Illusory streaks from corners and their perceptual integration

Perceptual grouping appears both as organized forms of real figural units and as illusory or “phantom” figures. The phenomenon is visible in the Hermann grid and in configurations which generate color spreading, e.g., “neon effects.” These configurations, generally regular repetitive patterns, appear to be crossed by illusory bands filled with a brighter shade or a colored tinge connecting the various loci of illusory effects. In this work, we explore a particular new illusion showing a grouping effect. It manifests as illusory streaks irradiating from the vertexes of angular contours and connecting pairs of figures nearby. It is only clearly visible when more than one figure is shown, and takes the shape of a net crossing their corners. Although the grouping effect is vivid, the local source of the illusion is completely hidden. Theories explaining this effect as due to the irradiation of illusory streaks (mainly that of Grossberg and Mingolla, 1985a,b) do not fully explain the figural patterns presented here. Illusory effects have already been documented at the angles of various figures, causing them to alter in amplitude and brightness; however, the figure illustrated here appears to have different features and location. Phenomenological observations and an experiment were conducted to assess the role played by geometric and photometric parameters in this illusion. Results showed that sharp angles, in low contrast with the surround, are the main source of the illusion which, however, only becomes visible when at least two figures are close together. These findings are discussed with respect to theories of contour processing and perceptual grouping, and in relation to other illusions

Orientation misperceptions induced by contrast polarity: comment on "contrast polarities determine the direction of caf\uc3\ua9 wall tilts" By Kitaoka et al

According to Kitaoka et al (2004, Perception 33 11 ^ 20), the Cafe\ua8 Wall illusion can be reduced to misalignment effects produced locally by a large shape on a line passing nearby. I demonstrate here that the interacting units are edges and not whole shapes, and that the source of the illusion does not consist in a local tilt but in a tendency of the edges to join when they have the same contrast polarity

The brightness of diamonds

A brightness illusion and transparency effects are described for configurations comprising alternating black and white bars on which are superimposed mid-luminance shapes (diamonds and hexagons). The resulting phenomena are predictable by Anderson's 'scission theory', which is thus given further suppor

The effects of luminance variation on edge misperception

A gray outline against a white (or black) ground appears to deviate when one of the divided regions turns into black (white). The direction of shift is not predictable on the basis of luminance profile and polarity contrast of this part of contour, called gray edge (to indicate a stepwise gradient from black to gray and from gray to white). Rather, it appears to depend on the luminance profiles of the collinear regions: A gray edge shifts toward the dark side whenever collinear with a gray line traversing a white ground. The same gray edge takes the opposite direction whenever it extends against a black ground. This rule proved to be successful in predicting the illusory convergence of the sides of a square that formed the stimuli of the first experiment, but the magnitude of the phenomenon was affected by luminance ratios and polarity contrasts of the gray edges, in agreement with the fmdings of the experiments on gray or blurred edge misalignment. A second experiment tested some hypotheses predicting the combined effects of two or more distorting sources. These hypotheses, suggested by the physical theory of vector sum, were partially disproved. A new model is proposed that assumes different ways ofintegrating local distortions. The third experiment tested predictions of how distorting pulses in opposite directions combine. The illusory misplacement of edge studied in this experiment is proposed as the underlying phenomena of the cafe wall illusion, the hollow square illusion, and other illusory phenomena observed with blurred areas. A connection with the induction grid phenomena is hypothesized

Piranesi and the infinite prisons

Piranesi was an extraordinarily talented artist who came to be considered the best known engraver and etcher of the 18th century. He spent his lifetime recording the magnificent buildings and ruins of ancient Rome. In his earlier work, he developed architectural fantasies and dark visions of imaginary prisons, the Carceri d\u2019Invenzione (Imaginary Prisons), which have fascinated people ever since they first appeared. They also made an impact in later artistic and literary contexts. Today they are considered the best expression of bewilderment, of the world\u2019s massive oppression of man. How did Piranesi generate such a powerful source of meaning? I have reviewed a number of contributions by art essayists in the search for what lies behind the fascination of the \u2018Carceri\u2019. Analysis of these texts provides a rich set of aesthetic and psychological attributes as well as detailed indications of where to seek the source of the meanings. I suggest here a fruitful integration of this analysis, developed by considering various particular perceptual and geometric cues. I discuss three possible sources of meaning: the combination of architectural elements out of proportion; the difficulty of building a unitary space; and the \u2018bottom perspective\u2019 that gives rise to a sensation of floating

Brightness Alteration with Interweaving Contours

Chromatic induction is observed whenever the perceived colour of a target surface shifts towards the hue of a neighbouring surface. Some vivid manifestations may be seen in a white background where thin coloured lines have been drawn (assimilation) or when lines of different colours are collinear (neon effect) or adjacent (watercolour) to each other. This study examines a particular colour induction that manifests in concomitance with an opposite effect of colour saturation (or anti-spread). The two phenomena can be observed when a repetitive pattern is drawn in which outline thin contours intercept wider contours or surfaces, colour spreading appear to fill the surface occupied by surfaces or thick lines whereas the background traversed by thin lines is seen as brighter or filled of a saturated white. These phenomena were first observed by Bozzi (1975) and Kanizsa (1979) in figural conditions that did not allow them to document their conjunction. Here we illustrate various manifestations of this twofold phenomenon and compare its effects with the known effects of brightness and colour induction. Some conjectures on the nature of these effects are discussed