Figure-Ground Organization and
Edge-Texture Grouping
Stephen
Palmer and Joseph Brooks
A new class of information about depth across an edge and figure-ground organization -- edge texture grouping -- is described and demonstrated. The central issue in both cases is whether the edge between two regions "belongs to" (or "is grouped with") one side or the other. The side that is grouped with the edge should be perceived as closer and shaped by the edge, whereas the ungrouped side should be perceived as farther away and extending (unshaped) behind the edge. This analysis implies that classic grouping factors that relate various edge properties to corresponding textural properties within adjacent regions should systematically influence perceived depth across an edge and figural status. We report the results of several experiments that strongly support this claim for the grouping factors of common fate, proximity, synchrony, and similarity of blur, color, and orientation. Further, we argue that evidence of grouping in certain moving and flickering displays that are ecologically unnatural suggest that these effects are mediated by grouping processes rather than by inferences based on simple ecological statistics. The edge-texture grouping hypothesis provides a unified account of the present phenomena and others previously reported in the depth perception literature (e.g., Yonas, Craton & Thompson, 1987) as well as a coherent explanation of Weisstein's anomalous findings about the effects of spatial frequency and flicker on depth and figural status (e.g., Klymenko & Weisstein, 1986). Edge-texture grouping effects have important implications for depth and shape algorithms in computer vision as well as for corresponding perceptual and neural processes in human vision.
See the Figure-Ground and Grouping Demos
Grouping Occurs Both Before and After
Constancy
Stephen Palmer and
Joseph Brooks
Previous results from our laboratory have shown that perceptual grouping occurs after various kinds of constancy processing, because perceived grouping can be strongly influenced by depth, post-constancy lightness perception, amodal completion, and illusory contours. We now report results showing that grouping also occurs before constancy processing. The general logic is to demonstrate that grouping both affects constancy and is affected by constancy. In shape constancy, for example, we first demonstrate that pictorial depth cues influence whether observers see a central column of ambiguous ovals as grouping with circles in the frontal plane or with ovals in the frontal plane. We then also show that whether observers perceive an ambiguous oval as a circle slanted in depth or as an oval in the frontal plane is strongly influenced by whether it is grouped with a surrounding trapezoid (consistent with the circular interpretation) or with a surrounding square (consistent with the oval interpretation). Proximity, common fate, and color similarity all have strong effects in our displays. We have also demonstrated similar kinds of grouping effects on edge assignment in depth perception for textured surfaces: the edge is assigned to the region whose texture elements group with the edge according to factors such as common fate, proximity, color similarity, and orientational similarity. Analogous grouping effects also appear to occur in displays involving lightness constancy. These results are inconsistent with aspects of Palmer and Rock's (1994) theory of perceptual organization, but consistent with an alternative formulation in which grouping occurs throughout perceptual processing.
See the Grouping and Constancy Demos
Contextual Influences on Figure-Ground
Organization
Joseph Brooks and
Sergei Gepshtein
The classic principles of figure-ground organization are based on properties of the regions at hand or the edges bounding those regions. Very little attention has been given to contextual cues to figure-ground organization. In a series of studies we are exploring the effect of flanking regions on the figure-ground organization of central ambiguous regions. In preliminary experiments, we have found that flankers influence the figure-ground organization of a target to the degree that those flanking regions group with the target. These results have implications for models of figure-ground organization.
See the Context and Figure-Ground Demos
The Occlusion Illusion
Stephen Palmer, Kevin Lai, and Joseph
Brooks
We investigated a size illusion first reported by Kanizsa (1979) in which a figure bounded by an occluding edge looks larger than the same figure not bounded by an occluder. We call this phenomenon an occlusion illusion. We conducted a series of experiments to document the magnitude of this illusion and distinguish between two hypotheses about its perceptual basis. The size-distance relation (Emmert's Law) predicts that the farther of two objects with equally-sized retinal projections should be seen as larger. Because occlusion implies relative distance order, the size-distance relation serves as one explanation of the occlusion illusion. Alternatively, the illusion may reflect an extension of the occluded surface at the occluding border without a uniform increase in the perceived size of the figure. The distinctive prediction between these two theories is whether the occlusion illusion reflects a change in the overall size of the figure (size-distance theory) or a shape change in which the occluded edges perceptually extend beyond their intersection points with the occluder. This extension would form a greater area of a slightly different shape. To test this, we first used a staircase procedure to measure the magnitude of the occlusion illusion under various occlusion conditions and various occluding and occluded shapes. We found that the strength of the illusion varied with the strength of the cue to occlusion. In a second experiment we specifically addressed the two hypotheses about the origin of the illusion. In the first phase of this experiment each participant completed two staircase procedures. One staircase measured the point of subjective equality (PSE) for the overall size of the figure in comparison to the partially-occluded figure. The other staircase measured the PSE for extension of the edges beyond their intersection points with the occluder. The two figures representing the PSEs for each of these procedures were then compared to the partially occluded figure showing the illusion. Participants were significantly more likely to judge the figure with the extended edges as more similar to the example of the illusion. This suggests that the occlusion illusion includes a modal extension of edges that intersect with an occluding object.
See a version of the occlusion illusion
Synchrony Grouping
Stephen Palmer and Rob
Meyerson
Synchrony grouping occurs when multiple elements within an array change at the same time, even if they do not change in the same way. It is a potent grouping factor, but its effect can be virtually eliminated simply by adding a brief blank frame between the frames that contain the elements, analogous to the effect of blank frames in the phenomenon of change blindness. Control conditions in which the blank frame was positioned within (rather than between) frames showed that these effects were not due simply to transients produced by the blank frames; they must be temporally aligned with the element changes. Drastic reductions in synchrony grouping can also be achieved without blanking the display, however, by changing the contrast of a square grid of lines that lie between each element and its neighbors, provided that the grid changes at the same time as the elements. This shows that the "change blindness" effect is not due merely to the absence of element changes, but to the presence of other synchronous changes in the display. When contrast changes in the grid are reduced, however, the change blindness effect gradually diminishes. We studied these effects psychophysically by having observers discriminate between perceived vertical vs. horizontal borders induced by synchrony grouping within square arrays of elements. The results thus far are consistent with an account of synchrony grouping in terms of spatially and temporally localized change signals that occur whenever and wherever changes take place and with a strength that depends on the magnitude of the change. By this account synchrony grouping is disrupted by blanking and by changes in the contrast of the grid because they effectively mask the contrast changes in the to-be-grouped elements.