Downhill slopes look shallower from the edge.

A dramatic failure of orientation constancy is documented in the perception of downhill slopes. Contrary to naïve expectation, steep downhill slopes look shallower from the edge than they do from back from the edge. Three experiments document and quantify this failure of constancy for real and virtual surfaces using a variety of dependent measures. Two additional studies document overestimation of both non-visually perceived head pitch and perceived gaze declination. A model of orientation constancy failure is fit to the data that combine exaggerations in perceived gaze declination with exaggerated scaling of perceived optical slant. These findings support a functional scale-expansion model of error in slope perception.