Illusory contours evoke, and filling elements reinforce the expansion of the perceived size

Abstract

In psychophysical experiments with illusory Kanizsa-type rectangles, observers perceived the distance between non-existent boundaries as longer than determined by the stimulus drawings. The elongation value gradually grew when the length of rectangles increased similar to experiments with real contoured and filled rectangles. In experiments with the Oppel-Kundt stimulus, according to the classics, the perceived length of the filled interval first increased from zero to maximum and then slightly decreased with the number of stripes (0–30). If the filling stripes were removed from the stimulus, but their endpoints remained, the perceptual errors did not disappear, only the experimental curve decreased somewhat, and the maximum smoothed. If two horizontal lines were added to the filled Oppel-Kundt stimulus interval, forming a contour rectangle, the perceptual errors were also pre-sent but remained approximately stable with various numbers of the filling stripes, and the curve approached the horizontal line. The data obtained showed that illusory contours could be as effective as real edges in the genesis of size expansion. The sense of illusory outline arose and intensified with increasing number of points in the line. The results confirmed the assumption that the Oppel-Kundt effect is an individual case of a general sensory phenomenon: the illusion of the size of visual objects. The decisive feature of the Oppel-Kundt object is horizontal illusory contours. The filling stripes themselves area uxiliary functional factors that, indicating the properties of the stimulus surface, emphasize its edges.