Most of us–bar a very few*–have two cerebral hemispheres in our brain. Our two cerebral hemispheres are connected by the corpus callosum, a fibrous tissues estimated to have around 200 million neurons.
In treating some cases of otherwise intractable epilepsy surgery is performed to cut the corpus callosum, leaving the person with what we might think of as two half brains – hence my title. (More accurately two cerebral hemispheres that no longer interact with each other via the corpus callosum.)
There is a lot of study of the consequences of the absence of a corpus callosum for brain function, both in patients of surgery and those that naturally lack a corpus callosum. Today I have been listening to Associate Professor Liz Franz speaking about her work investigating handedness. During her talk she reminded the audience that while with practice we can do different things with each hand simultaneously, when pushed we prefer to do the same thing – like the patting your head while rubbing circles on your stomach thing. (You can see a photo of Liz doing this in the link I gave earlier!)
I’d love to give you a run down on the ins and outs of work involving the corpus callosum, it’s fascinating stuff. Being short on time, I’ll instead bring you the video she showed a brief portion of in her talk. In it Alan Alda, of M*A*S*H fame,** presents in Scientific American Frontiers ten minutes on what they’ve titled the Severed Corpus Callosum. He talks with a patient of the surgery I mentioned and compares their performance on tests for independent cross-hemisphere activities. (Hint: the guy without a corpus callosum does better.)
* Aside from those who have undergone a hemispherectomy–removal of one hemisphere of the brain–there are rare cases of people with just one hemisphere.
** While he is best known for his M*A*S*H series, he has done extensive work on science communication programming too.