From Aquaman to Professor Xavier, mind reading is a staple for much of sci-fi. Yet it could be closer to reality than you might expect. In a series of experiments, researchers have been able to get two people to play a game similar to 20 questions, where one person asks a series of questions and then accurately guesses the object that another person is thinking of – despite not talking to each other, and being located over a kilometer apart.

“This is the most complex brain-to-brain experiment, I think, that's been done to date in humans,” said Andrea Stocco in a statement. Stocco is lead author of the study, which is published in PLOS ONE this week. “It uses conscious experiences through signals that are experienced visually, and it requires two people to collaborate.” The experiments indicate that two brains can be directly linked to allow one person to guess what the other is thinking of.

The experiment can be tricky to get your head around, so bear with me here. The first person involved, called the “respondent,” wears a cap connected to an electroencephalogram (EEG) machine, which reads their brain activity. They are then shown an object on a screen, such as a dog. The second person, the “inquirer,” who is sat in a room in a different building, is shown a list of possible objects that the first person is thinking about, and a list of simple yes or no questions (such as “can it fly?”). The inquirer asks these questions using a touch screen, which is then shown to the respondent on a computer. The respondent then answers not by talking, but by focusing on one of two flashing lights that represent either “yes” or “no.”

The EEG machine reads the respondent's brain activity, determining which answer they are thinking of, and then sends it via the internet, to the room of the inquirer. Here it activates a magnetic coil positioned behind the inquirer's head. If the respondent's answer was “yes,” then the coil generates a strong signal that stimulates the visual cortex of the inquirer's brain. This causes them to “see” a flash of light in their brain, in the form of blobs, waves, or lines, called a phosphene. This indicates to the inquirer that the other person has answered yes. After a series of questions, the inquirer can guess what object the respondent was thinking of.

They found that the participants were able to guess the correct object 72% of the time during the experiment, compared with only an 18% success rate in the controls (when, unknown to the participant, a plastic spacer prevented the generation of phosphenes). The researchers suspect that some of these wrong guesses were probably down to the participant never having experienced phosphenes before, and not knowing how to interpret them. “It's not something they've ever seen before,” said coauthor Chantel Prat, from the University of Washington.

The researchers next want to focus on whether it is possible to transfer signals directly from healthy brains to one damaged by a stroke, for example, in what they call “brain tutoring.” In addition to this, they want to see if they can also transmit brain states, such as sending signals from an awake brain to a sleepy one, or from a focused brain to one with attention deficit hyperactivity disorder (ADHD).