The man who did many of the experiments that first proved this was neurobiologist, Roger Sperry.
In a particularly striking experiment, which he filmed, we can watch one of the split brain patients trying to solve a puzzle. The puzzle required rearranging blocks so they matched the pattern on a picture.
First the man tried solving it with his left hand (controlled by the right hemisphere), and that hand was pretty good at it.
Then Sperry asked the patient to use his right hand (controlled by the left hemisphere). And this hand clearly did not have a clue what to do. So the left hand tried to help, but the right hand did not want help, so they ended up fighting like two young children.
Experiments like this led Sperry to conclude that “each hemisphere is a conscious system in its own right, perceiving, thinking, remembering, reasoning, willing, and emoting”.
In 1981 Sperry received a Nobel prize for his work. But in a cruel twist of fate, by then he was suffering from a fatal degenerative brain disease himself.
Most people who have had their corpus collosum cut appear normal afterwards. You could cross them in the street and you would not know anything had happened.
Karen was unlucky. After the operation, the right side of her brain refused to be dominated by the left.
She has suffered from Alien Hand Syndrome for 18 years, but fortunately for Karen her doctors have now found a medication that seems to have brought the right side of her brain back under some form of control.
The left hemisphere, which controls the right arm and leg, tends to be where language skills reside. The right hemisphere, which controls the left arm and leg, is largely responsible for spatial awareness and recognising patterns.
Usually the more analytical left hemisphere dominates, having the final say in the actions we perform.
The discovery of hemispherical dominance has its roots in the 1940s, when surgeons first decided to treat epilepsy by cutting the corpus callosum. After they had recovered, the patients appeared normal. But in psychology circles they became legends.
That is because these patients would, in time, reveal something that to me is truly astonishing – the two halves of our brains each contain a kind of separate consciousness. Each hemisphere is capable of its own independent will.
When this does not work, or when the damaged area cannot be identified, patients may be offered something more radical. In Karen’s case her surgeon cut her corpus callosum, a band of nervous fibres which keeps the two halves of the brain in constant contact.