Scientists have used a straight brain-to-brain connection to allow pairs of applicants almost 1.5 km apart to play a question-and-answer game by transmitting signals from one brain to the other over the Internet.

The experiment is thought to be the first to demonstrate that two brains can be unswervingly linked to allow one person to guess what's on another person's mind, researchers said.

"This is the most complex brain-to-brain experiment, I think, that's been done to date in humans," said lead author Andrea Stocco, an assistant professor of psychology and a researcher at University of Washington (UW)'s Institute for Learning and Brain Sciences.

"It uses conscious experiences through signals that are experienced visually, and it requires two people to collaborate," Stocco said.

The first "respondent," wears a cap connected to an electroencephalography (EEG) machine that records electrical brain activity.

The respondent is shown an object on a computer screen, and the second participant, or "inquirer," sees a list of possible objects and associated questions.

With the click of a mouse, the inquirer sends a question and the respondent answers "yes" or "no" by focusing on one of two flashing LED lights attached to the monitor, which flashes at unlike frequencies.

A "no" or "yes" answer both send a signal to the inquirer via the Internet but only a "yes" answer generates a response strong enough to stimulate the visual cortex and cause the inquirer to see a flash of light known as a "phosphene."

The experiment was carried out in dark rooms in two UW labs located almost 1.5 km apart and involved five pairs of participants, who played 20 rounds of the question-and-answer game. Each game had eight objects and three questions that would solve the game if answered correctly.

The sessions were a random mixture of 10 real games and 10 control games that were structured the same way. The researchers took steps to ensure participants couldn't use clues other than direct brain communication to complete the game.

Inquirers wore earplugs so they couldn't hear the different sounds produced by the varying stimulation intensities of the "yes" and "no" responses.

Participants were able to guess the correct object in 72 per cent of the real games, compared with just 18 per cent of the control rounds.

The research was published in the journal PLOS ONE.