Flashed Müller-Lyer and Poggendorff Virtual Illusions

A moving frame can dramatically displace the perceived location of stimuli flashed before and after the motion. Here, we use a moving frame to rearrange flashed elements into the form of classic illusions. Without the moving frame, the initial arrangement of the flashed elements has no illusory effect. The question is whether the frame-induced displacement of position precedes or follows the processes underlying the illusions. This illusory offset of flashed chevrons does generate a Müller-Lyer illusion and the illusory offset of two line segments does create a Poggendorff illusion. We conclude that the site where the frame-induced position shift emerges must precede the site at which the Müller-Lyer and Poggendorf illusions arise.

Susceptibility to Size Visual Illusions in a Non-Primate Mammal (Equus caballus)

The perception of different size illusions is believed to be determined by size-scaling mechanisms that lead individuals to extrapolate inappropriate 3D information from 2D stimuli. The Muller-Lyer illusion represents one of the most investigated size illusions. Studies on non-human primates showed a human-like perception of this illusory pattern. To date, it is not clear whether non-primate mammals experience a similar illusory effect. Here, we investigated whether horses perceive the Muller-Lyer illusion by using their spontaneous preference for the larger portion of carrot. In control trials, we presented horses with two carrot sticks of different sizes, and in test trials, carrot sticks of identical size were shown to the subjects together with arrowheads made of plastic material and arranged in a way meant to elicit the Müller-Lyer illusion in human observers. In control trials, horses significantly discriminated between the smaller and larger carrot stick. When presented with the illusion, they showed a significant preference for the carrot that humans perceive as longer. Further control trials excluded the possibility that their choices were based on the total size of the carrot stick and the arrowheads together. The susceptibility of horses to this illusion indicates that the perceptual mechanisms underlying size estimation in perissodactyla might be similar to those of primates, notwithstanding the considerable evolutionary divergence in the visual systems of these two mammalian groups.

An Upward-Facing Surface Appears Darker: The Role Played by the Light-From-Above Assumption in Lightness Perception

The human visual system can extract information on surface reflectance (lightness) from light intensity; this, however, confounds information on reflectance and illumination. We hypothesized that the visual system, to solve this lightness problem, utilizes the internally held prior assumption that illumination falls from above. Experiment 1 showed that an upward-facing surface is perceived to be darker than a downward-facing surface, proving our hypothesis. Experiment 2 showed the same results in the absence of explicit illumination cues. The effect of the light-from-left prior assumption was not observed in Experiment 3. The upward- and downward-facing surface stimuli in Experiments 1 and 2 showed no difference in a two-dimensional configuration or three-dimensional structure, and the participants’ perceived lightness appeared to be affected by the observers’ prior assumption that illumination is always from above. Other studies have not accounted for this illusory effect, and this study’s finding provides additional insights into the study of lightness perception.

Infants Perceive Three-Dimensional Subjective Contours

The addition of crossed horizontal disparity enhances the clarity of illusory contours compared to pictorial illusory contours and illusory contours with uncrossed horizontal disparity. Two infant-controlled habituation–dishabituation experiments explored the presence of this effect in infants 5 months of age. Experiment 1 examined whether infants are able to distinguish between a Kanizsa figure with crossed horizontal disparity and a Kanizsa figure with uncrossed horizontal disparity. Experiment 2 tested infants for their ability to differentiate between a Kanizsa figure with crossed horizontal disparity and a two-dimensional Kanizsa figure. The results provided evidence that the participants perceived the two- and the three-dimensional illusory Kanizsa contour, the illusory effect in which was strengthened by the addition of crossed horizontal disparity.

Effects of Color and Luminance Contrast on Size Perception—Evidence from a Horizontal Parallel Lines Illusion

The present study investigated a size illusion composed of two horizontal lines that were vertically separated and parallel to each other. When the two lines were of equal length, the upper line was consistently perceived to be a little longer than the lower line, therefore it was termed as horizontal parallel lines (HPL) illusion. We investigated the effect of color and luminance contrast on the HPL illusion by manipulating the color and luminance of the two lines. Results indicated the following: (1) differences in color between the two lines reduced the illusion; (2) differences in luminance between the two lines reduced the illusion; (3) Effect 1 was greater than Effect 2; (4) the illusory effect could not be affected as long as both of the lines were of the same color or luminance. The results suggest that the color or luminance contrast may contribute to the overall decrease in the illusory effect for lines with different colors/luminances, but generally the illusion decreases as the two lines are less similar to each other. These findings indicate that the similarity or ‘sameness’ effect dominates the effect of color/luminance contrast on the size illusion over the effect resulted from contrast difference or depth perception.

Exploring the Effects of Visual Frame and Matching Direction on the Vertical-Horizontal Illusion

The work presented here uses an adjustment method to test the vertical-horizontal illusion across four different configurations: a cross-shape, an L-shape, an inverted-T and a rotated-T. We examine the modulatory role of the variables visual frame and direction of the adjustment on the illusory effect. Two experiments were performed, one with rectangular and one with curvilinear visual frames. Our data show that in both experiments, the size of the expected illusion increases from the cross-shape to the L-shape and from the L-shape to the inverted-T, where it reaches its maximum. In the rotated-T, the illusion reverses reaching a significant effect in the opposite direction. This pattern of results appears consistently across different experimental conditions, although the variability in the amount of illusory effect seems to be modulated by the intervention of the two variables examined. A dissection of the vertical-horizontal illusion has been carried out in terms of a two-factor explanation – anisotropy and bisection – interacting in different ways across configurations.

Vasarely’s Nested Squares and the Alternating Brightness Star Illusion

The arts sometimes precede the sciences in the discovery of fundamental visual principles. Victor Vasarely’s “Nested Squares” show an illusory effect in which corners look brighter and more salient than straight edges, despite having equivalent luminance. This chapter summarizes recent research, originally based on Victor Vasarely’s Nested Squares illusion, to discover the related perceptual and underlying physiological principles. The results offer significant insights into how corners, angles, curves, and line endings affect the appearance of brightness, shape, salience, depth, and color in our brains. Concepts covered include the alternating brightness star illusion, center-surround simulations, brain activation, corner perception, and the redundancy-reducing hypothesis.

The Curved Grid Non-Illusions

The generally accepted explanation of the Hermann grid illusion is Baumgartner’s hypothesis that the illusory effect is generated by the response of retinal ganglion cells with concentric ON-OFF or OFF-ON receptive fields. To challenge this explanation, some simple distortions to the grid lines were introduced that make the illusion disappear totally, while all preconditions of Baumgartner’s hypothesis remained unchanged. Psychophysical experiments in which the distortion tolerance was measured showed the level of distortion at which the illusion disappears at a given type of distortion for a given subject. Statistical analysis shows that the distortion tolerance is independent of grid-line width within a wide range and of the type of distortion, except when one side of each line remains straight. The conclusion is the main cause of the Hermann grid illusion is the straightness of the edges of the grid lines. Similar results have been obtained in the scintillating grid.

The Enigmatic Enigma Illusion

As a visual illusion, the Enigma illusion is a pattern that in its original version consists of 120 black radial lines on a white background intercepted by three bicolored annuli and a central disk. The main illusory effect in the Enigma (leading to its name) occurs during fixation of the center of the static image. Then, quite intense streaming motion may be perceived on the different annuli. It is characterized by a traveling wave or some subtle motion on the annuli that may not be described in more detail by the observer. Sometimes the observers call it “a feeling of motion”. This perceived (illusory) motion can occur in either direction; clockwise or counterclockwise. This example shows that such illusions are an important means to psychophysically investigate human motion perception and its limits.

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. The curves obtained by two modified figures and by the original two-part stimulus were quite similar in shape, but the magnitudes of the illusions differed significantly. The figure with two filled intervals yielded about twice-stronger illusory effect than the contrasting figure with a single filled and two empty intervals. The two-part stimulus showed the illusion magnitudes in the midst. Our assumption suggests the illusory effect being related particularly to overestimations of the filled interval when compared with the empty interval displayed side-to-side. The unfilled interval might not contribute to the illusion.