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Abstract
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.
Highlights
Since the analyses of the early Gestaltists (e.g., Wertheimer, 1923), perceptual groups and objects are known to be the results of integration/interpolation processes occurring at different stages: stimuli in the visual field are either summarized or mediated, producing the perception of higherorder figural units.Vision research on figural grouping, and approaches based on phenomenological accounts of visual perception, have revealed the role played by several factors in perceptual grouping, such as the well-known gestalt rules of orientation, closure, and symmetry
Study of early processes in perceptual grouping has mainly focused on the role of contrast polarity, with a wide variety of tasks and stimuli: contour formation in Glass patterns (Glass and Switkes, 1976; Burr and Ross, 2006), illusory contour formation (Prazdny, 1983; Dresp and Grossberg, 1997; Spehar, 2000), line contrast detection (Wehrhahn and Dresp, 1998), orientation discrimination with co-linear lines (Brincat and Westheimer, 2000), perceptual closure (Spehar, 2002), contour detection (Field et al, 2000), illusory misalignment and tilt of contours (Kitaoka et al, 2004; van Lier and Csathó, 2006; Guidi et al, 2011), and contours reassembling in perceptual transparency (Kitaoka et al, 2001; Singh and Huang, 2003)
Polarity of the Illusory Effect The illusory streaks appeared to be lighter than the background when the inducer surfaces were the palest figural units in the configuration
Summary
Since the analyses of the early Gestaltists (e.g., Wertheimer, 1923), perceptual groups and objects are known to be the results of integration/interpolation processes occurring at different stages: stimuli in the visual field are either summarized or mediated, producing the perception of higherorder figural units. In this configuration, according to Grossberg and Mingolla (1985b), the isochromatic figural units should not generate illusory contours, but this prediction is not confirmed by the appearance of a lattice of gray streaks joining the corners of the figures. Luminance patterns at the corners trigger changes in brightness, taking the form of oriented lines or streaks connecting the vertexes They are not visible when the sources are in isolation but, once they are sufficiently close in distance and orientation, they emerge as a group, as Gabor shapes aligned along a contour become detectable and “pop out” within a field of identical, randomlyoriented, Gabor shapes (Field et al, 1993). A luminance gradient at the external profile of the star-like figure was predicted to strengthen the illusion; the opposite, i.e., a contour of extreme luminance, is expected to prevent surface color spreading
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