Illusion of the Size of Incomplete Contours

Abstract

The aim of the study was the illusion of the stimulus size depending on the configuration of the stimulus contour. Incomplete contour stimuli were obtained by removing from the pentagon: either a horizontal, vertical, or diagonal contour line, or a combination of lines forming the angle or rectangle. The expansion effect was always present without any exceptions, as in psychophysical experiments with stimuli of full contours. If the lower horizontal line was not present in the contour of the pentagon, the illusion did not change much, 8 to 9 arc min. When two lower lines, horizontal and diagonal were absent, there was an increase in strength of the illusion from 8 to 13 arc min. Apparently, this happened because the angular component was destroyed and the effect of the Müller-Lyer inner wings, causing a reduction in distance, weakened. If only the angular component, a pair of wings, was exposed, the strength of the illusion did not decrease, although the stimulus became shorter than the entire figure. On the contrary, there was an increase of the illusion from 8 to 13 arc min. Thus, the ratio of the effects of expansion and reduction changed. The expansion was on the decline, and the reduction was also declining, but to a higher degree. If the angular component was absent, and the contour of the open rectangle remained, the stimulus became shorter again. But the illusion significantly increased (8 to 17 arc min) referring to the strengthening of the expansion effect and weakening of reduction. For two horizontal lines, the result slightly increased. For the full rectangle, the illusion has also increased. In general, the ratio of expansion and reduction processes varied greatly in the responses to the stimuli during the length matching procedure. The local components of the stimuli were clearly affected by the configurations to which they belonged. The visual perception of the form cannot be divided into the perception of components and folded back like geometric drawings. When separated, any single segment, such as a line, angle, and the rest of the figure, become new visual wholes, regardless of the fact that the mosaic of excitations caused by them in the retina remains almost unchanged. The main factor of perceived distortions in the size of visual stimuli should be considered the functional relationships of the contour segments forming the shape, and not the integration of neural reactions for each segment.