It is well known that visible luminance gradients may generate contrast effects. In this work we present a
new paradoxical illusion in which the luminance range of gradual transitions has been reduced to make
them invisible. By adopting the phenomenological method proposed by Kanizsa, we have found that
unnoticeable luminance gradients still generate contrast effects. But, most interestingly, we have found
that when their width is narrowed, rather than generating contrast effects on the surrounded surfaces,
they generate an assimilation effect. Both high- and low-level interpretations of this ‘‘phantom” illusion
are critically evaluated.
