Oppel-Kundt illusion balance.

Perceptual estimates of spatial dimensions of visual objects depend on their shape and surface attributes. The present psychophysical study emphasizes two main contributors to the Oppel‑Kundt illusion: the outline of the filled space and the mode of filling. In past experiments, both factors have been considered significant.

Summation of two illusions of extent.

In the present visual psychophysical study, the Oppel‑Kundt and and Müller‑Lyer illusion magnitudes were measured separately (by single figures) and in combination (by two patterns superposed spatially). Data for 30 subjects revealed extensive variability both for the separate and combined illusion strength. Nevertheless, the effect of addition of the perceived length distortions was established.

Temporal dynamics of the Oppel-Kundt Illusion compared to the Müller-Lyer Illusion.

In psychophysical experiments, subjects reported whether the filled part of the Oppel-Kundt stimulus was longer than the empty one at different durations of the stimuli presentations. The experimental data yielded a smooth function indicating a gradual augmentation of the illusion strength within a relatively wide 100-1000 ms interval of the exposure durations. On the contrary, the experiments with the Müller-Lyer stimuli showed a gradual decrease of the illusion magnitude within the same interval of expositions.

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.

Three-part Oppel-Kundt illusory figure.

The Oppel-Kundt illusion was examined in the psychophysical experiments with the classical two-part stimuli and modified three-part figures. The modified versions comprised either one filled medial interval and two empty flanking intervals or one empty space situated in between two fillings. The illusion was measured as a function of the number of filling elements in the referential parts of the figures.

Centroid extraction and illusions of extent with different contextual flanks.

In the present study, a computational model of the automatic centroid extraction based on the processes of local integration of excitatory profiles in the visual pathways has been developed. The model predictions have been compared with the results of our psychophysical examination of the perceptual distortions of the spatial extent. In experiments, the subjects matched two spatial intervals flanked by one of the three types of the contextual objects: either the Müller-Lyer wings, or vertical stripes, or spot pairs.

Delboeuf illusion study.

In psychophysical experiments, the Delboeuf illusion was measured as a function of spatial parameters of the stimulus pattern. During the experiments, the stimulus shape, size, luminance, and the dimensions of inducing surround varied. Subjects were asked to change the size of the test part of the stimulus by adjusting its diameter to value that made the test part appear equal to the perceived size of the referent part.

[Müller-Lyer illusion and colour contrast].

In psychophysical experiments, subjects adjusted the test part of the Müller-Lyer figure to make it appear equal in length to the reference part of the figure in the absence and presence of luminance contrast. The illusion was measured as function of the length and internal angle of wings varying from 7 to 35 min of arc and from 40 degrees to 180 degrees, respectively. Müller-Lyer figures, 130 min of arc long with no shaft line, were generated by Cambrige Research Systems VSG2/3 and presented as stimuli on an EIZO T562 monitor with gamma correction.

Distracting effects in length matching.

In psychophysical experiments, subjects matched two spatial intervals of a three-spot stimulus into what appeared to them to be equal. The stimulus was flanked by stripes. The length matching errors increased in proportion to the referent interval of the stimulus and approached 6-12 percent of its length.

Superimposition of illusory patterns with contrast variations.

In psychophysical experiments, a perceived length matching task was performed. The stimuli were made of two spatially superimposed illusory figures that differed in structure and luminance contrast but had the same length and coincided precisely, with their ends matched. The contrast of one of the figures was fixed, and that of the other varied.

Psychophysical measurements of illusion of the puffy circle.

The strength of an illusion of curvature created by an equilateral triangle, square or pentagon inscribed in a circle has been measured in the psychophysical experiments. The arcs of the circle looked as if they were bowed outwards in the stimuli of various sizes, but, at a fixed diameter of the circumscribed circle, the triangles produced the strongest, and the pentagons, the weakest illusion. The strength of the illusion augmented with the stimulus diameter.

[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.