Does the visual system exploit projective geometry to help solve the motion correspondence problem?

Projective geometry determines how the retinal image of an object deforms as it moves through three-dimensional space. Does the visual system use constraints derived from this information, such as rigidity, to aid the tracking of moving objects? A novel psychophysical technique is introduced for assessing which of two competing motion transformations is 'preferred' by the visual system, in a two-frame sequence. In the first experiment, relative preference strengths for translations parallel and perpendicular to the major axis of a wire-frame object were measured by pitting the two against each other. It was found that parallel translations were preferred to perpendicular ones. On the basis of these data a proximity measure for normalising different transformations, independent of any effects of figural similarity, was developed. In the second experiment, two wire-frame planar structures were used to pit one of five transformations (rotation, expansion, vertical expansion, shear and random jitter) against a translation. Preference strength was measured as the translation distance at which the transformation and the translation were perceived with equal frequency. The PSEs were found to collapse on to a single line when plotted against the proximity magnitude, with the exception of a residual preference for pure translation over all other transformations. In general, these results suggest that preference strength for moving wire-frame figures is determined primarily by the proximity of local features on the displacing contour, with little regard for the projective shape transformation.