The effects of depth separation on lightness contrast and lightness assimilation

An intriguing paradox is created by the phenomena of lightness contrast, in which the difference in lightness between targets and inducers is over-estimated, and assimilation, in which this difference is under-estimated. Inducers’ spatial frequency is an important factor: inducers with low-spatial frequency seem to induce contrast whilst inducers with highspatial frequency seem to induce assimilation. The interaction of this factor with depth is less clear. Wolff (1934) found that contrast disappears when low-spatial frequency inducers are non-coplanar with targets, while Soranzo at al. (2010) found that contrast persists even with high-spatial frequency inducers when they are non-coplanar with the targets. The difference between these two studies is that Wolff manipulated actual depth, whilst Soranzo et al. manipulated stereoscopic depth. This study manipulated the actual distance between target and inducers, as well as Inducers’ Spatial frequency and Intensity. Participants indicated the target lightness by choosing the correspondent chip in a Munsell scale. Results suggest that high spatial-frequency inducers generate assimilation effects only when they are coplanar with the targets, but contrast effects when they are non-coplanar. Low-spatial frequency inducers generate contrast effects both when they are coplanar and at distance. We conclude that assimilation is more affected by distance than contrast.