Context effect in assimilation and contrast

Abstract

We have systematically studied the change of lightness appearance in an ambiguous visual pattern. Fuchs (1923 Zeitschrift fur Psychologie 92 249 - 325) found that differently coloured parts assimilate, whereas Agostini et al (1993 Perception 22 263 - 272) described the opposite effect: contrast between the subparts. To explore these quite different results in which the perceived organisation could trigger either assimilation effects or contrast effects, we have constructed an ambiguous pattern in which lightness differences between parts could lead to assimilation or contrast. More specifically, we have constructed a visual pattern that could be perceived as a pattern of adjacent straight grey stripes, or as a pattern of adjacent but phase-shifted grey vases. The ambiguous parts, that were darker or lighter than the grey, could belong perceptually to either the stripes or the vases. We asked the participants to judge the relative lightnesses of the grey parts in the stripes and vases. By means of a strong contextual and attentional cue, focusing on either the stripes or the vases, we have induced the perception of one of these structures. It turned out that for any pattern with fixed luminance relations between the different parts, participants could be divided in three classes: one for which contrast dominates, one for which assimilation dominates, and one mixed class. This division turned out to be consistent for each participant.