A new study published in The Lancet suggests that brain dead people might not be as unaware as most people would like to believe.

Researchers used an EEG machine to examine brain waves and found that three of 16 vegetative patients could understand what they heard and follow instructions. Here's more on the experiment from Smart Planet:
The “new” brain scanner is an electroencephalogram (EEG), which, ironically is an older, less expensive machine than the kind of brain scanner that has been used to look for consciousness in vegetative patients: a functional magnetic resonance imaging (fMRI).

The research team, led by Damian Cruse and Adrian M. Owen of the University of Western Ontario, hooked 16 people in a vegetative state up to an EEG, a common medical device that measures electrical activity in the  brain through electrodes placed on a person’s head.

The scientists then asked them to imagine that they were making a fist with their right hand whenever they heard a beep. In another exercise, they had them imagine that they were wiggling their toes when they heard a beep.

The scientists compared their EEG results to those of healthy people who were asked to imagine the same thing.

Three of the supposedly brain-dead people showed the same two brain patterns (one for the hand, one for the toes) that the healthy study subjects displayed during the experiments: Activity for both showed up in the premotor cortex, the region of the brain that sets physical movements in motion.

The subjects who displayed the brain activity of conscious people made up 20% of the study group. They were men (aged 29, 35 and 45) who had been pronounced vegetative for the past three months to two years.

Such findings could help scientists in deciding whether a gravely injured person is actually in a vegetative state or just unable to respond.