his left hand and shaken her violently, at which point his right hand came to her aid and grabbed the aggressive left hand. However, such conflicts are very rare. That being said, we still cannot with certainty derive existence of dual consciousness based on these case studies. Consciousness itself is something that neuroscientists are still trying to compherehend. We can’t have a subjective experience of what’s going on inside the mind of someone with disconnected hemispheres. We can only form objective opinions. That being said we need more novel experiments in this area as the true nature of our minds’ consciousness is definitely worth exploring.
Hope you find this blog informative as well as interesting.
