Academic journal article Attention, Perception and Psychophysics

Amodal Completion Is Modulated by Lightness Similarity

Academic journal article Attention, Perception and Psychophysics

Amodal Completion Is Modulated by Lightness Similarity

Article excerpt

Published online: 17 September 2013

(Q> Psychonomic Society, Inc. 2013

Abstract The strength of amodal completion is known to be modulated by contour relationships and global shape. Some researchers have shown that amodal completion also depends on surface similarity but they have not distinguished the relative importance of similarity in surface representations either pre or post lightness constancy. In the experiments reported here, we aimed to determine whether amodal completion depends on processes that occur either before or after the establishment of lightness constancy. We used computer rendering techniques to vary the consistency of a cast shadow with a decrement in luminance on one side of a partially occluded surface. We found that perceived completion depended on the consistency of the decrement in surface luminance with the orientation of a cast shadow (Exp. 1). In Experiment 2, we generated occluded surface fragments that could be either luminance-matched or lightness-matched to surface fragments on the opposite side of the occluder, and we found that the strength of amodal completion depended primarily on similarity in the perceived surface reflectance. In Experiments 3 and 4, we performed apparent-motion tasks to obtain converging evidence for grouping on the basis of perceived similarity in lightness. We found that matching surfaces in lightness significantly improved the apparent motion of surfaces behind an occluder, as compared with surfaces that were matched in contrast alone. These results suggest that amodal completion depends on the perceived similarity in lightness of partially occluded surface fragments.

Keywords 2-D shape and form * Apparent motion Scene perception * Amodal completion

Oiu visual system provides rich information about the siuface properties of objects across a wide variety of environmental conditions. Many objects tend to be partially obstructed from plain view by other objects in the foreground, but still appear as unified objects rather than independent image fragments. Amodal completion refers to the process by which the visual system integrates partially occluded image fragments into a percept of a unified siuface. Much of the previous research on amodal completion has focused on the role of edge contoius in determining when completion does and does not occiu. The consideration of siuface or material properties has been largely limited to manipulating the similarity in texture or color of siufaces in the image (Anderson, 2007a, b, c; Anderson, Singh, & Fleming, 2002; Kellman, Garrigan, & Shipley, 2005; Kellman, Garrigan, Shipley, & Keane, 2007a, b). Here, we explore the extent to which amodal completion depends on processes that occiu before or after processes involved in establishing lightness constancy.

The material properties of siufaces can vary along a variety of dimensions, including color and lightness. Previous research has shown that the perceived strength of siuface completion was sensitive to the similarity in luminance of solid and textured siufaces (Yin, Kellman, & Shipley, 1997, 2000). These studies revealed that the perceived path of tangent extensions depended not only on good continuation of their edges, but also on similarity in the color and textural detail of siuface fragments. Although the findings support the view that amodal completion depends on the similarity in color or lightness of the image fragments, siuface fragments that were matched in solid color were also matched in physical luminance and/or chromaticity in the image. It is therefore unclear whether amodal completion was mediated by low-level similarity in siuface luminance or color, or by postconstancy representations of siuface color and lightness.

Other researchers have obtained evidence suggesting that postconstancy processes play a strong role in the perceptual grouping of disjoint objects under conditions of transparency. Rock, Nijhawan, Palmer, and Tudor (1992) investigated whether similarity in surface luminance or perceived lightness had greater influence on grouping in conditions of transparency. …

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