Lightness or Brightness? Kanizsa’s paradox.

Kanizsa (1954 Archivio di Psicologia 15 251 - 264) demonstrated a paradoxical effect in the domain of surface colour. The demonstration consists of two parts: (i) two grays compared in a contrast configuration show the classical simultaneous lightness contrast effect, while the same grays in an assimilation configuration undergo an assimilation effect; (ii) when comparing the contrast configurations to those of assimilation, paradoxically, the gray judged darker in the contrast configuration is perceived more similar to the gray judged lighter in the assimilation configuration, and a symmetrical similarity is observed for the other two grays. According to Kanizsa, in this type of comparison, lightness and brightness are confounded. In this research, four experimental displays derived from Helson's contrast and assimilation configurations were judged separately on the continuum of lightness. A 2 × 2 design has been used: direction of physical contrast (increments or decrements) and physical contrast (high or low inducer luminance). Contrast has been found for increments and no effect for decrements in the assimilation configurations, while contrast configurations show contrast for both increments and decrements. In conclusion, lightness matches elicit contrast effects also in assimilation configurations; this fact and the simultaneous presence of both contrast and assimilation configurations, seem to be crucial factors for the paradox to occur.