ABSTRACT
A source of spatial information arises from the systematic gradients of projected texture that slanted surfaces produce in the optic array. The orientation of a surface with reference to a fixed environmental frame of reference (such as the horizontal plane) is normally referred to as geographical slant. While on the simulated slants angles ? ? and convey information with regard to the slant simulated, the rates of change of the angles ? and ?, produced by voluntary head movement, depend on the real location on the surface only. Bingham (in press) has argued that use of a constant head velocity for scaling distance is not without problems, as reliable information about head velocity may not be available. While his suggestion to use amplitude of movement can in fact resolve this problem, we note that in the present type of situation observers might move in such a way that they obtain a certain t-value for contact with the ground.
