Positional Shifts in the Visual Illusions of Extent / Aleksandr Bulatov, Lina Mickienė, Natalija Bulatova

Bibliogr.: p. 40

The present communications addresses a possible role of the perceptual position shifts of the stimulus parts in Brentano's illusion and related illusions of extent. The results of the present psychophysical experiments with detached Müller-Lyer wings are discussed in relation to the findings of some recent studies of full versions of illusory figures. It is shown that the effects obtained strongly support the "centroid" explanation of illusions investigated. According to explanation proposed by Morgan et al. (1900), the geometric illusions of extent of the Müller-Lyer type occur because the visual system fails to isolate the figure terminators (shaft end-points, or wings apexes) from the neighboring contextual flanks (wings themselves), and the judgments of the distances between the figure's terminators are biased toward the distances between the centroids of the adjacent flanks. Thus, the crucial point of the explanations is that it implies the perceptual positional shifts of the stimulus terminators in direction of centers-masses of the flanks. Recently, referring to the "centroid" hypothesis, a computational model of automatic centroid extraction has been developed and successfullu applied (Bulatov et al., 2009, 2010) to account the data obtained in experiments with full versions of illusory figures of the Brentano type; however, the issue concerning the manifestation of positional shifts for a single wings set remains unclear. In order to move forward in solving the problem and check the basic assumption of the "centroid" explanation of illusions of extent the present psychophysical study with detached Müller-Lyerwings was performed.