Changes in length judgments caused by rotation of the contextual distractor.

In the present study, we tested the applicability of the computational model of the illusion of interrupted spatial extent (Bulatov, Marma, & Bulatova, Attention, Perception, & Psychophysics, 82, 2714-2727, 2020) to account for the psychophysical data collected with three-dot stimuli containing a cross-shaped contextual distractor. In different series of experiments, the illusion magnitude changes caused by the rotation of distractors with different values of the internal angle (45°, 75°, and 90°) were quantitatively determined. It was shown that the data obtained for all modifications of stimuli can be rather well approximated by model functions proportional to the sum of the absolute values of cosines.

The filled-space illusion induced by a single-dot distractor.

In the present study, we tested the ability of our computational model of the filled‑space illusion to account for data collected in experiments with stimuli comprising single‑dot distractors. In three sets of experiments, we investigated this illusory effect as a function of distance between the distractor and lateral terminator of the reference spatial interval of the three‑dot stimulus. We found that the model calculations properly predicted all of the observed changes in magnitude of the illusion for stimuli with a single distracting dot placed both within and outside the interval, as well as, for stimuli with two distractors arranged symmetrically relative to the lateral terminator.

An effect of continuous contextual filling in the filled-space illusion.

In the filled-space (or Oppel-Kundt) illusion, the filled part of the stimulus for most observers appears longer in comparison with the empty one. In the first two experimental series of the present study, we investigated the illusory effect as a function of continuous filling (by a shaft-line segment) of the reference spatial interval of the three-dot stimulus. It was demonstrated that for the fixed length of the reference interval, the magnitude of the illusion increases non-linearly with the shaft length.

A quantitative analysis of illusion magnitude changes induced by rotation of contextual distractor.

In the present study, the predictions of the computational model of centroid extraction were verified in psychophysical examination of the length illusion induced by stimuli comprising the conventional or asymmetric Müller-Lyer wings as the contextual distractors. In experiments, the illusion magnitude changes evoked by rotation of distractors with different spatial parameters were quantitatively determined. It was demonstrated that the model calculations adequately account for the illusion magnitude variations shown by all the subjects for all modifications of stimuli.

Perceptual mislocalization of a single set of the Müller-Lyer wings.

In the present communication, a possible role of perceptual displacements of stimulus elements in the occurrence of visual illusions of extent has been considered. In psychophysical experiments with a single set of Müller-Lyer wings, subjects were asked to place an imaginary reference rectangle into a position that made the apex of the wings appear to be at the rectangle center. Three different stimulus parameters (the length, internal angle, or tilt angle of the wings) were used as independent variables in different series of experiments.

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.

Center-of-mass alterations and visual illusion of extent.

In the present communication, we have developed a computational model related to the conception of positional coding via centers-of-masses (centroids) of the objects' luminance distributions. The model predictions have been tested by the results of our psychophysical study of geometrical illusion of extent evoked by a modified Brentano figure consisting of three separate spots clusters. In experiments, the centroids of the clusters were manipulated by varying the positions of additional non-target spots flanking the stimulus terminators.

[Stimulus symmetry influence on curvature perception].

In the psychophysical experiments the symmetrical bright lines of various length and curvature are presented against the dark background on the monitor. The subjects are asked to adjust the test-spot by moving it horizontally or vertically into the invisible arc linking the endpoints of two visible segments of the given stimulus. The data have shown that the precision of curvature estimation depended on type of the curves: it is higher for the centered circle, less for the exponential spiral and still less for the shifted circle.