Distance Deception by Fragmented Objects: How Figures May Alter the Perceived Depth of the Background.

Illusory displacements in depth may be perceived in simple geometric configurations devoid of cues for spatial computation but also in real-world images where there is no shortage of information of this kind. Two of these different contexts drew the attention of vision scientists as sources of depth illusions: the Kanizsa square and the images of statues that Catalano's created with a part missing. Similar depth alterations occur in both cases: the portions of the background surrounded by "inducers" (pacmen or body parts) are perceived as coming to the foreground.

Saturn and its Rings: Four Centuries of Imperfect Amodal Completion.

The planet Saturn is a familiar image for us, but it presents perceptual peculiarities that impeded the discovery of its structure and which can still be misleading today. Saturn appears to be surrounded by rings which hide it to a certain extent and then continue behind the outline of the planet. What we perceive is the result of a double amodal completion in which the planetary globe and the rings exchange the roles of occluding and occluded surface.

An unexplained three-dimensional percept emerging from a bundle of lines.

Perceptual grouping has been extensively studied, but some areas are still unexplored-in particular, the figural organizations that emerge when bundles of intersecting lines are drawn. Here, we will describe some figural organizations that emerge after the superimposition of bundles of lines forming the profile of regular triangular waves. By manipulating the lines' jaggedness and junction geometry (regular or irregular X junction) we could generate the following organizations: (a) a grid, or a figural configuration in which both the lines and closed contours are perceived, (b) a figure-ground organization composed of figures separated by portions of the background, and (c) a corrugated surface appearing as a multifaceted polyhedral shell crossed by ridges and valleys.

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.

Brightness/darkness induction and the genesis of a contour.

Visual contours often result from the integration or interpolation of fragmented edges. The strength of the completion increases when the edges share the same contrast polarity (CP). Here we demonstrate that the appearance in the perceptual field of this integrated unit, or contour of invariant CP, is concomitant with a vivid brightness alteration of the surfaces at its opposite sides.

Not physically present contours can yield illusory motion.

We demonstrate a perceptual effect whereby contours not physically present in a visual scene can yield striking illusory motion. The not physically present contours are paths of invariant contrast polarity (CP). For example, when a square checkerboard composed of dark and light square checks with small black and white discs covering the vertices is put in lateral motion, there is the striking perception of vertical expansion/contraction.

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).

Arcs in perceptual space--going off at a tangent.

Underestimating the curvature of an arc may result from our tendency to view it as prolonged in the direction of the tangent to its ends. Support is given here for this hypothesis, but evidence is also provided that tangent deviation may alter an arc's circularity along its whole length.

Features of the selectivity for contrast polarity in contour integration revealed by a novel tilt illusion.

We studied a novel illusion of tilt inside checkerboards due to the role of contrast polarity in contour integration. The preference for binding of oriented contours having same contrast polarity, over binding of opposite polarity ones (CP rule), has been used to explain several visual illusions. In three experiments we investigated how the binding effect is influenced by luminance contrast value, relatability of contour elements, and distance among them.

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 support.

Draughtsmen at work.

To obtain the representation of a contour, the visual system integrates fragments of a pattern. One of the 'binding rules' governing this process requires that a path of conjunction in which contrast polarity is preserved be followed. Here we show that this rule has a corollary: where two alternative paths compete to emerge in opposite directions, the one with greater contrast luminance is likely to prevail.

Spatial distortions induced by dotted patterns.

Prolonged steady fixation of parallel, radial, or concentric lines positioned close to each other generates phenomena of illusory oscillation, waving, and shimmering. Purkinje reported a first observation of these phenomena almost two centuries ago (review by Wade, 1977 Perception 6 407-433). Here we show that illusory waving and oscillation phenomena arise at first glance when dotted lines are either interposed between continuous lines, or positioned very close to straight edges.

A new "tilt" illusion reveals the relation between border ownership and border binding.

The "association field" models of contour detection predict specific spatial conditions for linking or grouping neighboring elements into smooth contours. We previously suggested that the "association field" model may account for perceptual binding of near-collinear luminance edges of same contrast polarity and their consequent unification into a unique contrast border with illusory tilt. This approach is now developed into a new version of the tilt illusion, the seesaw illusion, in which the contrast border is perceived as inverting concave-convex illusory curvature when background luminance is inverted, indicating that contrast polarity must be incorporated into the notion of "association field" to account for the seesaw illusion.

Boundary completion, contrast polarity, and the perception of illusory tilt.

What we perceive as a unitary object can be the result of integrative processes that generate a whole from parts. Although this issue of visual perception has been widely explored, recent experimental findings demonstrate that our knowledge is still incomplete. In particular, the question whether contour binding is affected by the sign of contrast (contrast polarity) across edges requires more in-depth examination.

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'Invenzione (Imaginary Prisons), which have fascinated people ever since they first appeared.

Illusory boundary interpolation from local association field.

We previously showed that interpolation between vertically misaligned luminance edges of same polarity of contrast is preferred to that between co-linear edges of opposite polarity of contrast, although it results in illusory tilt (Roncato and Casco, 2003). We here analyze the spatial conditions that produce this illusory binding of vertically misaligned edges of light and dark tiles, alternated in a row, and in counterphase with those in the rows above and below. We find that, independently of scale and number of tiles in a row, the illusion is perceived when the vertical misalignment of more than three tiles is smaller than or equal to 9' and the horizontal separation between co-linear edges is smaller than or equal to 13'.

Texture segregation on the basis of contrast polarity of odd-symmetric filters.

This is the first study to demonstrate the selectivity of learning for contrast polarity. The finding is the main result of an investigation into the existence of central and peripheral vision mechanisms selective for contrast polarity within the texture-segregation process, using the perceptual learning paradigm in a detection task. Energy models (Malik & Perona, 1990) exclude segregation of textures composed of elements of odd-symmetric luminance profile by contrast polarity differences.

Orientation misperceptions induced by contrast polarity: comment on "Contrast polarities determine the direction of Café Wall tilts" by Akiyoshi Kitaoka, Baingio Pinna, and Gavin Brelstaff (2004).

According to Kitaoka et al (2004, Perception 33 11-20), the Café 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.

Lines that induce phenomenal transparency.

Three neighbouring opaque surfaces may appear split into two layers, one transparent and one opaque beneath, if an outline contour is drawn that encompasses two of them. The phenomenon was originally observed by Kanizsa [1955 Rivista di Psicologia 69 3-19; 1979 Organization in Vision: Essays on Gestalt Psychology (New York: Praeger)], for the case where an outline contour is drawn to encompass one of the two parts of a bicoloured figure and a portion of a background of lightest (or darkest) luminance. Preliminary observations revealed that the outline contour yields different effects: in addition to the stratification into layers described by Kanizsa, a second split, opposite in depth order, may occur when the outline contour is close in luminance to one of the three surfaces.

The influence of contrast and spatial factors in the perceived shape of boundaries.

When an edge can be perceived to continue either with a collinear edge of the opposite contrast polarity or with a noncollinear edge of the same contrast polarity, observers perceive an alignment between the edges of the same contrast polarity, even though they are noncollinear. Using several stimulus configurations and both free and tachistoscopic viewing, we studied the luminance and spatial factors affecting the perceived distortion and binding. The results showed that the two noncollinear edges tended to align when they had the same contrast polarity (Experiment 1A) and to misalign when they had opposite contrast polarity (Experiment 2), providing that (1) they were separated by a distance larger than 1 arcmin and smaller than 3-4 arcmin (for all configurations) and (2) they laterally overlapped for about 7 arcmin (Experiment 1B).