The role of amodal surface completion in stereoscopic transparency

Amodal Volume Completion and the Thin Building Illusion

We report results from an experiment showing that a tall pillar with a triangular base evokes radically different three-dimensional (3D) percepts depending on the vantage point from which it is observed. The base of the pillar is an isosceles right triangle, but the pillar is perceived as just a thin plane when viewed from some vantage points. Viewed from other vantage points, the perceived 3D shape of the pillar corresponds to a square or rectangular base. In general, our results suggest that the visual system uses a preference for rectangularity (or symmetry) to determine the 3D shape of objects. The amodal impressions of the invisible backside of the pillar are often quite compelling, and the corresponding illusions persist even when the observers know the true shape of the pillar. Interestingly, though, the compellingness and definiteness of the amodal impression of the pillar's backside depends on the vantage point. This is reflected in corresponding differences in the interobserver variability of the 3D shape judgments. We also discuss how variants of this illusion are used as a powerful tool in the art of magic.

Perceptual mechanisms underlying amodal surface integration of 3-D stereoscopic stimuli

The visual system can represent a partially occluded 3-D surface from images of separated surface segments. The underlying amodal surface integration process accomplishes this by amodally extending each surface segment behind the occluder (amodal surface extension) and integrating the extended surfaces to form a whole surface representation. We conducted five experiments to investigate how depth cues, such as binocular disparity, half-occlusion, and monocular depth cues (T-junctions and L-junctions), contribute to amodal surface extension, and how the geometrical relationship and image similarity among the surface segments affect surface integration. This was achieved by having observers adjust the stereoscopic depth and slant of a comparison stimulus to match those of the tested 3-D stimulus. We found that both binocular disparity and half-occlusion cues are used to determine border-ownership assignment of surface segments and for amodal surface extension. We also found that separated surface segments need to have the same luminance contrast-polarity for them to be integrated as a whole surface. Finally, we found that having the same motion direction, minimum misalignment between boundary contours, and proximity among separated segments facilitate their integration. Overall, our findings reveal a set of "perceptual factors" for amodal surface integration, which arguably reflects our visual system's built-in knowledge of the regularities in natural scenes.

Created unequal: Temporal dynamics of modal and amodal boundary interpolation

In this study we manipulate the distribution of contrast polarity reversals in inducing configurations to create novel variants of modal and amodal completion. The novel variants, better equated in their geometric and photometric characteristics offer a superior way to probe similarities and differences in the temporal dynamics that underlie different forms of perceptual completion. We use dot localisation to directly compare the spatial characteristics of modally and amodally interpolated contours at presentation durations ranging from 120 to 300ms and find robust differences in the spatiotemporal formation of modally and amodally completed boundaries. Modally completed contours are localised more accurately and with better spatial precision across all presentation durations. Our results challenge the assumption that the boundary interpolation system depends solely on the geometrical relatability of inducing fragments and suggest that boundary interpolation depends on the spatial distribution of local luminance relationships. As an alternative to the strong version of the identity hypothesis, we propose that modal and amodal completion are mediated by different mechanisms, triggered by particular configurations of contrast polarity.