Does the use of natural stimuli facilitate amodal completion in pigeons?

Three experiments were carried out to investigate whether amodal completion in pigeons can be facilitated by the use of colour photographs instead of highly artificial stimuli such as geometrical shapes. Ten pigeons were trained in a go/no-go procedure to discriminate between photographs of complete and of incomplete pigeon figures. In the subsequent test, the birds classified pictures of partly occluded pigeons as though they were complete (experiment 1). However, we found evidence that classification was based on spurious stimulus features that paralleled the intended class rule of figural completeness versus incompleteness. In particular, classification was shown to be guided by white background gaps that separated the parts of the fragmented pigeon figures (experiment 2), as well as by cues related to overall Gestalt (experiment 3). In summary, the present results indicate that the use of more natural stimuli such as photographs instead of geometrical shapes is insufficient for providing amodal completion in pigeons. It is suggested that a combination of various cues, including, eg, 3-D information and common motion in addition to surface and contour properties, may be required to induce a perceptual bias favouring visual completion of occluded portions.     

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.     

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.

     

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.     

Temporally unfolding neural representation of pictorial occlusion

The human visual system possesses a remarkable ability to reconstruct the shape of an object that is partly occluded by an interposed surface. Behavioral results suggest that, under some circumstances, this perceptual process (termed amodal completion) progresses from an initial representation of local image features to a completed representation of a shape that may include features that are not explicitly present in the retinal image. Recent functional magnetic resonance imaging (fMRI) studies have shown that the completed surface is represented in early visual cortical areas. We used fMRI adaptation, combined with brief, masked exposures, to track the amodal completion process as it unfolds in early visual cortical regions. We report evidence for an evolution of the neural representation from the image-based feature representation to the completed representation. Our method offers the possibility of measuring changes in cortical activity using fMRI over a time scale of a few hundred milliseconds.     

Amodal completion as reflected by gaze durations

In two experiments amodal completion of partly occluded shapes was investigated by recording eye movements in a directed visual-search task. Participants searched arrays of shapes in a prescribed order for target figures that could partly be occluded. Longer gaze durations were found on occlusion patterns than on truncated control patterns for targets but not for non-targets. This effect of occlusion was restricted to a subset of the stimuli. A second experiment was carried out to establish whether this restriction resulted from structural properties of the stimuli or their familiarity. Occlusion patterns in this experiment were ambiguous with respect to structure, allowing both local and global completions. One of the completions was always less familiar than the other. The results showed longer gazes only for the less familiar completions, irrespective of whether they were local or global.     

Perception of partly occluded figures by baboons (Papio papio)

Comparative literature provides conflicting findings whether animals experience amodal completion. Five experiments were conducted to verify if baboons perceive partly occluded objects as complete. The first three experiments used a go/no-go procedure and a video monitor for stimulus presentation. These experiments failed to reveal amodal completion, suggesting that the stimuli were processed as 2-D images rather than 3-D objects. In contrast, completion was demonstrated in a fourth experiment with cardboard stimuli in a two-alternative forced-choice (2AFC) discrimination task presented in a Wisconsin General Test Apparatus. Although in experiment 5 the same 2AFC procedure was used as in experiment 4, completion was absent when the stimuli were shown with a computer graphic system. The results suggest that baboons share with humans the ability for amodal completion, but also underline some procedural factors that might affect the elicitation of this capacity.     

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.     

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.     

Recognition of partly occluded objects by fish

The ability to visually complete partly occluded objects (so-called “amodal completion”) has been documented in mammals and birds. Here, we report the first evidence of such a perceptual ability in a fish species. Fish (Xenotoca eiseni) were trained to discriminate between a complete and an amputated disk. Thereafter, the fish performed test trials in which hexagonal polygons were either exactly juxtaposed or only placed close to the missing sectors of the disk in order to produce or not produce the impression (to a human observer) of an occlusion of the missing sectors of the disk by the polygon. In another experiment, fish were first trained to discriminate between hexagonal polygons that were either exactly juxtaposed or only placed close to the missing sectors of a disk, and then tested for choice between a complete and an amputated disk. In both experiments, fish behaved as if they were experiencing visual completion of the partly occluded stimuli. These findings suggest that the ability to visually complete partly occluded objects may be widespread among vertebrates, possibly inherited in mammals, birds and fish from early vertebrate ancestors.     

Tits use amodal completion in predator recognition: a field experiment

Amodal completion enables an animal to perceive partly concealed objects as an entirety, and to interact with them appropriately. Several studies, based upon either operant conditioning or filial imprinting techniques, have shown that various animals (both mammals and birds) can perform amodal completion. Before this study, the use of amodal completion by untrained animals in the recognition of objects had not been considered. Using two feeders, we observed in a field experiment the reaction of tits to the torso of a sparrowhawk (partly occluded or an ‘amputated’ dummy) in two different treatments (sparrowhawk torso vs. complete dummy pigeon; and torso vs. complete dummy sparrowhawk). It is clear that the birds considered the two torso variants as predators and kept away from both of them when the second feeder offered a ‘pigeon’ instead. On the other hand, when a ‘complete sparrowhawk’ was present on the second feeder, the number of visits to the occluded torso remained low; while the number of visits to the amputated one increased threefold. Birds risked perching near what was clearly an amputated torso; while the fear of a “hiding” (occluded) torso remained unchanged, when the second feeder did not provide a safe alternative. Such discrimination between torsos requires the ability for amodal completion. Our results demonstrate that in their recognition process, the birds not only use simple sign stimuli, but also complex cognitive functions.     

Investigating global effects in visual occlusion: from a partly occluded square to the back of a tree-trunk.

'Classic' occlusion examples, such as a square partly occluded by a rectangle, have given rise to so-called local and global accounts of amodal completion. Without denying the influence of local configurations, I take the position that, in the long run, any theory of amodal completion should account for global properties. After a brief review of local and global accounts, two extensions of the stimulus domain are proposed to further illustrate the necessity of global accounts. The first is the domain of so-called fuzzy regularities, i.e., regularities which are not based on metrical identities. It is argued and demonstrated that observers are even susceptible to these fuzzy regularities and that they complete partly occluded shapes accordingly. The second extension is towards 3D object completion. Theories of object representation that describe intrinsic regularities of objects appear to be most suitable to predict relative preferences of alternative object completions. Consequently, fuzzy object completions, such as the completion of the back of a tree-trunk, can be explained better by global constraints.     

What perceptual rules do capuchin monkeys (Cebus apella) follow in completing partly occluded figures?

Capuchin monkeys (Cebus apella) matched a variety of rodlike figures, distinguishable only by their central portions. The monkeys were then tested for perceptual completion rules by examining which comparison figures they would choose as matching the sample with the critical central portion occluded by a horizontal belt. In Experiment 1, the monkeys overwhelmingly chose a straight rod over disconnected rods and rods having irregular shapes at their center, irrespective of the presence-absence of common motion of visible parts of the samples. The monkeys chose a connected rod for relatable rods and disconnected rods for non-relatable ones in Experiment 2. In Experiments 3 and 4, some of the monkeys chose rods specified by the global regularity of the contour, whereas others did not. Experiment 5 showed that humans and capuchin monkeys basically follow similar perceptual rules in completing occluded contours, but the global regularity rule may be stronger in humans.     

Late influences on perceptual grouping: Amodal completion

Perceptual grouping is generally assumed to be an early visual process that operates on a previously unorganized image-based representation. The present experiment shows that elements perceived as occluded by a closer surface tend to be grouped with elements having the same shape as the amodally completed percept rather than with those having the same retinal shape as the incomplete stimulus. It is therefore concluded that perceptual grouping by shape similarity either occurs after amodal completion or is a temporally extended process that occurs both before and after it.     

Global visual completion of quasi-regular shapes

The topic of amodal completion has often been investigated by using partly occluded shapes that are regular. In research that has typically been done with displays such as these regular shapes, it has been shown that global aspects of a shape can determine completion. To see how robust these global influences in the completion process are, we investigated quasi-regular shapes, ie shapes with a certain overall regularity but not based on metrical identities. First, in experiment 1 participants had to complete quasi-regular shapes in a drawing task. Then, in experiment 2 the primed-matching paradigm was used. Results from both experiments provided evidence for global completions. In experiment 3 we found that multiple global completions can be primed, which, as a control experiment showed, cannot be explained by some inability of the visual system to see the difference between the different completions. These data support the notion that global influences on visual occlusion are apparent even when the partly occluded stimulus is outside the domain of regular shapes. Implications for a global approach are provided.     

Asymmetric priming effects in visual processing of occlusion patterns

In three experiments, we examined the effect of temporal context in amodal completion of partly occluded nontarget figures. In a primed same-different task, test pairs were preceded by a sequence of two primes, one of which was a single, the other a composite figure. Single figures reappeared in the composite ones, which also contained a square that could be viewed, alternatively, as an occluder or as yielding a mosaic fit to the other shape. To measure context influences between single and composite figures, both of which were nontargets, we studied their combined effect as primes on the test pairs of the same-different task, expecting that congruency between both primes should lead to a superadditive priming effect on the task. We found that single figures presented first provided facilitatory context for local and global occlusion as well as for mosaic interpretations of subsequently presented composite figures. These effects occurred only when the composite figure was presented briefly (50 msec). No superadditive facilitation occurred when composite figures were presented first and single figures followed them. The restriction of the effect to short presentations and its temporal asymmetry were taken as evidence that prior context biases possible occlusion interpretations during the process of completion, rather than afterward.     

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.     

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.     

The effects of occlusion and past experience on the allocation of object-based attention

There is considerable evidence indicating that cuing a specific portion of an object results in the entire object’s being attended to. In the present study, we examined whether previous experience with an object could halt perceptual (i.e., amodal) completion. In Experiment 1, two parallel rectangles were initially displayed, and then the middle portions of these objects were occluded. Attentional cuing effects were found for both discrete portions of the completed rectangles. In the final two experiments, four discrete objects were initially displayed, followed by the same occluder as that used in the first experiment. The appearance of the occluder (500 msec before the cue in Experiment 2, 100 msec before the cue in Experiment 3) allowed the four discrete objects to be completed into two rectangles. Attentional cuing effects were found for the completed rectangles in both experiments, indicating that previous experience was not sufficient to halt the amodal completion of objects.     

LOCAL AND GLOBAL PROCESSES IN VISUAL COMPLETION

In the natural environment, objects are frequently occluded, and people continuously complete partly occluded objects Do local processes or global processes control the completion of partly occluded objects? To answer this question, most previous studies simply asked subjects to draw the completions they "saw" Such drawing tasks are highly subjective, and they provide equivocal results Our studies are the first to use an objective, implicit paradigm (primed matching) to determine the extent to which local or global processes underlie the visual completion of partly occluded objects Our results suggest that global processes dominate perceptual completion, whereas local processes do not play a large role Therefore, local theories of completion, or theories in which local processes dominate, cannot be entirely correct