Brightness / lightness at a particular stimulus region depends on the properties (e.g. size or luminance) of the surrounding regions. Some surroundings induce spatial contrast, while others induce assimilation. Here we show that some patterns of spatial effects in the induction phenomena demonstrated in psychophysical studies by Rudd (2010) and Helson (1963) can be predicted by two biologically-plausible retinal models developed by Wilson (1997) and van Hateren (2005; 2007), each with a minimal yet critical modification.