ILLUSORY MODIFICATIONS OF AREA AND APPARENT LIGHTNESS.

A white disk when inside a small concentric circle (a display like Delboeuf figure) looks lighter than when surrounded by a larger one. The magnitude of the effect has been measured: PSE are reported for two different measures of the surrounding circle and three different grey backgrounds (EXP. 1a and b). The aim of a second experiment was to compare the increased lightness effect to the illusory size increment of the central area. Perceived area (EXP. 2a) and lightness magnitudes of a central white disk (diameter: 1.2 cm), surrounded by different circles (diameter: 3, 5, 7, 9 e 11 cm), on a middle grey background, were scaled, according to Pair Comparisons Method. The “distances ” of the targets on "area " and "lightness " continua have been determined. The aim of the present research was to check whether exquisitely phenomenal modifications of area affect perceived lightness, without any change in the luminance/reflectance of a surface. The Delboeuf illusion has been employed to alter the perceived size of a target. The complete Delboeuf pattern shows two pairs of concentric circles: when the diameter of the inner circle of the first figure geometrically matches the diameter of the outer circle of the second figure (Coren & Girgus, 1978), observers judge the inner circle of the first figure to be larger than the outer circle of the second figure. In this study the first part (I) only of the figure described by Delboeuf (1893) has been used (Piaget, 1961). Contemporary studies of the Delboeuf illusion use a single figure with two concentric circles and typically the inner one is judged (Jaeger & Long, 2007): “in current works on this illusion researchers compare a first circle without an outer paired circle with a second one with an additional outer paired circle