Abstract
Although the role of depth has been shown to play an important role in lightness constancy (Gilchrist, 1979, 1980), the effect of perceived depth on lightness illusions have been reported by some investigators, but not by others. Although Gilchrist's coplanar ratio principle was meant to apply to arrays in which the luminance range exceeds 30:1, some investigators have claimed that apparent depth relationships in lightness illusions can alter perceptual grouping, and as result, the magnitude of lightness illusions. For example, Wolff (1933) concluded that the magnitude of simultaneous lightness contrast (SLC) weakens as the targets and backgrounds become separated in three-dimensional space. However, Gibbs and Lawson (1974) found no such effect. Others have found that White's (1981) illusion can be weakened, or even reversed, by altering apparent depth relationships within a display. One possible reason for disagreements in the literature is that perceived depth relationships can be altered in a variety of ways. Some depth cues may be effective in altering the magnitude and direction of lightness illusions, whereas other cues may not. We presented observers with lightness illusions such as SLC, White's effect, and center/surround assimilation and isolated the specific depth cues available to those observers between conditions. Specific depth cues tested included shape-from-shading, T-junctions, stereopsis, vergence, and accommodation. Results show that not all depth cues are equally effective in altering the magnitude of lightness illusions. Discrepancies in the existing literature may be due to the differential effects of these cues.
Supported by National Science Foundations Grants BCS-0002620 and BCS-0196013.