Figure 1.

Our setup and the main variables recorded. Top left: Schematic side view of a head wearing SMI glasses to illustrate gaze parameters. The four markers on the glasses were used to calculate head orientation (the vertical component of which is plotted here in degrees as ”head-in-world,” or HiW_y) and the position of the cyclopean eye. Knowing the field of view of the SMI glasses (46${}^{\circ }$ vertically and 60${}^{\circ }$ horizontally for the head camera, as seen in the videos, and 60${}^{\circ }$/80${}^{\circ }$ for gaze tracking) allowed us to add the ”eye-in-head” or EiH_y gaze vector (also in degrees) to this vector and gave us ”eye-in-world,” EiW_y, when adding up the two parameters. Bottom left: Setup for our experiment. Participant wearing 39 retro-reflective markers and SMI glasses on a dual-belt treadmill, looking at a virtual road presented on a 240${}^{\circ }$ screen. Right: Gaze and gait parameters over two slip events from Experiment 1 as an example of the measured data. Top panel: Gaze-related parameters, including vertical coordinates of the head's pointing direction position of head-in-world (black), eye-in-world (red), and eye-in-head (blue). Time axis is relative to the initiation of one slip (i.e., a perturbation event); y-axis shows y-component of each parameter in degrees. Dashed vertical lines indicate time of perturbation. Middle: Movement-adjusted center of mass (black) compared to anterior and posterior base of support (gray), giving us the anterior–posterior margin of support ($MO{S}_{ap}$, green, in m; higher values indicate higher gait stability). Bottom: Vertical force in N on the left and right belt, respectively, which was used to detect steps online. Light blue and dark blue lines show the respective nominal belt speeds.