These findings appear to be
but a special case of a broader 
principle. That principle 
concerns how the edge between 
regions is perceived. Recent 
evidence suggests that the 
perceptual system only picks up 
the information about 
luminance difference at the 
edge between regions and then 
"assumes" that the difference 
at the edge applies throughout a 
region until another edge 
occurs. The evidence 
furthermore suggests that the 
perceptual system sorts the 
various edges in a scene into 
two categories: those in which 
the luminance ratio is based on 
lightness differences and those 
in which it is based on 
illumination differences. Once 
the two types of edges are sorted 
out, only the ratios from the 
entire set of regions between 
which there are surface-
lightness edges enter into the 
determination of specific 
lightness values. The 
illumination edges do not enter 
into this computation.