Diana Kwon in Scientific American:
Dreams are full of possibilities; by drifting into the world beyond our waking realities, we can visit magical lands, travel through time and interact with long-lost family and friends. The notion of communicating in real time with someone outside of our dreamscapes, however, sounds like science fiction. A new study demonstrates that, to some extent, this seeming fantasy can be made real. Scientists already knew that one-way contact is attainable. Previous studies have demonstrated that people can process external cues, such as sounds and smells, while asleep. There is also evidence that people are able to send messages in the other direction: Lucid dreamers—those who can become aware they are in a dream—can be trained to signal, using eye movements, that they are in the midst of a dream. Two-way communication, however, is more complex. It requires a person who is asleep to actually understand what they hear from the outside and think about it logically enough to generate an answer, explains Ken Paller, a cognitive neuroscientist at Northwestern University. “We believed that it was going to be possible—but until we actually demonstrated it, we weren’t sure.”
For this study, Paller and his colleagues recruited volunteers who said they remembered at least one dream per week and provided them with guidance on how to lucid dream. They were also trained to respond to simple math problems by moving their eyes back and forth—for example, the correct answer to “eight minus six,” would be moving your eyes to the left and right twice. While the participants slept, electrodes attached to their faces picked up their eye movements and electroencephalography (EEG)—a method of monitoring brain activity—kept track of what stage of sleep they were in.