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.

Research highlights
► Distracters tilting evokes changes of illusion magnitude by the cosine law.
► The cosine pattern of illusion magnitude changes does not depend on distracter shape.
► The results obtained support the “centroid” explanation of illusion of extent.