Abstract
Lightness contrast alters lightness of a target decreasing its similarity with neighbouring surfaces (inducers), while lightness assimilation has an opposite effect, similarity is increased. Previous studies emphasized some aspects of stimulation that favour occurrence of one or both of these two phenomena: spatial frequency of the inducers, magnitude and direction of the reflectance difference between the target and the inducers. More importantly, based on previous studies three precise hypotheses can be formulated that predict occurrence of the two phenomena: spatial frequency, differential stimulation and assimilation asymmetry.
 We manipulated target and inducers’ reflectance, and inducers’ spatial frequency. This enabled us not only to test the importance of these factors, but to predict lightness for each stimulus, according to all three hypotheses. Our results confirmed the importance of tested factors for both lightness contrast and assimilation.
 Unfortunately, the proposed hypotheses were poor in predicting the obtained data. Differential stimulation hypothesis correctly predicted obtained effect in less than half situations, since small reflectance differences produced contrast, and large differences produced assimilation. Spatial frequency hypothesis did not correctly predict the strength of obtained effects, and we obtained largest assimilation effects with low spatial frequency inducers. Finally, assimilation asymmetry hypothesis did not predict a single obtained effect. Contrary to this hypothesis predictions, we obtained contrast with decrement, and assimilation with increment inducers.
Highlights
The lightness of an object heavily depends on its context, physically identical targets presented in different contexts can have very different appearances
A target embedded among other surfaces will alter its lightness to decrease similarity with those other surfaces
In lightness assimilation, the lightness of a target is altered in a way that similarity with the inducers is increased (Figure 1B)
Summary
The lightness of an object heavily depends on its context, physically identical targets presented in different contexts can have very different appearances. In lightness assimilation, the lightness of a target is altered in a way that similarity with the inducers is increased (Figure 1B). The configuration of the inducers is different: in the contrast display, inducers are perceived as two “frames”, while in the assimilation display, inducers are perceived as “stripes”. Another important difference is that there is a significant disparity in the physical area that the inducers occupy: their area is much larger in the classical contrast display. The classical contrast display can be modified in a way that inducers occupy the same area as those in assimilation display, and are perceived as “stripes” (Figure 1C)
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