An unusual situation occurs in the realm of neuroscience that throws a wrench into many assumptions we make about ourselves.
This is the surprising case of the "split-brain" patient, who as a result of surgery, winds up with something that looks like two minds inside one head.
In extremely severe epilepsy, where a patient doesn't respond to medication and is incapacitated by seizures, surgeons can as a last measure give some relief by separating the two halves of the brain.
This is done by cutting the long strand of fibers that connects the hemispheres. Epileptic seizures are caused by excessive electrical discharges in the brain, which can jump from one hemisphere to the other. A separation of the hemispheres thus helps control the attacks.
However, when split-brain patients look into a machine that shows a different image to half of each eye, they say they saw the image shown to the right half only. That's what they say, but when asked to pick out the thing they saw from a series of drawings, their left hand points to the image shown to the left half.
What happened?
The left hemisphere, which receives information from the right half of each eye, also houses the speech centers. When asked to say what the image was, the left hemisphere speaks. The right hemisphere can't speak, however, it can still control one hand. When allowed to gesture with a hand, the right hemisphere points to the image that it saw.