Dependence of the Oppel-Kundt illusion on configuration of the components

We studied a size estimation of spatial intervals, which were limited by two straight or curved lines. Intervals widths varied from 2.56 to 3.12 deg. Additional lines could be within intervals. The task of the observers was to estimate the separation between the distant lines. Three experiments were carried out. In the first experiment, several additional lines divided spatial intervals into equal parts. Thus, we investigated the Oppel-Kundt illusion. We founded the invariant dependence of the illusion on number of lines with respect to the interval size. In the second experiment, only two additional lines were arranged inside the interval on different distances. Maximal illusion was obtained for the equal distance between the all four lines. In the third experiment, the two curved parallel lines limited spatial interval. Two other equidistant lines could be within the intervals. Revealed here illusions were greater than illusions for the intervals bounded by straight lines. Model of modules gave a good approximation to data of the Oppel-Kund illusion.