Shadows own the property that the luminance ratios along their edges are invariant. Shadows violating this property are defined as impossible shadows (ISs). Soranzo and Agostini (2004 Perception 33 1359 - 1368) found that ISs improve lightness constancy compared to possible shadows (PSs). They speculated that the luminance edges of ISs appear as illumination edges. In the present study, this hypothesis was further investigated by directly comparing the edges of PSs and ISs. In a CRT experiment, the screen was vertically divided into two frames of illumination by a contextual edge. Two adjacent rectangles created the standard edge ({ ratio=5:1}) which was collinear with the contextual one. Two additional adjacent rectangles produced the target edge. Observers were asked to equate the target edge contrast to the standard edge contrast. Two variables were systematically manipulated: contextual edge luminance ratio; 1:1 (base line), 5:1 (PS), 10:1 (IS1), 15:1 (IS2); and the additional rectangle position; light, shadow. We found that the apparent contrast of the standard edge was lower in ISs than in PSs. This effect was stronger in the light condition of the additional rectangle position variable. Outcomes corroborate the hypothesis that luminance edges produced by ISs are perceived as illumination edges.
The apparent luminance contrast in impossible shadow displays
GALMONTE, Alessandra;
2006-01-01
Abstract
Shadows own the property that the luminance ratios along their edges are invariant. Shadows violating this property are defined as impossible shadows (ISs). Soranzo and Agostini (2004 Perception 33 1359 - 1368) found that ISs improve lightness constancy compared to possible shadows (PSs). They speculated that the luminance edges of ISs appear as illumination edges. In the present study, this hypothesis was further investigated by directly comparing the edges of PSs and ISs. In a CRT experiment, the screen was vertically divided into two frames of illumination by a contextual edge. Two adjacent rectangles created the standard edge ({ ratio=5:1}) which was collinear with the contextual one. Two additional adjacent rectangles produced the target edge. Observers were asked to equate the target edge contrast to the standard edge contrast. Two variables were systematically manipulated: contextual edge luminance ratio; 1:1 (base line), 5:1 (PS), 10:1 (IS1), 15:1 (IS2); and the additional rectangle position; light, shadow. We found that the apparent contrast of the standard edge was lower in ISs than in PSs. This effect was stronger in the light condition of the additional rectangle position variable. Outcomes corroborate the hypothesis that luminance edges produced by ISs are perceived as illumination edges.
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