Figure 1.

Experimental Paradigm. (A, left) Screenshot of the game environment. Participants steered the ball (grey marble) to collect targets (golden coins) and avoid obstacles (red bars that slow down the ball to 10% of its speed). (A, right) Illustration of the BallGame setup: a pair of participants, each equipped with a 128-channel EEG cap, eye-tracker goggles and bimetal sensors attached to index fingers, sitting in adjacent rooms / sound-attenuated cabins. Note that analyses of the the EEG data are not included in the present article. (B, left) Demonstration of the game control—a bimetal sensor attached to the index finger translated bending and stretching of the finger into ball movement on the screen. (B, right) View of an example experience rating (bar filled by ‘left–right’ movement, answer confirmed with long ‘down’ movement). (C) Experimental protocol. After the instructions, participants were prepared (prep) for the game-concurrent data recording. The experiment began with baseline tasks and 10 trials of individual play. After further 20 trials of joint play, we took a longer break. Afterwards, participants played another 20 trials of joint, and 10 trials of individual play, and completed the baseline tasks. Finally, we conducted individual interviews. During this time, the other participant filled in personality questionnaires. (D) The two joint play conditions. In joint play DIFF (different), three of nine obstacles were visible to both players (dark grey bars), three only to player one or two (light grey bars). In joint play SAME, players saw the same six of nine obstacles (dark grey bars)—three obstacles remained invisible to the team (empty bars). The black dotted line indicates the path traveled by the ball in an example one-minute trial. (E) Experimental protocol of the joint play period. Joint play was structured in 12 blocks of three or four trials each, after which participants rated their experience in terms of their level of engagement, agreement and predictability (light-yellow boxes marked ‘QA’ (questions and answers)). In each session, participants played 10 trials of each condition (light grey boxes = joint play DIFF; dark grey boxes = joint play SAME). Note that the starting game condition alternated between pairs.