In a comatose patient, brain activity is "dampened and slowed," Nakaji said, and the "light switch of the brain" -- the reticular activating system -- is not on. While it might seem like the person is sleeping, the brain is in a much different mode. The brain activity of a sleeping person is much more active than that of someone in a coma.
"When you're asleep, you're actually much more active than you might think," Nakaji said. For example, if you poke a sleeping person, he or she might swat at your hand. Someone in a coma would not respond except with a reflex movement.
Most patients don't remain in a true coma: they either die or move into a vegetative state. A patient is described as in a vegetative state when the arousal mode starts to kick in, but they are not able to interact with their environment. A vegetative patient's eyelids might flicker when someone walks in the room, for example.
"By definition, vegetative patients are not able to give any meaningful responses," said Dr. Romer Geocadin, associate professor of neurology at Johns Hopkins. "Vegetative patients are able to regain the ability to ‘wake' up, but their brains are never able to interact with the environment or be aware. However, a vegetative patient can recover to a point that they can answer simple questions or follow simple commands. When they are able to do this, they are in a state known as minimally conscious ... at that time they are still severely incapacitated, but many may be able to respond to questions of prolonged life support."