The bridge between the halves
Cut down the middle, the brain shows a white arch between the halves, the corpus callosum. It is about 10 centimetres long and carries some 200 million nerve fibres from each hemisphere to the matching place in the other, so that the two halves work as one brain.
Median section of the brain: the corpus callosum arches over the third ventricle, with the pons, medulla and cerebellum below, Gray's Anatomy, 20th edition, 1918, figure 720Wikimedia Commons, Henry Vandyke Carter, Public domain (Gray720.png)
In the 1960s surgeons cut it in a few people whose epileptic seizures spread from one half to the other. Roger Sperry tested them. A word shown only to the right hemisphere could not be spoken, since speech lives in the left, yet the left hand could pick out the object it named. Sperry shared the Nobel Prize in 1981.
Sources
- Susan Standring (ed.), Gray's Anatomy, 42nd ed. (2020), chapter on the cerebral hemispheres.
- Roger W. Sperry, Some effects of disconnecting the cerebral hemispheres, Nobel lecture, 8 December 1981.
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