Depth Plane Separation Affects Both Lightness Contrast and Assimilation

Lightness contrast and assimilation are two opposite phenomena: contrast occurs when a gray target perceptually acquires a complementary color than the bordering, inducing, surfaces; assimilation is when a gray target perceptually acquires the same color 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 while assimilation reverses to contrast. Furthermore, interesting asymmetries between white and black inducers emerged with white inducers favoring contrast and black inducers favoring assimilation. These results provide further evidence that high-level processes of visual processing are involved in both phenomena, with important consequences for lightness theories.

Local computation of lightness on articulated surrounds

Lightness of a grey target on a uniform light (or dark) surround changes by articulating the surround (articulation effect). To elucidate the processing of lightness underlying the articulation effect, the present study introduced transparency over a dark surround and investigated its effects on lightness of the target. The transparency was produced by adding a contiguous external field to the dark surround while keeping local stimulus configuration constant. Results showed that the target lightness did not change on the articulated surround when a dark transparent filter was perceived over the target, although it did on the uniform surround. These results suggest that image decomposition into a transparent filter and an underlying surface does not necessarily change lightness of the surface if the surface is articulated. Moreover, the present study revealed that articulating the surround does not always enhance lightness contrast; it can reduce the contrast effect when the target luminance is not the highest within the surround. These findings are consistent with the theoretical view that lightness perception on articulated surfaces is determined locally within a spatially limited region, and they also place a constraint on how the luminance distribution within the limited region is scaled.