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.     

Visual evoked potentials elicited by subjective contour figures

In order to investigate the relationship between the appearance of illusory figures and the wave form of visual evoked potentials (VEPs), 8 different visual pattern stimuli were presented to 8 normal subjects. Four of the stimuli (experimental stimuli) produced subjective figures and contours (squares and discs). The 4 other stimuli (reference stimuli), although equal to the experimental stimuli in the amount of physical energy, did not produce the illusion of squares or discs. Electrodes were placed on the scalp at central and occipital locations. Three prominent peaks in the occipital record were observed in all subjects. An amplitude difference of VEP N180 (N2) between the subjective figures and the reference stimuli was found in the values for each subject. Enhancement of the VEP of the illusory figure stimuli was observed for a specific component (N2), whereas the amplitude values at the central components and the occipital P120 (P2) and P280 (P3) were almost the same as the reference values. The VEP (N2 component) amplitude enhancement at the occipital area for subjective figure stimuli suggests that illusory contour formation takes place at higher levels in the visual system. This was known from experiments using dichoptic presentation.     

Visual illusory productions with or without amodal completion.

A new type of illusory contour is presented whose appearance is generated by the graphic representation of groups of human figures interacting in a coordinated manner with external reality. When numerous pictorial indicators of cause-effect relationships are provided, and appropriate techniques and sufficiently ambiguous observation conditions are used, hallucinatory objects congruent with expectations linked to the meaning of the configurations appear. There is thus a high-level semantic component that is active in the formation of visual illusory contours and is even capable of interacting with other known factors: brightness contrast, the number of elements, the degree of alignment of the elements, etc. This new type of illusory contour fits current definitions and can be experimentally modified. The variations in subjective clarity scores are presented for a study in which twenty subjects observed nineteen experimental figures, certain variables of which were manipulated. The issue is worthy of further experimental investigation.     

Visual perception of smoothly curved surfaces from double-projected contour patterns

This research was designed to examine how human observers are able to perceive the 3-dimensional structure of smoothly curved surfaces from projected patterns of surface contours. Displays were generated by using a method of double projection that made it possible to cover a surface with a wide range of contour patterns of varying geometric structure and to eliminate systematic variations of image shading. The compelling impression of 3-dimensional form from these patterns provides strong evidence that the ability of observers to perceptually interpret surface contours is considerably less restrictive than would be reasonable to expect on the basis of existing computational models. Results suggest that the perceptual analysis of surface contours is able to exploit statistical regularities of contour structure over appropriately large regions of visual space.     

In search of closure: subjective contour illusions, Gestalt completion tests, and implications

This study investigates the construct validity of perceptual closure tests (CTs), and isolates a common processing demand from the right-hemisphere. Sixty-seven patients with focal unilateral lesions (34 right side, 33 left side), and 80 control subjects participated. Multivariate analyses indicated that there was substantial age-related variance in all CTs, while sex was variably significant; CTs are not uniform in their discriminating ability, and hence factorially complex; and their specifically right hemisphere-sensitive dimension was subjective contour illusions, and interestingly unrelated to facial discrimination ability. The methodological and theoretical implications are discussed.     

Visual cognition influences early vision: the role of visual short-term memory in amodal completion

A partly occluded visual object is perceptually filled in behind the occluding surface, a process known as amodal completion or visual interpolation. Previous research focused on the image-based properties that lead to amodal completion. In the present experiments, we examined the role of a higher-level visual process-visual short-term memory (VSTM)-in amodal completion. We measured the degree of amodal completion by asking participants to perform an object-based attention task on occluded objects while maintaining either zero or four items in visual working memory. When no items were stored in VSTM, participants completed the occluded objects; when four items were stored in VSTM, amodal completion was halted (Experiment 1). These results were not caused by the influence of VSTM on object-based attention per se (Experiment 2) or by the specific location of to-be-remembered items (Experiment 3). Items held in VSTM interfere with amodal completion, which suggests that amodal completion may not be an informationally encapsulated process, but rather can be affected by high-level visual processes.     

Volume completion.

The visual system completes image fragments into larger regions when those fragments are taken to be the visible portions of an occluded object. Kellman and Shipley (1991) argued that this "amodal" completion is based on the way that the contours of image fragments "relate." Contours relate when their imaginary extensions intersect at an obtuse or right angle. However, it is shown here that contour relatability is neither necessary nor sufficient for completion to take place. Demonstrations that go beyond traditional examples of overlapping flat surfaces reveal that "mergeable" volumes, rather than relatable contours, are the critical elements in completion phenomena. A volume is defined as a 3-D enclosure. Typically, this refers to a surface plus the inside that it encloses. Two volumes are mergeable when their unbounded visible surfaces are relatable or the insides enclosed by those surfaces can completely merge. Two surfaces are relatable when their visible portions can be extended into occluded space along the trajectories defined by their respective curvatures so that they merge into a common surface. A volume-based account of amodal completion subsumes surface completion as a special case and explains examples that neither a contour- nor a surface-based account can explain.     

Bayesian contour integration

The process by which the human visual system parses an image into contours, surfaces, and objects—perceptual grouping—has proven difficult to capture in a rigorous and general theory. A natural candidate for such a theory is Bayesian probability theory, which provides optimal interpretations of data under conditions of uncertainty. But the fit of Bayesian theory to human grouping judgments has never been tested, in part because methods for expressing grouping hypotheses probabilistically have not been available. This paper presents such methods for the case ofcontour integration—that is, the aggregation of a sequence of visual items into a “virtual curve.” Two experiments are reported in which human subjects were asked to group ambiguous configurations of dots (in Experiment 1, a sequence of five dots could be judged to contain a “corner” or not; in Experiment 2, an arrangement of six dots could be judged to fall into two disjoint contours or one smooth contour). The Bayesian theory accounts extremely well for subjects’ judgments, explaining more than 75% of the variance in both tasks. The theory thus provides a far more quantitatively precise account of human contour integration than has been previously possible, allowing a very precise calculation of the subjective goodness of a virtual chain of dots. Because Bayesian theory is inferentially optimal, this finding suggests a “rational justification,” and hence possibly an evolutionary rationale, for some of the rules of perceptual grouping.     

Amodal completion in bonobos

We trained two bonobos to discriminate among occluded, complete, and incomplete stimuli. The occluded stimulus comprised a pair of colored shapes, one of which appeared to occlude the other. The complete and incomplete stimuli involved the single shape that appeared to have been partially covered in the occluded stimulus; the complete stimulus showed the full shape, whereas the incomplete stimulus showed the same portion of the shape that was visible in the occluded stimulus. The correct response was to select the two-part occluded stimulus. Consistent with amodal completion, the bonobos committed a higher percentage of errors to the complete stimuli than to the incomplete stimuli. As well, the percentage of errors to the complete stimuli rose after repeated training with several different shapes. Extensive experience with two-dimensional images enhances amodal completion of partially occluded stimuli in bonobos.     

Amodal completion and relationalism

Amodal completion is usually characterized as the representation of those parts of the perceived object that we get no sensory stimulation from. In the case of the visual sense modality, for example, amodal completion is the representation of occluded parts of objects we see. I argue that relationalism about perception, the view that perceptual experience is constituted by the relation to the perceived object, cannot give a coherent account of amodal completion. The relationalist has two options: construe the perceptual relation as the relation to the entire perceived object or as the relation to the unoccluded parts of the perceived object. I argue that neither of these options are viable.

     

Bayesian contour integration.

The process by which the human visual system parses an image into contours, surfaces, and objects--perceptual grouping--has proven difficult to capture in a rigorous and general theory. A natural candidate for such a theory is Bayesian probability theory, which provides optimal interpretations of data under conditions of uncertainty. But the fit of Bayesian theory to human grouping judgments has never been tested, in part because methods for expressing grouping hypotheses probabilistically have not been available. This paper presents such methods for the case of contour integration--that is, the aggregation of a sequence of visual items into a "virtual curve." Two experiments are reported in which human subjects were asked to group ambiguous configurations of dots (in Experiment 1, a sequence of five dots could be judged to contain a "corner" or not; in Experiment 2, an arrangement of six dots could be judged to fall into two disjoint contours or one smooth contour). The Bayesian theory accounts extremely well for subjects' judgments, explaining more than 75% of the variance in both tasks. The theory thus provides a far more quantitatively precise account of human contour integration than has been previously possible, allowing a very precise calculation of the subjective goodness of a virtual chain of dots. Because Bayesian theory is inferentially optimal, this finding suggests a "rational justification," and hence possibly an evolutionary rationale, for some of the rules of perceptual grouping.     

Determinants of word fragment completion

A current debate in the memory literature concerns the validity of word fragment completion (WFC) as a test of implicit memory. It has been claimed that language variables exert a strong influence on the task, and that the task reflects memory only to a small extent (Hintzman & Hartry, 1990). For these reasons, the use of contingency analyses of recognition and WFC performance as a means of studying underlying memory processes has been criticized. The present study addressed this issue by examining the influence of language variables on completion of a set of computer generated single-solution fragments of Swedish words (Olofsson & Nyberg, 1992). It was found that language variables indeed had a large effect on completion performance, and that priming only accounted for a small portion of variance in the task. It is therefore suggested that the method of triangulation should be employed for contingency analyses involving WFC.     

The visual contour in depth

We asked subjects to match points on the surface of a smooth three-dimensional (3-D) shape with points on the surface of another object that was geometrically identical to the first object but was placed in a different pose, was differently textured, and was differently shaded. In all cases, the fiducial point was on the rim of one of the objects (i.e., the boundary of the visible region of the surface), whereas the matching point was well within the silhouette of the other object. This allowed us to draw (preliminary) conclusions concerning the way monocular human observers are able to handle the neighborhood of the rim, where the local slant assumes arbitrarily high values. All experiments were done in real space with real objects (no computer-simulated scenes), the points being indicated with laser beam illumination. The subject was given control over the direction of the laser beams and was thus able to perform the task by adjustment from the vantage position. We studied both consistency (whether the subject’s judgments were invariant against changes of relative pose) and veridicality (whether the depth of the visual contour as calculated from the settings agreed with the true distance as measured by mechanical means). Subjects caught much of the 3-D structure of the contour but did deviate appreciably and apparently idiosyncratically from the true geometry.     

Perceived orientation of contour quadrangles

One hundred subjects ranked the apparent tilt of ten quadrangles. A scale of perceived orientation was derived from a pair-comparison treatment of these data. The main characteristic determining estimated orientation of the quadrangles was the axis from which the sum of the squared distances to each point of the figure was minimal (the LS-axis). Judgements were also influenced by the orientation of an 'axis of balance', which ran through the centre of gravity and the lowermost apex of the quadrangle. Quadrangles with parallel LS-axes were systematically judged as differently tilted according to the difference in the orientation of their axes of balance. Both the LS-axis and the axis of balance are physical characteristics of an object that are of great importance for the optimal control of human action through vision.     

Binocular fusion and contour suppression

Data from three separate investigations are presented to support the conclusion that suppression of visual contours occurs during binocular fusion of identical figures. These data are discussed in relation to examples of confirming and disconfirming results of other investigators.     

Perceptual completion of surfaces in infancy

Two experiments provide evidence that 4-month-old infants perceive background surfaces as continuous behind occluding objects. Infants were shown a partly hidden background surface either for a brief period of familiarization (Experiment 1) or until they met a criterion of habituation (Experiment 2). The infants were then tested with nonoccluded surfaces that were either continuous or interrupted by a gap where the occluder had been. The infants in each study looked longer at the interrupted than at the continuous surface, relative to infants in baseline controls, which suggests that the partly hidden surface was perceived as continuous. Contrasting findings were obtained in a third experiment, in which infants were habituated to a partly hidden surface that stood in front of a background so that its edges were visible: Infants gave no evidence of perceiving the foreground surface as continuous behind the occluder. These experiments provide evidence that infants perceive a surface as continuous only if it serves as the background of a scene. The results are discussed in relation to figure-ground perception in pictures and surface layouts.