The veil between dreamworld and reality may be thinner than we thought. In a new study released Thursday, scientists in four countries say they’ve shown it’s possible to communicate with people while they’re lucid dreaming. At least some of the time, the dreamers were reportedly able to respond to yes-or-no questions and answer simple math problems through facial and eye movements; afterward, some recalled hearing the questions during their dream.

Cognitive neuroscientist and study author Ken Paller and his colleagues at Northwestern University in Chicago have been studying the connection between sleeping and memory for years. It’s commonly thought that sleep is crucial to the robust storage of memories created throughout the day. But little is still understood about this process and how dreams might play a role in it.

“We are investigating dreaming to learn more about why dreams happen and how they might be useful for mental function during waking,” Paller told Gizmodo in an email. “As in our other work, we hypothesize that events of sleep cognition can be beneficial for memory function.”