Illusory Modification of Lightness in “Anomalous” Figures

Observers see a white triangle on the top of three black disks; the white triangle appears lighter, although it has the same luminance of the white background. Also squared or irregular “anomalous” figures can be created (Kanizsa & Gerbino, 1987). Several competing explanations have been put forward for the effect (Dumais, & Bradley, 1976; Coren, 1991; Ramachandran & Rogers-Ramachandran, 2005), which will not be discussed in this context. The lightness/brightness of a surface is a function of the luminance of the area (candles/mq); when the same intensity is spread over a larger area the luminance decreases, and “normally” also the lightness is. Phenomenal modifications of lightness have been considered mainly in situations in which surfaces are affected by contrast, when different luminance/reflectance areas are simultaneously present (Li & Gilchrist, 1999; Gilchrist & Radonjić, 2009) Lightness can also be affected by illusory modification of the magnitude of an area: a white disk, when inside a small concentric circle (a display like those originally proposed by Delboeuf), looks not only larger but also lighter than when surrounded by a larger one. Increased size and lightness appear correlated (Zanuttini, Daneyko & Zavagno, 2009). This effect is counterintuitive, as the same intensity is spread over a larger area (Zanuttini & Danejko, 2010). The same lightness effect has been observed with the Ebbinghaus figure; moreover when the central disk is grey it looks darker (Danejko, Zavagno. & Zanuttini, 2011). What will happen of the lightness of an anomalous figure, when the size of the packs is manipulated in a Delboeuf or Ebbinghaus like manner?