Fig. 1.

a The phone-reading task. Participants look to the front, holding the phone with arms by the side in relaxed position, as in normal walking. After hearing the “beep” from the phone, participants raise their arm, look at the screen, and begin reading aloud the sequence of numbers. The presentation disappears after 2 s, and participants look again to the front with arms relaxed (free to swing). 5 s later, the next reading trial commences until a total of 5 trials is completed in each block. b Static calibration. Defining the position of the Eyes and reference frame relative to the tracked “head” segment (headband), undertaken whilst participants stood still and looked to a marker placed at 1.5 m in front of them. The marker was horizontally and vertically aligned with the midpoint of their eyes. On the right-hand side, defining Screen position in relation to the markers attached to the “phone”, Note Screen offset, from middle of screen, represents location of where text is displayed. c Kinematic outcome measures: D_RES is distance of the Screen from the Eyes, i.e., resultant displacement (D_x, D_y, D_z). UD_GAZE and RL_GAZE indicate gaze angles in the “yz” (up-down) and “xy” (left–right) planes, respectively, i.e., gaze angles relative to the head-segment’s reference frame (indicated by red [x] and blue [z] planes). The example in the figure shows the Screen displaced downwards and leftwards from the neutral (static calibration) alignment, resulting in negative UD_GAZE and RL_GAZE angles