Perceiving "Walk-on-able" Slopes

Prospective control of walking and running requires perceiving sloping surfaces that can and cannot support these activities. Two experiments on slant perception were conducted, motivated by the notion of geographical slant rather than optical slant, and by an affordance based description of surface layout. In Experiment 1, participants standing on a horizontal surface (the ground) adjusted the inclination in the frontal plane of a large wooden platform at 0, 2, or 4 m from them until satisfied that it was at the maximal slope at which ordinary walking could be conducted. Subsequent to their judgments, the actual maximum of a "walk-onable" slope was determined for each participant. In Experiment 2, participants adjusted the inclination of the visible platform to match the inclination of the right foot, which was occluded and resting against a small ramp inclined between 0' and 50' to the ground plane. They then judged the visible platform for its walk-onability and were subsequently tested for the maximal slope that each could in fact walk on in the ordinary manner. In both experiments, perception of the maximal slope that is walk-on-able conformed closely to the actual slope maximum permitting ordinary walking. The discussion of the results addresses limitations of previous slant perception research and underscores the importance of actionrelevant measures to investigations of the perception of surface layout.