If you were having brain surgery, would you either be out cold, or awake during the procedure?
Of course, unconscious, right?
Well, your surgeon may disagree. More often than you'd think, brain surgeons like to keep their patients awake during the operation.
To prevent pain, patients are numbed up, or even knocked out while doctors cut through the skin and skull. However, the brain itself has no pain receptors, so after the brain has been exposed, the surgeon will often bring the patient back to consciousness.
This is partly because we know so little about the brain. Scientists have a rough sense of which areas of the brain control functions like speech, hearing, muscle control, and so on, but there is no precise map. This means that a surgeon mucking around in the brain can easily damage a critical area without knowing it. However, if the patient is awake, and reacting to what's happening to his brain, the surgeon has a better sense of what he is doing.
For example, if a surgeon is removing a tumor near a part of the brain that controls speech, and the patient is awake, the doc will notice a sudden slurring, or difficulty forming words. Both of these reactions are signals that the surgeon should stop and back away.
Though it may seem strange, being awake during brain surgery is really a way to protect the brain.