Amodal completion versus induced inhomogeneities in the organization of illusory figures.

An analysis is presented of a phenomenological model of illusory contours. The model is based on amodal completion as the primary factor giving rise to the illusory figure. In the experiment, conducted by the method of paired comparisons, the same parameter was manipulated in two series of equivalent configurations. The first series yielded examples of amodal completion, the second examples of illusory figures. Three groups of subjects evaluated the magnitude of completion, the brightness contrast of the illusory figure, and the contour clarity of the illusory figure. A control experiment was conducted, which demonstrated that in these configurations amodal completion and amodal continuation behave in the same way. Line displacement did not influence the brightness or the contour clarity of the illusory figures, though it influenced the magnitude of amodal completion. These results are in agreement with the energetic model developed by Sambin.     

Early demonstrations of subjective contours, amodal completion, and depth from half-occlusions: "stereoscopic experiments with silhouettes" by Adolf von Szily (1921).

If all-black figures are used, certain monocular appendages to binocular shapes are seen in depth, either nearer (when in a medial position) or further (when in a lateral position) than the binocular shape itself. These appendages also link to form subjective contours in front of the binocular shape or amodal completions behind it. These and other discoveries by von Szily, made before 1921, anticipate a number of modern findings.     

Illusory contour figures are perceived as occluding surfaces by 8‐month‐old infants

Infants have been demonstrated to be able to perceive illusory contours in Kanizsa figures. This study tested whether they also perceive these illusory figures as having the properties of real objects, such as depth and capability of occluding other objects. Eight‐ and five‐month‐old infants were presented with scenes that included a Kanizsa square and further depth cues provided by the deletion and accretion pattern of a moving duck. The 8‐month‐old infants looked significantly longer at the scene when the two types of occlusion cues were inconsistent than when they were consistent with each other, which provides evidence that they interpreted the Kanizsa square as a depth cue. In contrast, 5‐month‐olds did not show this difference. This finding demonstrates that 8‐month‐olds perceive the figure formed by the illusory contours as having properties of a real object that can act as an occluder.     

Evidence relating subjective contours and interpretations involving interposition

Subjective contours, according to one theory, outline surfaces that are apparently interposed between the viewer and background (because of the disruption of background figures, sudden termination of lines, and other interposition 'cues') but are not explicitly outlined by intensity discontinuities. This theory predicts that if the cues are not interpreted as evidence of interposition, no intervening surface need be postulated, hence no subjective contours would be seen. This prediction, however, is difficult to test because observers normally interpret the cues as interposition evidence and normally see the subjective contours. Tests are reported on a patient with visual agnosia who is unable to make the usual interposition interpretations and unable to see subjective contours, but has normal ability to interpret standard visual illusions, stereograms, and in particular, stereogram versions of the standard subjective contour figures, which elicit to him strong subjective edges in depth (corresponding to the subjective contours viewed in the monocular versions of the figures.     

Modal and amodal completion generate different shapes

Mechanisms of contour completion are critical for computing visual surface structure in the face of occlusion. Theories of visual completion posit that mechanisms of contour interpolation operate independently of whether the completion is modal or amodal--thereby generating identical shapes in the two cases. This identity hypothesis was tested in two experiments using a configuration of two overlapping objects and a modified Kanizsa configuration. Participants adjusted the shape of a comparison display in order to match the shape of perceived interpolated contours in a standard completion display. Results revealed large and systematic shape differences between modal and amodal contours in both configurations. Participants perceived amodal (i.e., partly occluded) contours to be systematically more angular--that is, closer to a corner--than corresponding modal (i.e., illusory) contours. The results falsify the identity hypothesis in its current form: Corresponding modal and amodal contours can have different shapes, and, therefore, mechanisms of contour interpolation cannot be independent of completion type.     

Simultaneous completions of modal and amodal figures: Visual evoked potentials reveal asymmetrical interference effects

Modal and amodal completion processes are thought to affect the emergence and the potency of each other. To see whether one dominates over the other, we measured the Npd, that is, the negativity of visual-evoked potentials whose amplitude increases with perceptual difficulty. In the first experiment, the Npds to illusory (modal) squares and diamonds placed over amodal diamonds and squares, respectively, were found to be greater when targets were the amodal figures than when targets were the modal figures, thus suggesting that modal figures created more interference. A second experiment showed that greater Npds were specific to interference and not to the greater difficulty at focusing on amodal figures rather than on modal figures. A third experiment showed that the interference also occurs with real figures in replacement of modal ones. Overall, the results suggest a certain dominance of modal completion over amodal completion when both occur in the same display.     

Is amodal completion necessary for the formation of illusory figures?

Kanizsa's hypothesis suggests that the creation of an anomalous surface is due to the amodal completion of the inducers. In the present paper a new pattern that is able to disconfirm this explanation is presented. According to the Helmholtz-Ratoosh law amodal completion only occurs when the borders of two adjacent surfaces meet forming T-shaped junctions. When the borders of the two adjacent surfaces have Y-shaped junctions, amodal completion is absent. However, when a pattern inducing an anomalous figure has the latter figural characteristics, in spite of the absence of amodal completion, an illusory figure is still visible. In this paper a set of experimental results (carried out by means of a magnitude estimation procedure as well as the method of constant stimuli) supporting the aforementioned observations is presented.     

错觉轮廓的适应效应

错觉轮廓反映知觉的主动建构过程, 考察其是否存在适应效应有助于理解视觉系统反馈调节的特性。我们采用Kanizsa这种典型的错觉轮廓来研究其适应过程, 结果发现：Kanizsa错觉轮廓具有适应效应, 并且这种适应主要是由主观形成的整体轮廓造成的, 而不是由Pac-Man上的线条引起的。表明依赖于高级视觉皮层反馈调节的主观建构过程和自下而上的神经元信息一样, 会随呈现时间的增加, 神经活动减弱, 体现为适应效应。     

Amodal completion and the perception of depth without binocular correspondence

Half-occlusions and illusory contours have recently been used to show that depth can be perceived in the absence of binocular correspondence and that there is more to stereopsis than solving the correspondence problem. In the present study we show a new way for depth to be assigned in the absence of binocular correspondence, namely amodal completion. Although an occluder removed all possibility of direct binocular matching, subjects consistently assigned the correct depth (convexity or concavity) to partially occluded 'folded cards' stimuli. Our results highlight the importance of more global, surface-based processes in stereopsis.     

Perceptual Grouping: It's Later Than You Think

Recent research on perceptual grouping is described with particular emphasis on the level at which grouping factors operate. Contrary to the standard view of grouping as an early, two-dimensional, image-based process, experimental results show that it is strongly influenced by binocular depth perception, lightness constancy, amodal completion, and illusory figures. Such findings imply that at least some grouping processes operate at the level of conscious perception rather than the retinal image. Whether classical grouping processes also operate at an early, preconstancy level is an important, but currently unanswered question.     

The role of junctions in surface completion and contour matching

It has been suggested that contour junctions may be used as cues for occlusion. Ecologically, T-junctions and L-junctions are concurrent with situations of occlusion: they arise when the bounding contour of the occluding surface intersects with that of the occluded surface. However, there are other image properties that can be used as cues for occlusion. Here the role of junctions is directly compared with other occlusion cues--specifically, relatability and surface-similarity--in the emergence of amodal completion and illusory contour perception. Stimuli have been constructed that differ only in the junction structure, with the other occlusion cues kept unchanged. L-junctions and T-junctions were eliminated from the image or manipulated so as to be locally inconsistent with the (still valid) global occlusion interpretation. Although the other occlusion cues of relatability and surface similarity still existed in the image, subjects reported not perceiving illusory contours or amodal completion in junction-manipulated images. Junction manipulation also affected the perceived stereoscopic depth and motion of image regions, depending on whether they were perceived to amodally complete with a disjoint region in the image. These results are interpreted in terms of the role of junctions in the processes of surface completion and contour matching. It is proposed that junctions, being a local cue for occlusion, are used to launch completion processes. Other, more global occlusion cues, such as relatability, play a part at a later stage, once completion processes have been launched.     

Filling-in models of completion: rejoinder to Kellman, Garrigan, Shipley, and Keane (2007) and Albert (2007)

There has been a growing interest in understanding the computations involved in the processes underlying visual segmentation and interpolation in conditions of occlusion. P. J. Kellman, P. Garrigan, T. F. Shipley, and B. P. Keane and M. K. Albert defended the view that identical contour interpolation mechanisms underlie modal and amodal completion. In the current rejoinder, the author provides further psychophysical evidence against this view and argues that no physiological data support the claim that modal and amodal contours are represented identically at any stage of processing. The author also shows that the illusory glass surfaces that Kellman et al. and Albert upheld as evidence against his arguments about luminance constraints in completion are explained by theoretical principles that he has previously articulated, and variants of these illusions receive no explanation within either of the models Kellman et al. and Albert propose. The author shows that the principles needed to explain these percepts embody fundamental asymmetries in the way that relative depth shapes segmentation and interpolation processes and that models of completion that lack these constraints--such as P. J. Kellman, P. Garrigan, and T. F. Shipley's and M. K. Albert's --cannot account for a host of documented completion phenomena.     

The effects of perceptual set on the shape and apparent depth of subjective contours

Two experiments showed the influence of perceptual set on the perception of subjective contours. In the first, the perceived shape of a subjective-contour figure (a minimal version of the Ehrenstein configuration) was varied by altering the observer’s viewing set. The second experiment showed that apparent depth emerged in subjective-contour figures when observers were set to perceive the illusory contours.     

Illusory form with inducers of opposite contrast polarity: Evidence for multistage integration

The perception of brightness differences in Ehrenstein figures and of illusory contours in phaseshifted line gratings was investigated as a function of the contrast polarity of the inducing elements. We presented either continuous lines or line-like arrangements composed of aligned dashes or dots whose spacing was varied. Ayes/no procedure was used in which naive observers had to decide whether or not they perceived a brightness difference in a given Ehrenstein figure or an illusory contour in a phase-shifted line grating. The results show that brightness differences are perceived to some extent in Ehrenstein figures with inducers of opposite polarity of contrast; however, the percentage ofyes responses was systematically lower and response times were longer than for figures with inducers of the same polarity. Phase-shifted line gratings with lines of opposite polarity of contrast yielded stronger illusory contours and shorter response times than those with lines of the same polarity. When the sign of contrast was not the same within a given line of induction, neither differences in brightness nor illusory contours were perceived. The results suggest that the mechanisms that lead to apparent differences in brightness are more sensitive to input of the same contrast polarity, the mechanisms generating illusory contours more sensitive to input of opposite polarity. The data are discussed in the light of a multistage approach to illusory form perception and some implications for cortical models of illusory contour integration are discussed.     

Illusory contours and stereo depth

A theory for illusory contours, which fill gaps in certain figures, is proposed and subjected to an experimental test. We suggest that nearer masking objects are perceptually postulated to “account” for gaps when these are unlikely. The experiment shows that when stereoscopic depth information incompatible with this “perceptual hypothesis” is presented, the illusory contours are reduced in intensity or disappear.     

When does grouping happen?

Recent research on perceptual grouping is described with particular emphasis on identifying the level(s) at which grouping factors operate. Contrary to the classical view of grouping as an early, two-dimensional, image-based process, recent experimental results show that it is strongly influenced by phenomena related to perceptual constancy, such as binocular depth perception, lightness constancy, amodal completion, and illusory contours. These findings imply that at least some grouping processes operate at the level of phenomenal perception rather than at the level of the retinal image. Preliminary evidence is reported showing that grouping can affect perceptual constancy, suggesting that grouping processes must also operate at an early, preconstancy level. If so, grouping may be a ubiquitous, ongoing aspect of visual organization that occurs for each level of representation rather than as a single stage that can be definitively localized relative to other perceptual processes.     

Surface contours, Glass patterns, and a slant illusion

A surface contour pattern constructed from continuous sine waves is subject to several visual interpretations, whereby the separate regions containing the maxima and the minima of the sine waves may be seen as representing either convex or concave areas of a three-dimensional surface. In a pattern of segments of contours comprising only the regions containing the maxima and minima of the sine waves, a set of surfaces is perceived, each of which tends to be seen as convex, and which possesses an illusory slant which is different for columns of contour segments containing maxima as compared with columns containing minima. It is conjectured that the slant illusion is a manifestation of the processes by which depth is derived from surface contour information. It is demonstrated that corresponding figures constructed from sinusoidal Glass patterns produce similar effects. From this it is concluded that the structure of Glass patterns provides a sufficient input representation for the processes by which surface shape is recovered from surface contours.     

Late influences on perceptual grouping: illusory figures

Two experiments demonstrate that grouping can be strongly influenced by the presence of figures defined by illusory contours. Rectangular arrays were constructed in which a central column of figures could group either with those on one side, on the basis of perception of figures defined by illusory contours, or with those on the other side, on the basis of physically present inducing elements. In all displays, subjects grouped according to the illusory figures significantly more often than for control displays that contained the same inducing elements, but rearranged so that illusory contours were degraded or eliminated. A second experiment showed that in objectively defined grouping tasks, subjects grouped faster by illusory figures than by inducing elements. These results indicate that grouping can occur after illusory contours have been perceived.     

Stereopsis and subjective contours

Ten Ss rated perceived depth and contour clarity of figures containing binocularly disparate subjective contours. There was no tendency for stereoscopic depth cues to enhance the perceived clarity of subjective contours. Disparity cues that were incompatible with monocular depth cues reduced the depth sensation but did not affect contour clarity. Although subjective contours can be perceived stereoscopically, they are seen in less depth than real contours with the same degree of horizontal disparity.     

Directional harmonic theory: a computational Gestalt model to account for illusory contour and vertex formation

Visual illusions and perceptual grouping phenomena offer an invaluable tool for probing the computational mechanism of low-level visual processing. Some illusions, like the Kanizsa figure, reveal illusory contours that form edges collinear with the inducing stimulus. This kind of illusory contour has been modeled by neural network models by way of cells equipped with elongated spatial receptive fields designed to detect and complete the collinear alignment. There are, however, other illusory groupings which are not so easy to account for in neural network terms. The Ehrenstein illusion exhibits an illusory contour that forms a contour orthogonal to the stimulus instead of collinear with it. Other perceptual grouping effects reveal illusory contours that exhibit a sharp corner or vertex, and still others take the form of vertices defined by the intersection of three, four, or more illusory contours that meet at a point. A direct extension of the collinear completion models to account for these phenomena tends towards a combinatorial explosion, because it would suggest cells with specialized receptive fields configured to perform each of those completion types, each of which would have to be replicated at every location and every orientation across the visual field. These phenomena therefore challenge the adequacy of the neural network approach to account for these diverse perceptual phenomena. I have proposed elsewhere an alternative paradigm of neurocomputation in the harmonic resonance theory (Lehar 1999, see website), whereby pattern recognition and completion are performed by spatial standing waves across the neural substrate. The standing waves perform a computational function analogous to that of the spatial receptive fields of the neural network approach, except that, unlike that paradigm, a single resonance mechanism performs a function equivalent to a whole array of spatial receptive fields of different spatial configurations and of different orientations, and thereby avoids the combinatorial explosion inherent in the older paradigm. The present paper presents the directional harmonic model, a more specific development of the harmonic resonance theory, designed to account for specific perceptual grouping phenomena. Computer simulations of the directional harmonic model show that it can account for collinear contours as observed in the Kanizsa figure, orthogonal contours as seen in the Ehrenstein illusion, and a number of illusory vertex percepts composed of two, three, or more illusory contours that meet in a variety of configurations.