A Computational Account of a Class of Orientation Illusions

Contrast-dependent orientation illusions are phenomena in which the appearance of the illusion depends not only on geometrical arrangements of the constituents of illusory configurations, but also on their luminance levels. Whereas certain standard configurations may evoke strong illusory effects, their contrast-manipulated variants (configurations in which only the luminance contrast polarity of some of their elements is manipulated, while retaining the geometry of the standard versions) may show weakened or no illusory effects, or even reversed illusions. Although generally rather salient, the contrast-dependent illusions have not been researched in much detail, except for the well-known Münsterberg (Café Wall) illusion. Here I report how this class of effects may be accounted for by a simple model involving banks of oriented filters. Given a 2D visual image (luminance distribution) as input, the model produces as output corresponding neural images (simulated 2D neural activity distributions). The simulations show that the model’s reactions to illusory tilt stimuli contain characteristic sub-patterns which are structurally similar to sub-patterns of its reactions to actually tilted stimuli. Furthermore, such ‘tilted’ sub-patterns are less prominent, absent, or even reversed in neural images of corresponding contrast-manipulated configurations. Thus the explanation why such configurations induce illusory appearance of tilt is that they are partial metamers of configurations that induce veridical appearance of tilt. In addition to now classical effects such as the Café Wall and the Twisted Cords illusion, this account also applies to some more recent, salient effects devised by Akiyoshi Kitaoka and Baingio Pinna