Marked effects of global orientation on appearance and perceived direction of motion

Purpose. The perceived form of objects can change dramatically as a function of their global orientation (e.g., a square vs. a diamond). We explored the effects of global orientation on the perceptual organization of form-and-motion stimuli. Methods. The stimuli were single lines, or line gratings, moving within differently shaped apertures (cf. Wallach 1935). The global orientation of the stimuli was varied from trial to trial, but the relative orientation between aperture and lines was fixed. Global rotations were by either 45 or 90 deg. We used square and 'kite-shaped' apertures (see Fig. Below), and circular apertures as controls. Results. Orientation affected perceived direction and appearance. In square/diamond apertures, perceived direction of motion followed the cardinal directions. Thus, in a 'diamond', the perceived direction followed the line (1D) motion, whereas in a 'square', terminator (2D) motion dominated. Along with perceptual changes in direction of motion, the appearance of the lines' rigidity and even the shape of the aperture changed. In the kite-shaped aperture, perceived direction of motion was always horizontal when the aperture (and line) were oriented vertically. However, when rotated by 90 deg., the motion deviated from the cardinal (vertical) direction: instead, the line was seen to move obliquely, and change its direction as it passed through the middle of the aperture. Such effects of the global orientation were not observed when the aperture was circular-i.e. when only the orientation of the lines varied. Conclusions. There is a strong bias to perceive motion in the cardinal directions, especially horizontal. However, for this tendency to reveal itself, either the 1D or 2D motion must be along a cardinal direction. In such cases, this bias can determine the outcome of the competition between the effects of 1D and 2D motion signals.