Anomalous induction of brightness and surface qualities: a new illusion due to radial lines and chromatic rings.

When a chromatic (eg light-blue) annulus surrounds the central gap of an Ehrenstein figure so as to connect the inner ends of the radial lines, a striking new lightness effect emerges: the central white disk has both a self-luminous quality (brighter than in the regular Ehrenstein figure) and a surface quality (dense, paste-like). Self-luminous and surface qualities do not ordinarily appear co-extensively: hence, the brightness induction is called anomalous. In experiment 1, subjects separately scaled self-luminous and surface properties, and in experiment 2, brightness was nulled by physically darkening the central gap. Experiments 3 and 4 were designed to evaluate the importance of chromatic versus achromatic properties of the annulus; other aspects of the annulus (width or the inclusion of a thin black ring inside or outside the chromatic annulus) were tested in experiments 5-7. In experiments 8-12, subjects rated the brightness of modified Ehrenstein figures varying the radial lines (number, length, width, contrast, arrangement). Variation of these parameters generally affected brightness enhancement in the Ehrenstein figure and anomalous brightness induction in a similar manner, but was stronger for the latter effect. On the basis of these results, anomalous brightness induction is attributed to a surface induction process triggered by an interaction between illusory brightness enhancement (due to the radial lines) and border ownership (due to the blue annulus).