https://www.the-scientist.com/news-opinion/researchers-exchange-messages-with-dreamers-68477

Dreamers answered experimenters’ questions or solved simple math problems, showing that complex two-way communication between the dreaming and waking world is possible.

...

In response to yes/no questions and simple math problems, six participants correctly answered a total of 29 questions with pre-arranged eye signals.

...

TS: What was the main finding?

KA: The main finding was that it is possible to interact with a sleeping person, to exchange messages . . . and have the answer in real time, without [the dreamer] waking up, transferred back to the sleep laboratory experimenter. And of course this has quite large implications. So this view that people are shut off from the waking world during sleep, this has to be updated. This is basically a new method for investigating and understanding dreams.

...

TS: What’s next for your research on lucid dreaming?

KA: One thing that I find very exciting is how you can actually increase message complexity. Instead of just asking yes/no questions or math questions, you might think of having answers be more complex, like reporting really what you’re seeing in your dream. You might be able to report things like, ‘I see a house,’ ‘I see faces,’ ‘I see the beach,’ ‘I’m terrified,’ ‘I’m happy.’ There are many things that you might want to communicate which go behind beyond yes/no. So I’m working on a method for that, which uses more sophisticated eye movements for transferring words and sentences to the waking world.