What happens if you cut a brain in half?

Split brain photo 1

This brain has been cut away to leave the corpus callosum visible in the centre.

During an epileptic seizure, brain cells fire at a higher rate than normal and begin to synchronise with each other, leading to a wave of abnormal activity which spreads through the cortex and disrupts normal functioning. It can be difficult to control, even with modern medicine, and the drugs can have unpleasant side-effects, but today’s interventions are nothing in comparison with the experimental treatments carried out in the 1940s by William Van Wagenen, who sliced his patients brains in half right down the middle! He cut along the corpus callosum, a dense bundle of nerve fibres which connects the two hemispheres of the brain and allows them to communicate with each other, reducing the frequency and severity of seizures, and their spread to the other hemisphere.

Information from the left side of the visual field crosses over into the right side of the brain, and vice versa.

Information from the left side of the visual field crosses over into the right side of the brain, and vice versa.

The patients appeared remarkably normal, despite having two almost isolated brains in the same skull, and apart from occasional language difficulties (and sometimes alien hand syndrome) were able to lead independent lives. It is only once we restrict the sensory input to a single hemisphere that we can see the bizarre reality of what is going on in the split-brain person’s head. The brain is wired so that the left hemisphere has control over the right half of the body, and receives visual input from the right side of the field of vision, while the opposite is true for the right hemisphere. If an object is flashed on the right side of a screen, entering the left hemisphere, the person can easily name it, as it is the left hemisphere which is most important for language and vocabulary. If an object is presented on the left side, however, entering the right hemisphere, then the person will claim that they saw nothing at all! If a pen is put in their left hand, then they will be able to draw the object they saw, and can then look down at the drawing and name it. This bizarre situation emerges because the right hemisphere cannot express itself verbally, and only the left hemisphere could answer the question with words. Only once the left hemisphere has seen the right hemisphere’s non-verbal answer can it answer the question verbally!

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Tobias Webber


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