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Two experiments are described, the purpose of which was to investigate the presence of a misalignment illusion caused by Poggendorff-like conditions in two paintings by Peter Paul Rubens, both depicting the Descent from the Cross, one located in Antwerp (Belgium), the other in Lille (France). The first shows a geometrical misalignment made by Rubens in a minor detail, which is considered proof that the artist observed the Poggendorff illusion. The second painting, instead, shows a perfect geometrical alignment in a similar detail. In experiment 1, participants were asked to align a top segment to a lower one in two types of stimuli: a full-size digitally manipulated reproduction of the painting and a Poggendorff-like configuration that recalled the painting's lines displacement and tilt. Adjustments were performed from two distances, one up close (painting distance) and one from below and far (observation distance). Results confirmed the presence of the Poggendorff illusion, but mean adjustments significantly differed from the misalignment perpetrated by Rubens. Experiment 2 was set up in a similar fashion with the Lille painting. Results confirmed the presence of the Poggendorff illusion also in this painting; however, the alignment by Rubens coincides with the geometrical one. Results from both experiments do not support the claim that Rubens observed the Poggendorff illusion and therefore corrected for it in the Antwerp painting. An alternative account is discussed, which relates to the structural layout of the painting.

The glare effect is a brightness illusion that has captured the attention of the vision community since its discovery. However, its photometrical reversal, which we refer to here as photometrical reversed glare (PRG) stimuli, remained relatively unexplored. We presented three experiments that sought to examine the perceived brightness of a target area surrounded by luminance gradients in PRG stimuli and compare them with conventional glare effect configurations. Experiment 1 measured the brightness of the central target area of PRG stimuli through an adjustment task; the results showed that the target appeared brighter than similar, comparative areas not surrounded by luminance gradients. This finding was unexpected given the recent report that PRG stimuli cause pupil dilation. Meanwhile, Experiments 2 and 3 implemented a rating task to further test the findings in Experiment 1. Again, the study found a robust brightening illusion in the target area of PRG stimuli in a wide range of target and background luminance. The results are discussed in comparison with the brightness enhancement of the glare effect.

Dilating the pupils allow more quanta of light to impact the retina. Consequently, by “forcing” dilation of one pupil with a pharmacological agent (Tropicamide), the brightness of a surface under observation should increase proportionally to the dilation of the eye pupil. In a psychophysical procedure of brightness adjustment and matching, we presented to one eye geometrical patterns with a central square (the reference pattern) that differed in physical brightness within backgrounds of constant luminance. Subsequently, with the other eye, participants adjusted to the same luminance a similar pattern (target) whose central square luminance was randomly set higher or lower in brightness than the reference. As only one eye was treated with Tropicamide, we assessed whether the subjective brightness of the target square shifted in a consistent direction when viewed with the dilated pupil compared to the untreated eye. We found that, as the eye pupil increased post drug administration, so did the sense of brightness of the pattern (i.e., higher brightness adjustments followed viewing the reference with the treated eye). A reversed effect was observed when the untreated (control) eye viewed the reference first. The results confirm that even slight dilations of the pupil can result in an enhanced perceptual experience of brightness of the attended object, corresponding to an average increase of 2.09 cd/m2 for each 1 mm increase in pupil diameter

Pupil diameters were recorded with an eye-tracker while participants observed cruciform patterns of gray-scale gradients that evoked illusions of enhanced brightness (glare) or of enhanced darkness. The illusions were either presented as static images or as dynamic animations which initially appeared as a pattern of filled squares that—in a few seconds—gradually changed into gradients until the patterns were identical to the static ones. Gradients could either converge toward the center, resulting in a central region of enhanced, illusory, brightness or darkness, or oriented toward each side of the screen, resulting in the perception of a peripheral ring of illusory brightness or darkness. It was found that pupil responses to these illusions matched both the direction and intensity of perceived changes in light: Glare stimuli resulted in pupil constrictions, and darkness stimuli evoked dilations of the pupils. A second experiment found that gradients of brightness were most effective in constricting the pupils than isoluminant step-luminance, local, variations in luminance. This set of findings suggest that the eye strategically adjusts to reflect in a predictive manner, given that these brightness illusions only suggest a change in luminance when none has occurred, the content within brightness maps of the visual scene

In his 1975 paper, Bozzi presented a sample of transparency effects obtained with achromatic line drawings, that is with drawings made only of black lines and dots. Commenting his work, I will follow the structure of his paper. Without the time to run appropriate experi - ments, and having shown the demos proposed in this work only to a few expert observers, whose descriptions are however consistent with my own impressions, when describing the effects I will rely on what I see, aware of the need for proper experiments to be carried out.

A white surface surrounded by luminance gradients which are darker in the outer ends appears brighter than an equiluminant surface surrounded by uniform gray (i.e., the glare effect). Although the glare effect has been a subject of much research, its photometrical reversal (i.e., the center is dark and the outer ends of the surrounding gradients are lighter), which we dubbed as photometrical negative glare (PNG), is still left surprisingly unexplored. In the present study, perceived luminance in the center of PNG was examined. Twenty participants compared equiluminant central areas of a PNG target and a comparative one, and rated how much the latter appeared to be brighter or darker than the former. Five luminance values were employed for the equiluminant (black and four levels of gray) and three for the background (black, dark gray, and light gray). Similar conditions were set up for the glare effect, and all stimuli were repeated ten times in random order. We expected a darkening effect with PNG stimuli compared to brightness enhancements experienced with glare stimuli. Results showed instead a substantial brightness enhancement for PNG, with the exception of the black target, which showed a tendency to appear darker than the comparative one.

The Glare Effect is an illusion in which a white region appears self-luminous when surrounded by linearly decreasing luminance ramps (R). The Glare Effect Test (GET) was constructed as a paper test to easily measure luminosity thresholds within such illusion in different age groups with the following hypotheses: (i) GET can actually measure a perceptual process that changes along the lifespan; (ii) given that older adults present more symptoms of discomfort glare, they should show a lower threshold compared to young adults. Two groups of adults were recruited: young (20–30 years old) and senior (60–75 years old). Participants were assessed with GET and a battery of visual tests: acuity, contrast sensitivity, BHT, Ishihara, and the Navon effect. GET comprised 101 cards in which R changed gradually from solid black (card 0) to full black-to-white ramp (card 100). Subjects were required to classify cards on the basis of those two reference cards. PSEs show no correlation with the other visual tests, revealing a divergent validity. A significant difference between the groups ‘‘young’’ and ‘‘senior’’ was found: contrary to our expectations, luminosity thresholds for ‘‘senior’’ were higher than those for ‘‘young’’, suggesting a non linear relationship between luminosity perception and discomfort glare

This paper is about the Poggendorff illusion in pictorial artworks. In particular, it deals with the role played by the Poggendorff illusion in the making of the geometrical misalignments in the mosaic known as Lunetta di San Lorenzo in Ravenna (Italy) and in Rubens’s oil on panel Descent from the Cross in Antwerp (Belgium). Three experiments are presented. Experiment 1 tested the role of viewing distance in perceived collinearity in the mosaic by asking participants to adjust two ends of a cross in silhouette renderings of the saint projected so to mimic its original size. Results confirm an effect of viewing distance and support the hypothesis that the author of the mosaic became aware of the illusion and attempted to ‘fix’ it. Experiment 2 aimed at verifying perceived collinearity in a book-size version of Rubens’s Descent from the Cross by employing a similar adjustment method. Results demonstrate that the Poggendorff illusion affects perceived collinearity in the painting, but perceptual collinearity does not agree with the actual geometrical misalignment in book-size reproductions of the masterpiece. Experiment 3 tested the role of viewing distance in a classic Poggendorff illusion configuration by employing the same setup as in experiment 1. Contrary to what was found in experiment 1, viewing distance did not modulate the magnitude of the illusion in the classic display, suggesting that the outcome of perceived collinearity is sensitive to the complexity of the visual scene. The difference in results between experiments 1 and 3 underscore the necessity of new tests before any reasonable conclusion can be drawn about the whys and wherefores of Rubens’s misalignment.

In 1949 the Georgian psychologist Uznadze described a simultaneous haptic size contrast illusion that occurs after adapting, out of vision, one hand to a small sphere and the other to a large one. After the adaptation process (10-15 simultaneous exposures), participants are suddenly exposed to two spheres equal in size and weight, but the impression is that the sphere held by the hand adapted to the large sphere is much smaller than the other one. Here we experimentally sought for the mechanisms underpinning the illusion. One experiment tested the possibility of measuring the illusion’s magnitude with a matching method in which right-handed participants were asked to indicate the size of each hand-held sphere on a 12 step visual scale made of spheres painted matt black. Variables were: (a, within) size of the adapting spheres; (b, between) evaluation direction (right-left, left-right); (c, between) position of the small adapting sphere (right, left). Only variables a and b produced significant effects, confirming the possibility to use a visual scale to measure the illusion, however suggesting also a role of handedness in the evaluation process. Further experiments assessed the role of handedness and of simultaneous vs single-hand adaptation in the evaluation process.

We investigated lightness perception under different illuminations with observers adapted to the ambient light. Four simultaneous lightness contrast paper displays were sequentially presented in random order; one display was the classic textbook illusion (1), the other three were contrast displays with four squares per background arranged in such a way to form a cross with a central gap around the target. The crosses for the three modified contrast displays were characterized as follows: white cross on black background and black cross on white background (2); dark grey cross on black background and light grey cross on white background (3); crosses with luminance ramps (4). Participants were randomly assigned to one of three illumination conditions – scotopic, normal, and Gelb, which yielded for the identical targets the following luminance: 0.82, 6.81, and 560 cd/m2 – and were asked to find for each target the corresponding “color” on a 16-step achromatic Munsell scale inserted inside a box whose illumination was unaffected by stimulus illumination. A main effect of illumination on lightness matching was found (p<0.0001), with targets appearing lighter as stimulus luminance is increased, regardless the type of configuration. Results are discussed with reference to existing theories in lightness perception.

In the staircase Gelb effect, five squares cut from achromatic Munsell (aM) papers 2.0, 4.0, 6.0, 8.0, and 9.5 are arranged in a row from the darkest to the lightest and illuminated by a spotlight, often referred to as “Gelb illumination”. The perceptual outcome is a compressed lightness range, from middle grey to white, or super-white. The illusion has been extensively used as a case study for the Anchoring Theory (Gilchrist et al, 1999, Psychological Review, 106, 786-834). According to such theory, the highest luminance (hL) of the configuration is assigned the value of white in the local framework. We studied the role played by such hL in the compression rate of the illusion, by manipulating the hL target – four levels: 9.5, 9.25, 9.0, and pastel yellow (Munsell 5Y 9/4 with luminance between the values for 9.25 and 9.0). Results show that the achromatic hLs are off the aM scale, appearing either luminous or super-white; the brightness of the yellow hL target appears also greater than 9.5. The compression effect drops as the hL is lowered. This “decompression” is statistically significant for targets 2.0 and 4.0 with hL 9.0 and yellow

Among monocular depth cues, ocular parallax (first described formally by Brewster in 1844) remains mostly unknown, its role in perception still not investigated scientifically. Given that every single eye movement induces ocular parallax, it is a potentially useful depth cue. This paper is an attempt to revive interest in the topic. As a monocular depth cue, ocular parallax naturally leads us to consider its benefit for a monocularly enucleated individual. Throughout history, numerous illustrious personalities coped with this fate in various ways. Here, we consider some historical insights into the visual life of the erstwhile duke of Urbino, Federico da Montefeltro (1422–1482), warlord and patron to the painter Piero della Francesca, and the Japanese warlord, Masamune Date (1567–1636), a vocal patron of exploration and the arts.

In 1975 Paolo Bozzi (1930-2003) published a paper in which he showed the possibility of creating impressions of transparencies with simple achromatic line drawings. Being this year the 10th anniversary of his passing, a selected core of his works – originally written in Italian – are now being translated into English, to be published in a book with original commentaries by other fellow scientists who were his students and/or interested in the topics of those papers. To commemorate Bozzi, I present a sneak preview of my comments written for his line drawing transparency paper, in which I discuss Bozzi’s findings and extend them to the domain of colour. By introducing colour, I demonstrate the generality of Bozzi’s findings and the strength of the effects, which are determined solely by figural structure, articulation, and good continuation, three themes that underlie Bozzi’s original work.

The erstwhile Duke of Urbino Federico da Montefeltro (1422–1482), Warlord and patron to the painter Piero della Francesca, suffered various scars in his military career; among these a few that, already while he was still a young man, affected his visual ability. Here, we present these historical details—including portraits of the Duke at two different stages in his life—to obtain insight into his vision, that is, what his visual capabilities were, which visual cues he most relied on, and the strategies he devised to improve this residual vision. Specifically, the evidence suggests that the Duke was willing to sacrifice his facial appearance for the sake of improved vision, ie that he had his nose surgically altered to enlarge his field of view. It also served him in his role as a horse-mounted military leader, and in some aspects may have enhanced some of his depth cues beyond the capacity of a person endowed with normal vision. We finally show how Piero della Francesca, in his masterful 1465–66 profile portrait of Federico, gives us an intuitive but uncannily personal glimpse of the way in which the Duke of Urbino looked out onto the world.

In a previous study (Daneyko et al, 2011 i-Perception 2 503–507) we argued that the geometrical misalignment of two parts of the long cross in the Byzantine mosaic known as Lunette of San Lorenzo (Ravenna) is related to the Poggendorff illusion, which induced the artist to prefer a perceptual adjustment over a geometrical one. However, the geometrical misalignment was greater in the original mosaic than the average alignment determined by participants to our previous experiment. In the present work we addressed this difference by projecting a silhouette rendering of the mosaic on a large screen, simulating both size and height of the saint viewed from ground level. Participants were asked to align one end of the cross to the other end from two different viewpoints: (1) from a distance of 50 cm frontoparallel to the projection, and (2) from a distance of 750 cm, 450 cm below the projection. Results from condition 1 were similar to those obtained in our previous experiment, while results from condition 2 are consistent with the geometrical misalignment of the original mosaic. Our findings support the hypothesis that the artist was aware of the misalignment illusion, noticed when he inspected the mosaic from ground level.

In the Mausoleum of Galla Placidia (Ravenna, Italy), the San Lorenzo lunette shows two peculiar visual effects: a transparency effect of gold seen through gold and perceptual collinearity between two parts of a cross which are physically misaligned. Both effects are found within the area of the halo surrounding the saint’s head. In this work we addressed the problem posed by the physical misalignment of the cross. Our hypothesis is that the physical misalignment went unnoticed throughout history because the artist produced a perceptual alignment to correct for the Poggendorff illusion. Hence, we asked observers to align two ends of a cross in a reproduction showing the silhouette of San Lorenzo’s torso holding the cross. Results support our hypothesis: both direction and magnitude of adjustments comply with the alignment in the original mosaic

We examined lightness effects observed in Delboeuf and Ebbinghaus size-contrast illusions. Results of four experiments are reported. Experiment 1 was conducted with Delboeuf- like stimuli and shows that the disk that appears bigger appears either lighter or darker than the disk that appears smaller, depending on the contrast polarity between disks and background. Experiment 2 shows that the direction of these lightness effects is not influenced by the lumi- nance of the size-contrast inducers. Experiment 3 shows that a similar lightness effect is also observed in modified Ebbinghaus size-contrast displays. Experiment 4 tested the presence of the size-contrast illusion in the stimuli used in experiments 2 and 3. © 2011 a Pion publication.