There is yet another state of conscious for our collection that are not the same as normal active wakefulness. Some people that appear to be in a vegetative state are actually conscious but unable to respond. The BPS Research Digest has a report on diagnosing this condition (here).
when she was instructed to pay special attention to her own name, the woman’s EEG signal showed an exaggerated P300 response to her name, compared with when she was instructed to just listen passively. The P300 is a spike of activity recorded from the parietal lobe, which is thought to be a marker of consciousness or decision making.
Brain imaging has been used before but using EEG is much more cheap and convenient. The protocol is also an advantage.
this active auditory event-related paradigm (requiring explicit comprehension of auditory-verbal instructions) provides an interesting tool for detecting voluntary brain activity in patients that behaviourally would be diagnosed as comatose
Wikipedia points out the difference between lock-in syndrome and persistent vegetative state.
Unlike persistent vegetative state, in which the upper portions of the brain are damaged and the lower portions are spared, locked-in syndrome is caused by damage to specific portions of the lower brain and brainstem (ventral part of the pons) with no damage to the upper brain.
What is this state like? People who have survived describe it as ‘being buried alive’.