cal in reflectance to the ones on the left (that is, they were made of the same
shade of gray paper) but were seen under a shadow cast by an opaque vertical
strip hanging nearby. As a result, their luminance—the amount of light reach-
ing the observer’s eye after being reflected by the central squares—was identi-
cal to the luminance of the squares on the right. Therefore, if grouping were
based on relatively early processing of image structure, the central squares
would be grouped with the luminance-matched ones on the right. If it were
based on relatively late processing after perception of shadows had been ach-
ieved, they would group with the reflectance-matched ones on the left. The
results showed that grouping followed the predictions of the postconstancy
grouping hypothesis: Similarity grouping was governed by the perceived light-
ness of the squares rather than by their retinal luminance. Other conditions
ruled out the possibility that this result was due to simple luminance ratios of
the squares to their backgrounds.
Perceptual grouping is also affected by visual completion (Palmer, Neff, &
Beck, 1996). Visual completion refers to the fact that when observers see an
Figure 8.12
Retinal versus perceived distance in proximity grouping. Luminous beads spaced as shown in part
A appear to be organized in columns when viewed in the frontal plane (B), because of proximity.
When slanted in depth (C) and viewed with both eyes, they are still seen as organized into columns,
even when they are closer together horizontally on the retina. This result shows that proximity
grouping is influenced by stereoscopic depth processing.
Organizing Objects and Scenes 203