Stimulus determinants of achromatic constancy.

Achromatic constancy refers to the invariance of either lightness or brightness despite changes in illumination intensity. Previous results disagree as to the extent to which constancy occurs with fixed luminance ratio conditions. This may reflect a lack of distinction between lightness and brightness. We measured both lightness and brightness constancy with fixed luminance ratios between test and annular fields under conditions varying in stimulus duration and adaptation. Lightness constancy occurred for all ratios under sustained viewing conditions and irrespective of adaptation level. Brightness constancy occurred for high-contrast stimuli and under sustained viewing conditions. These results support a bidimensional description of achromatic stimuli and suggest that lightness and brightness are the outcomes of two functionally distinct processes.