Contrast Illusion: Old Facts in a New Light

Abstract

The article deals with the effect of contrast illusion. This experimental fact was discovered by N. N. Uznadze, but it has not yet received proper explanation. We review classical experiments by N. N. Uznadze, where a set display was established under conditions in which conscious control is assumed. The procedure and the results of the experiment that tests the possibility of contrast illusion under conditions of forming a fixed set on irrelevant stimulus (out of the attention focus) are described. Participants perceived geometrical figures on a screen divided into two windows. In the left window there were always 20% fewer points than in the right window. The task for the participant was to determine in which window the presented geometrical figure was displayed higher in the frame. Under control conditions participants compared an equal number of points in each window. In most cases (84%) the participants were wrong. Contrast illusions were dominant (58 % of all cases). The proposed explanation of the contrast illusion effect presumes two stages of unconscious processes that precede the effect of the illusion, namely, the stage of parallel collation and the stage of comparison of the results of that collation. Refs 12. Figs 3. Tables 2.