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Branka Spehar, Monica Iglesias, Colin W. G. Clifford; Assimilation and contrast in complex configurations. Journal of Vision 2005;5(8):241. doi: 10.1167/5.8.241.
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Lightness induction is the shift in surface appearance caused by adjacent or nearby surfaces. The spatial arrangement of the surround can make the appearance of the surface more different from (contrast) or more similar to (assimilation) the surround. Although most research on lightness induction has focused on contrast effects, it is effects of assimilation that have generally proven more challenging for theories of lightness perception. Although assimilation effects tend to occur with more complex contexts, often containing repetitive patterns, we are still generally unable to ascertain the particular conditions that determine whether assimilation or contrast effects will occur. The present study is motivated by our earlier findings with White's effect, where assimilation seems to be restricted to specific luminance conditions (Spehar, Gilchrist & Arend, 1995: Spehar, Clifford, & Agostini, 2002). A matching procedure and a forced-choice judgment procedure were used to investigate the influence that the luminance relationships between target and inducing surfaces had in four different configurations: checkerboard, bullseye, White's effect and Todorovic's effect. The results demonstrate that under classical luminance conditions, when target luminance is intermediate in luminance to the different inducing regions, all four configurations produce assimilative effects. However, when target luminance is either higher than or lower than that of all the inducing regions, the direction of the illusion reverses and all four configurations show contrast effects. These results provide strong evidence for the importance of qualitative luminance relations in the emergence of assimilation and contrast effects in lightness induction.
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