Superimposition of illusory patterns with contrast variations.

In psychophysical experiments, a perceived length matching task was performed. The stimuli were made of two spatially superimposed illusory figures that differed in structure and luminance contrast but had the same length and coincided precisely, with their ends matched. The contrast of one of the figures was fixed, and that of the other varied. In experiments with stimuli viewed monocularly, the combined patterns produced illusions of perceived length, the strength of which varied with alterations of contrast of one of the figures. If the figures were presented separately to different eyes of the same subject, changes of contrast did not have a noticeable influence on the illusion's strength. When the two stimulus components were displaced spatially and shown side by side, the monoptic and dichoptic stimuli yielded different results as well. The illusion's strength increased with an increase of the distance between the figures when viewed monocularly, but remained invariable if the figures were presented separately to each eye. The results obtained in experiments with dichoptic stimuli suggest that stimulus length distortions may occur in the monocular retinocortical pathways.