Contextual flanks' tilting and magnitude of illusion of extent.

The "centroid" explanation of the Müller-Lyer and similar illusions of extent supposes the perceptual positional shifts of the stimulus terminators in direction of the centers-of-masses of adjacent contextual flanks. In the present study, the validity of the assumption was tested in psychophysical examination of illusory figures comprising the Müller-Lyer wings or arcs of a circle as the contextual objects. In experiments, the illusion magnitude changes evoked by the tilting of stimulus flanks have been measured. A good resemblance between the experimental data and theoretical predictions was obtained that strongly supports the idea of local positional shifts and serves in favor of "centroid" explanation of illusions investigated.