Tilting of the contextual filling line and the magnitude of the filled/unfilled illusion

In the filled/unfilled (or the Oppel-Kundt) illusion, for most observers the empty part of the stimulus appears shorter than the filled one of the same length. Although the phenomenon of the filled/unfilled illusion is rather well studied, there is still no consensus among researchers regarding its occurrence and features. It is known that uniformity and regular distribution of the filling elements can be critical for the illusion manifestation: if the elements are irregularly spaced or unequal in height, thickness, or luminance, then the effect of the illusion is less apparent. This tilting of the contextual filling line should significantly influence the magnitude of the filled/unfilled illusion, what, in turn, can provide a valuable information concerning the properties of spatial distribution of contextual interactions within the relevant visual area. There was orginized experiments and results confirmed the validity of our model. The data of the psychophysical experiments showed the curve similar to a Difference-of-Gaussians function. Such the result is consistent with our modeling and supports the suggestion on the presence of some area of spatial integration in closed vicinity of the stimulus axis.