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Engel found that at the deepest levels of anaesthesia, the primary sensory cortex was the only region to respond to the electric shock. "Long-distance communication seems to be blocked, so the brain cannot build the global workspace," says Engel, who presented the work at last year's Society for Neuroscience meeting in San Diego. "It's like the message is reaching the mailbox, but no one is picking it up."

What could be causing the blockage? Engel has unpublished EEG data suggesting that propofol interferes with communication between the primary sensory cortex and other brain regions by causing abnormally strong synchrony between them. "It's not just shutting things down. The communication has changed," he says. "If too many neurons fire in a strongly synchronised rhythm, there is no room for exchange of specific messages."