Why are lightness values compressed in abnormal illumination? A L Gilchrist1, S Ivory |
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1Psychology, Rutgers University, NJ, United States
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| Failures of lightness constancy always take the form of gamut compression. To exploit this important clue, we measured the compression for a row of 5 target squares standing in a spotlight (30 X ambient) within a checkerboard-covered vision tunnel. Varying the luminance range of the 5 squares and the checkerboard walls produced 6 conditions that we used to test 5 stimulus metrics potentially underlying the compression. The amount of compression was predicted by the ratio of highest target luminance to highest checkerboard luminance (equivalent to the perceived illumination difference), but not by overall luminance range or by the formula in anchoring theory or by two other metrics. Compression was identical for a row of squares suspended in midair within the tunnel or seen through an aperture on the far wall, showing that border ownership at the boundary enclosing the squares is not critical. However, substantially more compression was produced when the row was placed within a rectangular beam of light projected onto the far wall. This suggests that an occlusion boundary segregates frameworks better than a cast illumination boundary, even though the cast illumination edge reveals the illumination difference between squares and tunnel. |
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