BINOCULAR PERCEPTION OF SLANT ABOUT OBLIQUE AXES RELATIVE TO A VISUALFRAME OF REFERENCE

From the literature it is known that the processing of disparity for s lant is different in the presence and in the absence of a visual frame of reference. The experimental finding that vertical disparity is not processed for slant perception in the presence of a visual reference is elaborated. This theoretical analysis results in a reduction of the three basic first-order transformations between the retinal half imag es (divergence, rotation, and deformation) to only two basic orthogona l transformations. The first of these, horizontal scale, results in sl ant perception about the vertical axis, whereas the second, horizontal shear, results in slant perception about the horizontal axis. These t ransformations are based primarily on horizontal disparity. It is show n experimentally that in the presence of a frame of reference the amou nt of vertical transformation that is added to the two basic transform ations (horizontal scale and shear) of a random-dot stimulus is indeed irrelevant for slant perception. It is suggested that, in the presenc e of a visual reference, slant perception about oblique axes is based solely on linear combinations of the horizontal-scale and horizontal-s hear transformations. Subjects are able to reproduce slants about obli que axes experimentally merely by combining horizontal scale and shear .