Two minds, but a single brain

On the outside, our brain looks a lot like a walnut: two halves with twisted surfaces and a connective band in between. Both brain halves contain essentially the same components, but they are not identical. The size of the different components varies and dissimilarity also exists for the tasks each hemisphere is performing. During evolution, some specialisation of each hemisphere turned out to be advantageous.

The corpus callosum connects the left and the right hemispheres possible.  (© decade3d -

The corpus callosum connects the left and the right hemispheres. (© decade3d –

The left hemisphere controls the right side of our body and the right hemisphere controls the left side. For most brain functions, areas in both hemispheres are being activated. Information exchange takes place via the connective band, the corpus callosum, so both hemispheres can contribute together to the correct execution of a task.

In exceptional situations, this corpus callosum is being severed in a surgical procedure. This is done in people who suffer a severe form of epilepsy. When all other treatments have failed to counteract the seizures, an operation may be the last resort. By severing the corpus callosum and thus disconnecting the two brain halves, an epileptic seizure won’t spread out over the entire brain, but will stay limited to the one hemisphere where the seizure started. In most cases, the operation does indeed decrease the seizures. Sometimes, they even disappear completely.

The brain of which the corpus callosum is severed partly or completely, is called a split-brain. Although the name of the condition leads one to suspect otherwise, there is no connection whatsoever between a split-brain and a split personality. The latter is the popular name for a psychiatric disorder called dissociative identity disorder. People suffering this disorder have at least two distinct and relatively enduring identities that alternately control their behaviour. It is a controversial disorder with some psychiatrists denying the existence of split personality. Scientific research into this topic is needed. But split personality patients certainly don’t have a split-brain.


This is how the visual system works. Images from the right visual field are processed in the left hemisphere and vice versa. (courtesy The brain from top to bottom)

Daily life
What does it mean for the owner of the brain when the corpus callosum is severed? How does each hemisphere function when it’s on its own and can’t consult or cooperate with the other half directly anymore? Exactly the same, say the patients with a split-brain. Not quit so, say the neuroscientists who examined the patients before and after the surgical procedure. Both hemispheres sort of start living their own lives. But the owner of the brain hardly notices it in daily life. The brain of split-brain patients develops strategies to get around the absence of information transfer between the two hemispheres. The difference only will surface during specialised research, in which each hemisphere separately has to execute tests.

Thanks to split-brain research, we have come to know a lot about the division of tasks between the two hemispheres. In general, the right hemisphere is dominant for spatial relationships and the left hemisphere is dominant for language. This goes true even for the vast majority of left-handed people. Since the right hemisphere controls the left hand which left-handed people use for writing, it would be expected the right hemisphere would also be more involved in their language capacity. But that is not so.


If the name of an object is projected so that the split-brain patient sees it with the right hemisphere only, he will say that he doesn’t see anything, because the severed connection has in fact prevented his left hemisphere, which is dominant for language, from doing so. But if the experimenter then asks the subject to use his left hand to choose a card with a drawing of the object of which he saw the name, or to identify this object by feeling it with his left hand, he will have no problem in performing the task. The right hemisphere clearly can recognise words although it cannot express itself in complex sentences. (courtesy The brain from top to bottom)

After split-brain surgery, when tested in a laboratory setting, patients can only produce speech when their left hemisphere is approached. With the passing of time, a few split-brain patients learn to produce speech not only in the left, but also in the right hemisphere up to a limited capacity. This is a good example of the flexibility of the brain. After all, both hemispheres contain all the necessary components, but they haven’t developed to the same extent. So, the right hemisphere also possesses the language centers, but in a more rudimentary form. When stimulated these components could still develop. Sometimes, this reorganisation within the brain takes as long as ten years, sometimes it never happens. The brain’s flexibility is greatest in children who’s brain hasn’t developed completely yet.

A very exiting discovery in the split-brain research is that each hemisphere seems to have its own, independent consciousness. Since the hemispheres are not connected and cannot consult one another, they both come up with their own explanation of an activity.

That story is not unambiguous. The right hemisphere appears to stick to the facts to a greater extent, whereas the left hemisphere loses itself quickly in speculation to try to make a logical whole out of events.


In this experiment, the patient’s left hand is pointing at the card with a picture of a snow shovel, because the right hemisphere, which controls this hand, has seen the projected image of a winter scene. Meanwhile, his right hand is pointing at the card with a picture of a chicken, because his left hemisphere has seen the image of a chicken’s foot. But when the patient is asked to explain why his left hand is pointing at the shovel, his talking hemisphere— the left one —has no access to the information seen by the right, and so instead interprets his behaviour by responding that the reason is that you use a shovel to clean out the chicken house. (courtesy The brain from top to bottom)

This has been demonstrated in beautiful laboratory settings managed by the most famous split-brain researcher Michael Gazzanigga. He made sure the left hemisphere could not see what the right hemisphere was doing. When asked why the left hand – controlled by the right hemisphere to which the left hemisphere had no access – was picking up a certain object from the table, as requested by the researcher, the language center in the left hemisphere confabulated a nice story to make the facts fit.

So, split-brain is not only the term used for people having undergone the operation to sever the corpus callosum, but is also in fact the real consequence of the procedure. But even people with a normally functioning corpus callosum suffer the consequences sometimes of a discrepancy between the two hemispheres. We all have learned from experience you cannot always accept indiscriminately the nice words the language centers in the left hemisphere produce. And those memories that seem to be engraved crystal-clear in our brain, should also be looked at with distrust. A memory is not a photograph of the moment, but is biased bij the ‘interpreter’ in the left hemisphere.

“The left hemisphere interpreter tries to make sense of the world around us.”
Michael Gazzanigga, neuroscientist

The possibility could exist, in normally cooperating hemispheres, the sober right hemisphere keeps the unbridled interpretations of the left hemisphere in check to a certain degree. This does not mean it would be useful to accept invitations for training programs to find out which of the two is your dominant hemisphere, to develop your right hemisphere more, or to bring both your hemispheres in balance. So far, no scientific evidence has come up to corroborate the improvements these programs promise.


Making up stories © Pete Saloutos –

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