Stimuli and data collection. a, Visual stimuli presented on a head-mounted visual display superimposed on a background made of 50% dark and 50% light triangles. The human subjects fixated a central dot (fixation dot) and pressed a button to indicate whether a square stimulus was presented on the left or right hemifield (stimulus target). Before the button press, eye movements had to be restricted within ±2.5° (fixation window). b, Example recordings from the eye tracker and head motion sensors of the head-mounted visual display. From top to bottom, recordings of horizontal eye position, vertical eye position, and pupil diameter for right eye (black) and left eye (orange), occasionally interrupted by eye blinks. Below the pupil traces, the figure shows recordings from head motion in the yaw (red), pitch (green), and roll (blue) axes, and time stamps for each stimulus presentation (black) and button-press responses (magenta).