What is it?
A persistent vegetative state (commonly, but incorrectly, referred
to as "brain-death") sometimes follows a coma. Individuals
in such a state have lost their thinking abilities and awareness of their
surroundings, but retain non-cognitive function and normal sleep patterns.
Even though those in a persistent vegetative state lose their higher
brain functions, other key functions such as breathing and circulation
remain relatively intact. Spontaneous movements may occur, and the eyes
may open in response to external stimuli. They may even occasionally
grimace, cry, or laugh. Although individuals in a persistent vegetative
state may appear somewhat normal, they do not speak and they are unable
to respond to commands.
http://www.ninds.nih.gov/disorders/coma/coma.htm
New York Times review of cases (Carey, B., 2005) . Discusses "minimally conscious state."
But -- what counts as a "response?" Or more relevantly, an intentional "response?"