A Day at the Beach: Does the energy cost of walking influence visually perceived distance?

Abstract

It takes less effort to walk from here to the Tiki Hut on the brick walkway than on the sandy beach. Does that influence how far away the Tiki Hut looks? Last year at VSS we ran the experiment to find out. The energy cost of walking on dry sand is twice that of walking on firm ground (Lejeune, et al., 1998). If distance is perceived in units of energetic cost or ‘behavioural potential’ (Proffitt, 2006), then visual distances over sand should be double those over brick. If distance is perceived in constant units (e.g. eye-heights as specified by declination angle; Ooi, et al., 2001), then the visual distances should be similar. Participants (N=13) viewed a target at a distance of 5, 7, 9, or 11m over sand or brick, were blindfolded and turned toward the same or different terrain, then blind-walked to match the visual distance. Walked Distance was analyzed using mixed effects regression with Target Distance, Viewed Terrain, and Walked Terrain as fixed-effect predictors. The energetic hypothesis predicts that walked distance should be greater after viewing over sand than over brick. But walked distance was actually shorter after viewing over sand (β = -0.59, SE = 0.21, χ2(1) = 6.05, p = 0.01). There was no difference between walked distance on sand and brick (p = 0.71), with moderate evidence for the null hypothesis (BFJZS = 3.38). Thus, perceived distance did not increase with the energy cost of the viewed terrain. Moreover, walked distance was no different on sand and brick, indicating an accurate locomotor calibration to each terrain. The results imply that distance is not perceived in terms of energetic cost. Rather, visually perceived distance is constant, and walking is guided by a visual-locomotor mapping (e.g. from declination angle to locomotor distance on the terrain) (Warren, 2019).