In the 1960's a certain type of severe epilepsy was only cured by a major surgery that cut this connection, and in doing so stopped the excess signalling from nerve cells spreading to the opposite hemisphere of the brain. This made it possible for the patients who had previously lived with constant seizures, to live a life almost seizure free, whilst having one hemisphere of the brain fully disconnected from the other.
It wasn't after a few experiments held by Roger Sperry that psychologist found some odd side effects to this procedure.
In a normal brain, when we view an image in our left visual field it is sent to the right hemisphere of our brain and vise versa. If an image is sent only to one specific visual field, that particular hemisphere sends the information to its other half for us to be able to fully understand what we are looking at. Roger Sperry and his colleagues conducted an experiment that allowed us to understand hemispherical specialisation in detail. Persons who had undergone the "Split Brain" procedure were perfect for this study as it allowed him to look at the two halves of the brain separately.
The experiment consisted of a screen with a dot in the centre. The experimenter asked the participants (those with a "split brain") to focus on the dot but press a button when ever they saw an image. The researchers then flashed images of different stimuli (images, letters, mathematical problems etc.) in the the subjects left or right field of view. As each half of the visual field projects to the opposite side of the brain it is possible to show a picture to either just the right hemisphere or just the left hemisphere.
When participants were flashed images to the right of the dot, the information was sent to there left hemisphere, and when asked what they saw, there was no problems naming the item. When they were asked to pick up the the object from a line of random objects that they could not see, they had no problem receiving what ever stimuli they saw. When the an image was flashed to the left, and therefore right half of the brain, participants said they saw nothing, although the left hand kept pressing the button to signal that an image was, indeed, flashed. What was even more bizarre, was that in spite of the fact that subjects could not name the object, they could draw it and also pick it out of a line of random items, but only with their left hand, and have no idea as to why they drew or picked that certain thing up. This showed scientists that they did actually see