A dreamer can signal, with eye movements, that the dream is lucid

People trained to notice they are dreaming can, during confirmed REM sleep, make a prearranged pattern of eye movements. The pattern shows up on the recorder. The dream continues.

In 1981 Stephen LaBerge and colleagues asked practiced dreamers to do a specific thing if they realised they were dreaming: move their eyes in an agreed pattern. Five subjects did it, during sleep that was unmistakably REM — the state in which the body’s other muscles are slack and the eyes are not. The polygraph recorded the pattern. The subjects, woken, reported that they had been lucid and had sent the signal on purpose. Later work has used the same channel to mark the moment of a dreamed action and compare it with the brain’s record.

The channel is narrow. Eye movements are available because the eyes are not paralysed with the rest of the body. There is no sentence, no picture of the dream, no window. There is a bit, sent from inside a scene the machine cannot see, saying that the person in the scene knows it is a scene. That is already a change in kind. A report after waking can be doubted, embroidered, or forgotten. A signal during the REM that matches an agreement made while awake is a measurement.