Depth Plane Separation Affects Both Lightness Contrast and Assimilation

Lightness contrast and assimilation are two opposite phenomena: contrast occurs when a grey target perceptually acquires a complementary colour than the bordering, inducing, surfaces; assimilation is when a grey target perceptually acquires the same colour component as the inducers. Previous research has shown that both phenomena are affected by the manipulation of depth between the inducers and target. However, different results have been reported; it is not clear whether contrast persists when inducers are non-coplanar with the target. Previous studies differ for the spatial configuration of the stimuli and the technique adopted to manipulate depth. The aim of this research was to measure the effects of manipulating the depth between inducers and target in comparable conditions. Results show that contrast persists, but largely reduces, after depth manipulation whilst assimilation reverses to contrast. Furthermore, interesting asymmetries between white and black inducers emerged with white inducers favouring contrast and black inducers favouring assimilation. These results provide further evidence that high-level processes of visual processing are involved in both phenomena; with important consequences for lightness theories.