Only "a living human being and what resembles ... a living human being" can sensibly be thought of as a person - so thought the philosopher Ludwig Wittgenstein. It is the inability of those in Persistent Vegetative States (PVS) to respond like other humans that drives debates about who can decide whether they are kept alive. But remarkable brain scans of a woman stuck in this terrible condition last week revealed new ways in which some such patients do resemble other people. The 23 year-old had shown no awareness for over a year, yet when doctors asked her to imagine situations - such as walking around her house - her cerebral electrical activity mirrored that in healthy people having the same thoughts, suggesting she may have an active mental life.

The evidence may give hope to those with close relatives imprisoned in a coma. It also invites reflection on the question of when it can be right to end a life, although the scientists behind the study said it would be wrong to rush to conclusions by generalising from just one case. More research is needed to establish how many (or how few) other such patients' brains show a similar response, but as knowledge builds there could be profound implications. These will not be simple, however.

Court rulings that allow the withdrawal of support where it will lead to death are already unfailingly controversial. They may become more so. Yet even if in future cases victims' brains can be shown to respond to speech, it would not follow automatically that withdrawing support was wrong. For if an individual is conscious but suffering and has no prospect of recovery, they should not be kept alive against their wishes.

Through living wills, healthy people are increasingly setting out the circumstances in which they would rather treatment stopped, but medics dealing with those who cannot speak still often lack such guidance. The suggestion that consciousness may be silently endured through coma may encourage others to set out when they would rather it ended. It also suggests that equivalent Declaration specifying a preference for treatment to continue should be given a legal force that they currently lack. Eventually doctors might pose questions directly and analyse patients' brain activity to establish whether they wished for changes to treatment - or even whether they wished to be kept alive. That remains science fiction, but it is an attractive prospect, for it could give back a measure of control to individuals who have had it completely taken from them.