Lightness contrast & assimilation: testing the hypotheses

In lightness contrast, a target will alter its lightness to decrease similarity with neighbouring surfaces, while in lightness assimilation similarity is increased. Previous studies emphasized some aspects of the stimulation pattern that seem to favour the 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 the 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 to both tests the importance of these factors, and to predict lightness for each stimulus, according to all three hypotheses. Our results confirmed the influence of the tested factors on target lightness. The proposed hypotheses were poor in predicting the obtained data. The differential stimulation hypothesis correctly predicted the obtained effect in less than half our stimuli. The spatial frequency hypothesis did not correctly predict the strength of obtained effects, as we obtained the largest assimilation effects with low spatial frequency inducers. Finally, the assimilation asymmetry hypothesis did not predict a single obtained effect. Contrary to this hypothesis prediction, we obtained contrast with decrement and assimilation with increment inducers.