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Stephen Ivory, Alan Gilchrist; The staircase Kardos effect: An anchoring role for lowest luminance?. Journal of Vision 2010;10(7):414. doi: 10.1167/10.7.414.
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© ARVO (1962-2015); The Authors (2016-present)
In the staircase Gelb effect, a black surface in a spotlight appears white and becomes darker as four lighter shades of gray are added within the spotlight. Each new square is seen as white until the next square is added confirming that lightness values are anchored by the highest luminance. To explore whether the lowest luminance plays any anchoring role, we tested an inverted version of the staircase Gelb effect. We started with a white target square in a hidden shadow that appeared black (Kardos illusion). Then successively 4 darker squares were added in a row within the shadow: light gray, middle gray, dark gray, and black, each new configuration viewed by a separate group of 15 observers, who matched each square using a 16 step Munsell chart. The target square appeared lighter as each darker square was added, suggesting that the lowest luminance may play some anchoring role. However, as darker squares were added, not only did lowest luminance decrease but the number of squares (articulation) increased as well. In subsequent experiments we varied lowest luminance while holding articulation constant and we varied articulation while holding lowest luminance constant. Separate groups of 15 observers each viewed 6 different displays: 2, 5, and 30 squares with a reflectance range of 30:1 (white to black) and 2, 5, and 28 squares with a reflectance range of 2.25:1 (white to light gray). Articulation level had a major effect on target lightness while lowest luminance affected target lightness in the 5-square configuration, but not in the 2- or 28/30- square configurations. Our results suggest that the staircase Kardos effect is due to the increasing articulation, not the decreasing lowest luminance, consistent with other evidence of the asymmetry between highest and lowest luminance values in anchoring lightness.
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