In the wake of the bitterly contested death of Terri Schiavo, the New York Times has surveyed how much doctors know about the type of brain injury she sustained. In recent years researchers have done a lot of work to categorise unconscious states. The most familiar, of course, is sleep, whose deepest phases show little electrical activity in the brain and almost complete unresponsiveness.

Then there are states of impaired unconsciousness known as comas. There are a range of states of comas, but patients usually emerge from them gradually within two to three weeks. Recently doctors agreed on another category which they call the minimally conscious state. Patients with this condition appear to be severely brain- damaged, but from time to time they respond to stimuli. Doctors compare the electrical activity in their brains to an old set of Christmas lights which blink on and off in unpredictable ways. This can last a lifetime, but some patients have made astonishing recoveries after many years. A mechanic in Arkansas regained awareness in 2003 after 18 years of unconsciousness, for instance. And in February, a Kansas woman, Sarah Scantlin, recovered consciousness 20 years after being hit by a drunken driver. She is now undergoing intensive rehabilitation.

For patients in a persistent vegetative state, however, only one or two bulbs in the Christmas lights work. Apparently this is the state in which Terri Schiavo lived. The eyes are open and there is some reflex activity, but there are no signs of psychological responsiveness. A 1994 study of 700 patients in a persistent vegetative state found that none had recovered after two years.

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