Contrast Polarities Determine the Direction of Café Wall Tilts

Extension of a computational model of a class of orientation illusions

Polarity-dependent orientation illusions constitute a class of illusions in which the impression of orientation does not depend only on geometrical relations between its elements, but also on the relations between their luminances. Several examples of such figures are presented in the paper. Todorović (2021a) presented a simple computational model of such phenomena. Simulations of the model indicated that a common feature of the neural basis of these illusions is the presence of certain neural structures called 'oblique clusters'. A limitation of the model was that it used a restricted set of parameters. In this paper a generalization of the model is introduced involving types of receptive fields, their orientation sensitivity and their size or spatial frequency tuning. The simulations of the new model indicated that oblique clusters were present in the reaction patterns under a much wider set of conditions, though not all. The original hypothesis that oblique clusters constituted the neural foundations of impressions of tilt in this class of illusions was vindicated.

Polarity-dependent orientation illusions: Review, model, and simulations

Polarity-dependent orientation illusions are manifested in figures in which the impression of target orientation does not depend only on geometrical relations between the elements of the figure, but also on the relations between their luminances, that is, on luminance polarities. The best-known phenomenon belonging to this class of effects is the Münsterberg/Café Wall illusion. In this paper a considerable number of examples of this type of illusions are presented, many of which are novel variants. A two-level convolutional model of such illusions is introduced, in which the first level corresponds to the stimulus input and the second level contains units fashioned after simple cells in V1, whose spatial patterns of activity represent the model's reaction to the stimulus. The main finding of numerous simulations of the model is that the figures inducing illusory impressions of tilt share a common spatial pattern of neural activation, labeled 'oblique clusters', which is absent in related non-illusory figures. Furthermore, a similar pattern is also present in simulations of figures which induce veridical impressions of tilt. The simulations suggest that the neural basis of perception of a specific degree of tilt may not be the activity of neurons tuned narrowly to that particular degree of tilt, but rather the presence of certain signature spatial patterns of activity of populations of neurons.

Motion Aftereffects From Moving Illusions

Lines in the café wall illusion, and motion trajectories in the furrow illusion, appear to be tilted away from their true orientations. We adapted to moving versions of both illusions and found that the resulting motion aftereffects were appropriate to their perceptual, not their physical, orientations.

Bioplausible multiscale filtering in retino-cortical processing as a mechanism in perceptual grouping

Why does our visual system fail to reconstruct reality, when we look at certain patterns? Where do Geometrical illusions start to emerge in the visual pathway? How far should we take computational models of vision with the same visual ability to detect illusions as we do? This study addresses these questions, by focusing on a specific underlying neural mechanism involved in our visual experiences that affects our final perception. Among many types of visual illusion, 'Geometrical' and, in particular, 'Tilt Illusions' are rather important, being characterized by misperception of geometric patterns involving lines and tiles in combination with contrasting orientation, size or position. Over the last decade, many new neurophysiological experiments have led to new insights as to how, when and where retinal processing takes place, and the encoding nature of the retinal representation that is sent to the cortex for further processing. Based on these neurobiological discoveries, we provide computer simulation evidence from modelling retinal ganglion cells responses to some complex Tilt Illusions, suggesting that the emergence of tilt in these illusions is partially related to the interaction of multiscale visual processing performed in the retina. The output of our low-level filtering model is presented for several types of Tilt Illusion, predicting that the final tilt percept arises from multiple-scale processing of the Differences of Gaussians and the perceptual interaction of foreground and background elements. The model is a variation of classical receptive field implementation for simple cells in early stages of vision with the scales tuned to the object/texture sizes in the pattern. Our results suggest that this model has a high potential in revealing the underlying mechanism connecting low-level filtering approaches to mid- and high-level explanations such as 'Anchoring theory' and 'Perceptual grouping'.

The Café Wall Illusion: Local and Global Perception from Multiple Scales to Multiscale

Geometrical illusions are a subclass of optical illusions in which the geometrical characteristics of patterns in particular orientations and angles are distorted and misperceived as a result of low-to-high-level retinal/cortical processing. Modelling the detection of tilt in these illusions, and its strength, is a challenging task and leads to the development of techniques that explain important features of human perception. We present here a predictive and quantitative approach for modelling foveal and peripheral vision for the induced tilt in the Café Wall illusion, in which parallel mortar lines between shifted rows of black and white tiles appear to converge and diverge. Difference of Gaussians is used to define a bioderived filtering model for the responses of retinal simple cells to the stimulus, while an analytical processing pipeline is developed to quantify the angle of tilt in the model and develop confidence intervals around them. Several sampling sizes and aspect ratios are explored to model variant foveal views, and a variety of pattern configurations are tested to model variant Gestalt views. The analysis of our model across this range of test configurations presents a precisely quantified comparison contrasting local tilt detection in the foveal sample sets with pattern-wide Gestalt tilt.

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.

Dancing Shapes: A Comparison of Luminance-Induced Distortions

We present an illusory display in which a grid of outlined squares is positioned in front of a moving luminance gradient. Observers perceive a strong, illusory, ‘wavelike’ motion of the superimposed squares. We compared luminance effects on dynamic and static aspects of this illusion. The dynamic aspect was investigated by means of a temporal gradient, which induced an illusory pulsing of the outlined squares. The static aspect was investigated in two different ways. In one experiment, the outlined squares were positioned on a spatial gradient, which caused the squares to look like trapezoid shapes. In another experiment, the squares were positioned on different luminance fields, which affected their apparent size. In all experiments, luminance settings were the same, and observers were asked to indicate the direction and strength of the induced distortions. The overall results show large agreements between the dynamic distortion and the first-mentioned static distortion, whereas different tendencies emerged for the second static distortion. In a second series of experiments, we examined these distortions for various ranges of the luminance gradient and for border gradients as well. On the basis of these data, we explored how the directions of the perceived distortions of the single-gradient displays examined in this paper could be related to each other.

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, Perception33 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.

Perceptual Experience of Visual Motion Activates hMT+ Independently From the Physical Reality: fMRI Insights From the Looming Pinna Figure

The human motion processing area, hMT+, has been labeled the critical neural area for processing of real and illusory visual motion in radial 2D patterns. However, the activation in hMT+ during perception of illusory rotation in the looming double-circular Pinna Figure (PF) generated in 3D space has not been observed yet. To do so, an optic-flow like motion of rings (looming) in PF was generated on a computer screen. A psychophysically precise nulling procedure allowed quantifying the individual amount of the perceived illusory rotation in PF (PI) for each participant. The interpolation of the individual illusory motion parameters created a subjectively non-rotating PF and a physically rotating control stimulus of identical rotary strength as the PI. The physically rotating control was a double-circular figure which diverged from PF only in its arrangement of luminance gradients. In a 3-Tesla scanner, participants were presented with a random order of rotating and non-rotating figures (illusory, real, no rotation, and nulled PI). Both types, illusory and real rotation, when equal in perceptual strength for the observer, were found to be processed by hMT+.

Does Alicante have the longest urban geometric illusion in the world?

Interest in geometric illusions in architectural design was restimulated in the 1970s by the publication of a paper by Gregory and Heard on the Café Wall illusion. Illusory patterns have been used in urban design for centuries. This brief note identifies two further examples of urban illusions found in Alicante, Spain and suggests that they may represent currently the longest examples of urban geometric illusion in the world.

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. Experiment 1 showed that the effect was indeed present only when the CP rule was satisfied, and found it to be stronger when the luminance contrast values of the elements are more similar. In experiment 2 the illusion was reported only with relatable edges, and its strength was modulated by the degree of relatability. The CP-rule effectiveness, thus, seems to depend on good continuation. The intensity of contrast polarity signals propagating from an oriented contour might be the less intense, the more its direction deviates from linearity. In experiment 3 we estimated the distance threshold and found it to be smaller than the one found for other illusions, arising with collinear fragments. This seems to show that the reach of the contrast polarity signal inside the association field of a contour unit is shorter along non-collinear orientations than along collinear ones.

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.

The Jaggy Diamonds Illusion

We report a new illusion in which the edges of diamonds placed at the intersections of grids are perceived to be jaggy (the jaggy diamonds illusion). Interestingly, the illusion disappears when the stimulus is rotated by 45°, when the stimuli are observed at a close distance, and on the diamond at which the observers stare. Luminance contrast between diamonds, grids, and background is a strong determinant for this illusion.

Three-dimensional shape from second-order orientation flows

In images of textured surfaces, orientation flows formed by perspective convergence invariably convey 3D shape. We show that orientation flows formed by contrast-modulated (CM) and illusory contours (IC) convey 3D shape, and that both stimulus types induce 3D shape aftereffects on CM and IC test stimuli. Adaptation to luminance-modulated (LM) orientation flows induce robust 3D shape aftereffects on CM and IC tests, however, aftereffects using CM/IC adapting stimuli on LM tests were substantially weaker. These results can be explained by the adaptation of 3D shape-selective neurons that invariantly extract first- and second-order orientation flows from striate and extra-striate signals, which receive stronger input from neurons selective for first-order orientation flows.

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. Here we show the effects of edge bindings that originate from the merging of laterally displaced edges with the same contrast polarity. We have studied a particular context in which such effects may emerge: a checkerboard with a series of alternated dark and light shapes superimposed on the corners of the squares. The phenomenal observations and experimental findings support the theories according to which boundary completions are originated by phenomena of edge propagation within a 'field of completion' (eg Shipley and Kellman, 2003 Perception 32 985-999) adjacent to an edge ending. Our findings conform to the Shipley and Kellman theory that boundary completion results from the interaction of edges as well as from edges and shapes lacking in oriented contours, the latter serving as 'receiving units', anchoring the paths of activations generated by oriented edges. We propose to integrate this theory with the hypothesis that interactions sensitive to the contrast sign generate conjunction paths of edges that alter their perceived orientation. Based on this perspective we propose an alternative account for the Café Wall illusion that can be extended to other phenomena of orientation misperception and to a Café Wall inversion effect that has not been observed previously.

Neural basis of 3-D shape aftereffects

We used selective adaptation to identify the neural mechanisms responsible for 3-D shape perception from orientation flows in retinal images [Li, A., & Zaidi, Q. (2000). Perception of three-dimensional shape from texture is based on patterns of oriented energy. Vision Research 40 (2), 217-242)]. Three-dimensional shape adaptation could involve stages from photoreceptors to non-oriented retinal cells, oriented cells in striate cortex, and extra-striate cells that respond to 3-D slants. To psychophysically isolate the relevant stage, we used 3-D adapting stimuli created from real and illusory orientations, and test stimuli different from the adapting stimuli in phases and frequencies. The results showed that mechanisms that adapt to 3-D shapes combine real and illusory 2-D orientation information over a range of spatial frequencies.

Reversed Café Wall illusion with missing fundamental gratings

The direction of Café Wall illusion was measured for ordinary Café Wall figures comprised of blocks with a square wave profile and for those with a missing fundamental (MF) profile. For the MF version, it was found that the illusion direction alternates according to the patterns' main component frequency when the shift between adjacent rows was systematically varied. The direction of illusion for the MF version was opposite to that for the original version when the phase shifts of adjacent rows were between 60 and 120degrees of the original grating. The results indicate that the illusion depends on the physical component rather than the appearance of the pattern. The possibility of the Café Wall illusion being mediated by low-level luminance mechanisms each tuned to a different spatial frequency is discussed based on the results.

Configurational coincidence among six phenomena: a comment on van Lier and Csathó (2006)

This study focuses on a configurational coincidence among six different illusions, two being motion illusions (reversed phi movement and phi movement), two being binocular stereo effects (Anstis-Rogers type and Gregory-Heard type), one being a position illusion (Gregory-Heard illusion), and one being an orientation illusion (Café-Wall-like tilt illusion). The stimuli of these six illusions or effects share the same configuration, in which a thin region is flanked by thick regions of different luminances. A phenomenological comparison is conducted with the use of dynamic demonstrations as well as static ones. In particular, the reversed phi movement is extensively discussed.