Adding 3D space to human navigation: Humans stop short of home after walking over a bridge

Human navigation has mostly been studied on flat ground, leaving open how elevation changes affect path integration. In two experiments, blindfolded participants reproduced the horizontal distance home after traversing flat paths, a bridge or stairs. Experiment 1 showed no detectable mean error after a straight flat path, but a substantial undershoot after the bridge. In Experiment 2, bridge and stair paths produced shorter returns relative to a straight-flat baseline than did a flat path matching the bridge’s profile; crawling over the bridge increased this undershoot. Participants overestimated bridge inclination and height. In Experiment 1, a horizontal projection based on perceived slopes descriptively covered approximately 74% of the group mean undershoot, without a detectable individual association. In Experiment 2, an exploratory perception–behaviour correlation did not survive correction for multiple comparisons, and homing errors showed no detectable inclination effect despite different perceptual predictions. Slope overestimation may therefore contribute to the undershoot, but its causal role remains unresolved. One interpretation is that humans integrate vertical movements metrically using biased inputs. The findings do not distinguish a 3D representation from projection onto a 2D representation.