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You might think consciousness is like a light switch, either on or off. But the true picture now seems rather murkier, challenging our notions of awareness and free will, as well as raising issues of consent in coma patients.

People used to think just three states of consciousness existed, says Adam Zeman of the Peninsular Medical School in Exeter, UK. "You're either awake or asleep - and if you're asleep you're either in dream or non-dream sleep."

But brain imaging suggests there are more. Take sleepwalking, which affects as many as 1 in 20 adults. "The sleepwalking brain is literally in a half-awake, half-asleep state," says Zeman. Researchers once manoeuvred a sleepwalker into a brain scanner, and while they saw that much of the cortex - involved in awareness and consciousness - was offline, other brain areas were active, including those linked with emotion.

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