Split brain: are we two brains and one mind?

Evelina Petitto

“Scientists have often wondered whether split brain patients, who have had the two hemispheres of their brain surgically disconnected, are ‘of two minds’” (Zilmer, 2001)

It is quite trivial to say: we only have one brain. However, our brain is composed of two hemispheres, each one specialised to perform certain tasks and process certain pieces of information. These two parts of the brain normally interact and cooperate in our everyday life, but what happens when they can’t communicate with each other?

This is what happens in the so called split brain patients, whose hemispheres are surgically disconnected with a procedure called corpus callosotomy. This procedure is usually undertaken as the last resort to treat severe epilepsy, and consists of the dissection of the corpus callosum, the structure in the brain where the connections between the two hemispheres are; as a result, the left and the right brain can process information and behave independently from each other. The consequences of the procedure can sometimes affect the everyday life of the patients, who can develop a split personality. For example, one of them reported her difficulties in completing tasks each one of us normally accomplishes without even thinking about it, such as choosing what to buy in a grocery store or getting dressed. In fact, this patient described how her left and right hand were competing with each other, each one of them trying to decide on its own what to do: she consciously wanted to buy something, reached it with her right hand to put it in the trolley, whilst the left hand put it back where it was on the shelf. Similarly, her left and right hands also choose different dresses to wear, and she often found herself wearing two or three outfits at once, before throwing everything on the bed and start to get dressed again. Another patient had difficulties in getting dressed too, with his hands not agreeing on what to do: one hand would button his shirt, the other would unbutton it. Another one, tried to strike his wife with his left hand, while the right hand grabbed the left to stop it. Although curious and fascinating, these disorders are quite rare among split brain patients, and tend to disappear with time.

Nonetheless, split brain patients have been very important for neuroscience. When tested in experimental settings, in fact, they appear to have two separate consciousnesses; in addition, they helped to prove that the two hemispheres of the brain have different specialisations. To understand the results of the experiments we will describe, it is important to have a general idea on how brain and body interact; the left part of our body is controlled by our right brain and vice versa. So, our right brain moves the muscles on the left and the left brain moves the muscles on the right; in the same way, sensitive information from the left part of our body are processed by the right brain and those from the right part are processed by the left brain. Our brain usually unifies the information it receives through the connections via the corpus callosum. However, in split brain patients this can’t happen, and the information processed in one hemisphere can’t be shared with the other one. So how do split brain patients perform in artificial settings able to stimulate only one hemisphere at a time?

The two main types of experiments performed on split brain patients are visual and tactile. In visual experiments, patients are presented with a series of visual stimuli in the left or in the right visual field (so that they couldn’t see them with both eyes) and are asked to ring a bell when they see a stimulus. All patients report correctly the fact that a stimulus is presented, however they can verbalise what they saw only when the stimulus is presented in the right visual field (and therefore processed by the left side of the brain). When the stimulus is presented in the left visual field, the patients simply respond that they didn’t see anything, despite the fact that they correctly rang the bell. When asked to choose a random object among others, however, they always choose the one correspondent to the stimulus presented (but without consciously knowing the reason), or, in the same way, they are able to draw it. It is to notice that both the choice of the object and the drawing have to be performed with the left hand. In fact, we can say that the right hand doesn’t know what the patient saw: the stimulus was presented to the left eye, the information processed in the right brain, and the right brain controls the left muscles of the body, that can activate to produce the according behaviour (pointing or drawing).


(from:Split brain: a tale of two halves. http://www.nature.com)

Similarly, in tactile experiments the patients hold an object with one hand without being able to see it. When the hand touching the object is the right one, patients can easily name what it is. On the contrary, when they touch the object with their left hand, the patients can’t say what is that they are touching. At the same time, however, they can choose with the left hand the object correspondent to the one they were touching among other distractive stimuli. These experiments importantly helped to prove that the left hemisphere is dominant for language and speech. Furthermore, they also point out another intriguing fact: although the two hemispheres behave independently and don’t communicate with each other, the patients don’t lose their unified sense of self. To explain this phenomenon, Gazzaniga (the pioneer investigator of split brain together with Perry) formulated the “interpreter theory”. When asked to explain with words actions performed by the left part of the body, split brain patients make up stories to justify their behaviour. In one example, the word “smile” was presented to the right hemisphere of a patient, and the word “face” was presented to the left hemisphere. When asked to draw what he saw, the patient drew a smiley face; asked why he drew that, the patient responded that nobody likes to see a sad face. Another time, the picture of a naked man was presented to the right hemisphere of a patient. The patient (who was a girl) started to laugh but she couldn’t explain why; she then said she was probably laughing because of the machine that projected the images. Even more significant for this theory are the results from a slightly more complex experiment. Two different pictures were projected at the same time, one to the left brain, the other to the right. The patient then has to point at the picture that has a connection with what they saw on the screen. In one of these experiments, the image of a winter scene was projected to the right hemisphere, while a chicken’s foot was presented to the left one. When asked to choose a picture, the left hand correctly chose a snow shovel, whilst the right hand correctly chose a chicken. But when the patient was asked about the reason his left hand was pointing at a shovel, he answered that the shovel is used to clean the chicken’s house. In other worlds, his left hemisphere, which didn’t have any access to what the right one had seen, observed what the left hand was doing and then tried to interpret that behaviour according to the information that it had. These experiments showed that the left brain is the interpreter and narrator of reality, what people use to order the information we receive and create narratives that give order and meaning to the word.

For more informations: https://www.youtube.com/watch?v=aCv4K5aStdU


One thought on “Split brain: are we two brains and one mind?

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