Prior experience affects amodal completion in pigeons

In a three-alternative forced-choice task, 4 pigeons were trained to discriminate a target stimulus consisting of two colored shapes, one of which partially occluded the other, from two foil stimuli that portrayed either a complete or an incomplete version of the occluded shape. The dependent measure was the percentage of total errors that the birds committed to the complete foil. At the outset of training, the pigeons committed approximately 50% of total errors to the complete foil, but as training progressed, the percentage of errors to the complete foil rose. When the pigeons were given a second exposure to the initial set of stimuli, they committed 70% of total errors to the complete foil, suggesting that they now saw the complete foil as more similar to the occluded target than the incomplete foil. These results suggest that experience with 2-D images may facilitate amodal completion in pigeons, perhaps via perceptual learning.     

Amodal completion of moving objects by pigeons

In a series of four experiments, we explored whether pigeons complete partially occluded moving shapes. Four pigeons were trained to discriminate between a complete moving shape and an incomplete moving shape in a two-alternative forced-choice task. In testing, the birds were presented with a partially occluded moving shape. In experiment 1, none of the pigeons appeared to complete the testing stimulus; instead, they appeared to perceive the testing stimulus as incomplete fragments. However, in experiments 2, 3, and 4, three of the birds appeared to complete the partially occluded moving shapes. These rare positive results suggest that motion may facilitate amodal completion by pigeons, perhaps by enhancing the figure - ground segregation process.     

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.     

Time course of amodal completion revealed by a shape discrimination task

We measured the extent of amodal completion as a function of stimulus duration over the range of 15–210 msec, for both moving and stationary stimuli. Completion was assessed using a performancebased measure: a shape discrimination task that is easy if the stimulus is amodally completed and difficult if it is not. Specifically, participants judged whether an upright rectangle was longer horizontally or vertically, when the rectangle was unoccluded, occluded at its corners by four negative-contrast squares, or occluded at its corners by four zero-contrast squares. In the zero-contrast condition, amodal completion did not occur because there were no occlusion cues; in the unoccluded condition, the entire figure was present. Thus, comparing performance in the negative-contrast condition to these two extremes provided a quantitative measure of amodal completion. This measure revealed a rapid but measurable time course for amodal completion. Moving and stationary stimuli took the same amount of time to be completed (≈ 75 msec), but moving stimuli had slightly stronger completion at long durations.     

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.     

Investigating local and global effects of surface colours and contours in amodal completion

ABSTRACT

We studied interpretations of partly occluded shapes. Models that account for amodal completion mostly deal with local and global contour characteristics. In the current study, we were interested in the effects of colour on local and global contour completions. In our stimuli, local contour completions comprised simple linear extensions of the partly occluded contours, whereas global contour completions accounted for global shape regularities. Our stimuli were designed such that the visible surface colour could also be completed in a local or global fashion, being consistent or inconsistent with contour completions. We tested the preferred interpretations of the partly occluded shapes by using a sequential matching task. Participants had to judge whether a test shape could be a previously shown partly occluded shape. We found that interpretations of partly occluded shapes depend on both colour and contour characteristics. Additional time bin analyses revealed that for fast responses colour and contour completions already depend on the visible context of the partly occluded shapes, while for slow responses the congruency between colour and contour completions play a role as well.     

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.     

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.     

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.     

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.     

Amodal completion is modulated by lightness similarity

The strength of amodal completion is known to be modulated by contour relationships and global shape. Some researchers have shown that amodal completion also depends on surface similarity, but they have not distinguished the relative importance of similarity in surface representations either pre or post lightness constancy. In the experiments reported here, we aimed to determine whether amodal completion depends on processes that occur either before or after the establishment of lightness constancy. We used computer rendering techniques to vary the consistency of a cast shadow with a decrement in luminance on one side of a partially occluded surface. We found that perceived completion depended on the consistency of the decrement in surface luminance with the orientation of a cast shadow (Exp.?1). In Experiment ?2, we generated occluded surface fragments that could be either luminance-matched or lightness-matched to surface fragments on the opposite side of the occluder, and we found that the strength of amodal completion depended primarily on similarity in the perceived surface reflectance. In Experiments 3 and 4, we performed apparent-motion tasks to obtain converging evidence for grouping on the basis of perceived similarity in lightness. We found that matching surfaces in lightness significantly improved the apparent motion of surfaces behind an occluder, as compared with surfaces that were matched in contrast alone. These results suggest that amodal completion depends on the perceived similarity in lightness of partially occluded surface fragments.     

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.     

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.     

Kanizsa's shrinkage illusion produced by a misapplied 3-D corrective mechanism

In order to include the monocular areas from the left and the right eye in the cyclopean view, the visual system displaces the occluded elements which would result in a horizontal elongation of the shape but does not occur thanks to a correction mechanism which preserves the shape. We hypothesised that this mechanism causes Kanizsa's amodal shrinkage illusion (the apparent elongation of a partially occluded square) when it is incorrectly applied by the visual system to a two-dimensional stimulus. Four experiments tested this hypothesis: (i) one-eyed observers were less susceptible to the illusion than people with normal binocular vision because, for them, the correction for shape is unnecessary; (ii) the illusion was stronger with binocular than with monocular vision since binocularity induces the visual system to correct for the shape distortion; (iii) the illusion diminished when the stimulus was rotated 90 degrees given that displacement and compression are not required for vertical occlusion; (iv) the magnitude of the illusion was a function of the width of the occluder because, as previous research has shown, the edges of a partially occluded square are less displaced the farther they are from the edges of the occluder. The data from the four experiments support our hypothesis even though no condition was able to eliminate the illusion; other possible causes are discussed.     

Complete mergeability and amodal completion.

When image fragments are taken to correspond to the visible portions of a single occluded object, the object is said to 'amodally complete' behind the occluder. Kellman and Shipley (Kellman, P. J., & Shipley, T. F. (1991). A theory of visual interpolation in objective perception. Cognitive Psychology, 23, 144-221) argued that when the virtual contour extensions of such image fragments subtend an obtuse or right angle, the contours are 'relatable' and therefore complete. However, edge and surface relatability are neither necessary nor sufficient for completion to be perceived (Tse, P. U. (1999) Volume completion. Cognitive Psychology). Evidence is offered that completion is not driven directly by image cues such as contour relatability, but is driven, rather, by intermediate representations, such as volumes that are inferred from global image cue relationships. Evidence suggests that several factors, none of which is necessary for amodal completion to occur, contribute to the perceived strength of amodal completion, including similarity of pattern or substance, proximity, and good volume continuation or complete mergeability. Two partially occluded volumes are completely mergeable when they can be extended into occluded space along the trajectory defined by their visible surfaces such that they merge entirely with each other. Mergeability is not measurable in the image because it describes an inferred relationship among volumes that must themselves be inferred from the image.     

Perception of partly occluded objects by young chicks

Completion of partly occluded objects is a ubiquitous phenomenon in human visual perception. It is unclear, however, whether it occurs at all in other species: Studies on visual discrimination learning have revealed that animals usually attend to parts and features of the discriminative stimuli rather than to global object properties. We provide here the first demonstration of recognition of partly occluded objects in a bird species, the domestic chickGallus gallus, using the naturalistic setting made available by filial imprinting, a process whereby young birds form attachments to their mothers or some artificial substitute. In Experiment 1, newborn chicks were reared singly with a red cardboard triangle, to which they rapidly imprinted and therefore treated as a social partner. On Day 3 of life, the chicks were presented with pairs of objects composed of either isolated fragments or occluded parts of the imprinting stimulus. Chicks consistently chose to associate with complete or with partly occluded versions of the imprinting object rather than with separate fragments of it. Similarly, in Experiment 2, chicks reared with a partly occluded triangle chose to associate with a complete triangle rather than with a fragmented one, whereas chicks reared with a fragmented triangle chose to associate with a fragmented triangle and not with a complete one. Newborn chicks thus appear to behave as if they could experience amodal completion.     

Development of modal and amodal completion in infants

Visual completion has been divided into two types: modal and amodal. While psychophysical studies with adults provided several common properties between modal and amodal completion, studies with infants showed differential trends in the development of these perceptual abilities. In the present study, we further examined the development of these two kinds of visual completion in infants aged 3 to 6 months. We created a display composed of a partially overlapping circle and square. The display induced either modal or amodal completion depending on the colour. Infants were familiarised with either the modal or the amodal display. After this familiarisation, the infants were tested on their discrimination between the complete figure and the broken figure. If the infants could perceptually complete the figures in the familiarisation display, they were expected to show a novelty preference for the broken figure. A total of thirty-two infants participated in the present study. Our results suggest that modal completion develops by 3-4 months of age, whereas amodal completion develops by 5-6 months of age.     

Volume completion in 4.5-month-old infants

In this study, we examined 4.5-month-old infants' visual completion of self-occluding three-dimensional objects. A previous study on this topic reported that 6-month-old, but not 4-month-old infants extrapolate a convex, symmetric prism from a limited view of its surfaces (Soska & Johnson, 2008). As of yet, studies on the development of amodal completion of three-dimensional, self-occluding objects are scarce. Given 4-month-old infants' abilities to derive three-dimensional shape from a variety of visual cues, three-dimensional amodal completion may well depend on the perceptual strength of three-dimensionality in the stimulus displays. The first experiments (1A and 1B) tested this hypothesis by means of a habituation paradigm and showed that 4.5-month-old infants are indeed able to amodally complete the back of a self-occluding object when sufficient three-dimensional cues are available. Further support for volume completion in 4.5-month-old infants was found in a second experiment, again using a habituation paradigm, that measured perceived connectedness between two visually separated, self-occluding, three-dimensional objects.     

Investigating metrical and structural aspects of visual completion: Priming versus searching

Earlier research on visual occlusion showed some flexibility in the formation of visual completions, as long as the structural aspects (e.g., symmetry) of the visible part of the partly occluded shape were preserved in the completion (de Wit & van Lier, ). In this study, we examined whether changing the overall size of the occluded shape would preserve the overall structure. In Experiment 1, using the primed‐matching paradigm, we found evidence for relative size invariance in the completion process. To investigate whether changes in the structural aspects of shape are generally more salient than those of size, we employed the same stimuli in visual search and change detection paradigms. Experiment 2 demonstrated effects of completion in both paradigms. Experiment 3 showed that the metrical aspects of the shapes used in Experiment 1 are nevertheless detected faster than the structural aspects under search conditions. Because the variation in structural aspects is not more salient than in metrical aspects, we conclude that for these shapes, visual completion is indeed size‐invariant. The relations between performances in the three paradigms are discussed.