Figure 2. Gaze behavior depends on terrain.

(a) Example images of the five terrain types. Sections of the hiking path were assigned to one of the five terrain types. The Pavement terrain included the paved parts of the hiking path, while the Flat terrain included the parts of the trail which were composed of flat packed earth. The Medium terrain had small irregularities in the path as well as loose rocks and pebbles. The Bark terrain (though similar to the Medium terrain) was given a separate designation as it was generally flatter than the Medium terrain but large pieces of bark and occasional tree roots were strewn across the path. Finally, the Rocks terrain had significant path irregularities which required attention to locate stable footholds. (b) Histograms of vertical gaze angle (angle relative to the direction of gravity) across different terrain types. In very flat, regular terrain (e.g. pavement, flat) participant gaze accumulates at the horizon (90°). With increasing terrain complexity participants shift gaze downward (30°–60°). Data are averaged over 10 subjects for rocky terrain and 8 subjects for the other terrains. Shaded error bars are ±1 SEM. Individual subject data are shown in ‘Methods’.