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.     

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.     

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.     

Curved apparent motion induced by amodal completion

We investigated whether amodal completion can bias apparent motion (AM) to deviate from its default straight path toward a longer curved path, which would violate the well-established principle that AM follows the shortest possible path. Observers viewed motion sequences of two alternating rectangular tokens positioned at the ends of a semicircular occluder, with varying interstimulus intervals (ISIs; 100-500?ms). At short ISIs, observers tended to report simple straight-path motion-that is, outside the occluder. But at long ISIs, they became increasingly likely to report a curved-path motion behind the occluder. This tendency toward reporting curved-path motion was influenced by the shape of tokens, display orientation, the gap between tokens and the occluder, and binocular depth cues. Our results suggest that the visual system tends to minimize unexplained absence of a moving object, as well as its path length, such that AM deviates from the shortest path when amodal integration of motion trajectory behind the curved occluder can account for the objective invisibility of the object during the ISI.     

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.     

Shrinkage and expansion by amodal completion: a critical review

The whole literature on the so-called dimensional effects of amodal completion (shrinkage of a partially occluded figure and expansion of modally visible parts of the same figure) is critically reviewed. It is claimed that phenomenal shrinkage is the sum of at least two illusions independent of figure ground distinction: (1) shrinkage of spaces divided into a few parts, and (2) shrinkage of empty spaces. It is also shown that a partially occluded figure can widen, rather than shrink. It is concluded that the stratification in two phenomenally superimposed figures accompanies the shrinkage effect, but is irrelevant to its generation. Further experiments are needed in order to draw a well-grounded conclusion on the expansion effect.     

Perceptual causality in the amodal completion of kinetic structures

Perceptual causality as an amodal completion of kinetic structures was demonstrated. The experimental situation was as follows: a square appeared on the left, moved from left to right till it touched one side of a stationary rectangle (a perceptual screen), and disappeared; after a while, another square appeared at the other side of the rectangle and moved right. The first square moved faster than the second one. Sometimes, two moving objects were perceived, with an amodal launching effect: the first moving object seemed to collide with the second one behind the screen. When both the screen and the occluding interval were small, despite the discrepancy of velocities, a single moving object was perceived, but the object seemed to abruptly change its speed behind the screen. The perception of an amodal completion of movement raises some theoretical issues: the discrepancy between amodal completion of kinetic structures and that of static ones, the problem of perceptual identity, and figure-ground segregation in dynamic scene. The dilemma of the naive subject will also be discussed. Received: 11 June 1997 / Accepted: 14 October 1998     

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.     

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.     

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.     

The effects of kinetic occlusion and categorization on amodal completion. A comment on Gerbino and Salmaso (1987)

The experimental study of Gerbino and Salmaso (1987) focused on the process of amodal completion, i.e. the process that leads to the impression of a total pattern in the absence of information about all parts of the pattern. One of the questions that they have attempted to answer with respect to this process is whether or not it depends on the categorization of the modal part (i.e. the part about which visual information is present) as a modification of a complete prototype. The major conclusion of their experiments is that amodal completion does not depend upon categorization. Furthermore, they interpret their results as indicating that neither the truncated form of the occluded part nor the unification/segregation of contour segments are phenomenally real. We designed some variants of the tunnel effect to demonstrate (1) that the explicit categorization of some parts of the input can have an influence on amodal completion and (2) that the truncated form of a pattern can be phenomenally real. Our results corroborate these hypotheses and can, therefore, interpreted as refuting two rather central claims of Gerbino and Salmaso (1987). The conclusion of our experiment must be that amodal completion is not as primary as they have supposed but can be influenced by kinetic occlusion and categorization stored in visual memory.     

Actuality and the amodal perspective

In this paper, I examine our intuitive understanding of metaphysical contingency, and ask what features a metaphysical picture must possess in order to satisfy our intuitions about modal matters. After spelling out what I think are the central intuitions in this domain, I examine the debate between the two most widely held views on the nature of modality, namely, modal realism and modal actualism. I argue that while each of these views is able to accommodate some of our intuitions, it leaves others unsatisfied. I then present an alternative metaphysical picture, which I argue can accommodate our intuitions in a way that the traditional views cannot. More specifically, I argue that our intuitions about modality call for a pluralist view of the structure of reality—a view on which there is more than one ultimate ‘shape’ to the fundamental facts, each corresponding to a distinct metaphysically privileged perspective on reality.     

Infants' perceptual differentiation of amodal and modality-specific audio-visual relations.

Ninety-six infants of 3 1/2 months were tested in an infant-control habituation procedure to determine whether they could detect three types of audio-visual relations in the same events. The events portrayed two amodal invariant relations, temporal synchrony and temporal microstructure specifying the composition of the objects, and one modality-specific relation, that between the pitch of the sound and the color/shape of the objects. Subjects were habituated to two events accompanied by their natural, synchronous, and appropriate sounds and then received test trials in which the relation between the visual and the acoustic information was changed. Consistent with Gibson's increasing specificity hypothesis, it was expected that infants would differentiate amodal invariant relations prior to detecting arbitrary, modality-specific relations. Results were consistent with this prediction, demonstrating significant visual recovery to a change in temporal synchrony and temporal microstructure, but not to a change in the pitch-color/shape relations. Two subsequent discrimination studies demonstrated that infants' failure to detect the changes in pitch-color/shape relations could not be attributed to an inability to discriminate the pitch or the color/shape changes used in Experiment 1. Infants showed robust discrimination of the contrasts used.     

Mechanisms of modal and amodal interpolation

P. J. Kellman and T. F. Shipley (1992) and P. J. Kellman, P. Garrigan, and T. F. Shipley (2005) suggested that completion of partly occluded objects and illusory objects involve the same or similar mechanisms at critical stages of contour interpolation. B. L. Anderson, M. Singh, and R. W. Fleming and B. L. Anderson (2007) presented a number of arguments against this view. The author analyzes 3 of these arguments, as well as B. L. Anderson's ecological justification for believing that these mechanisms must be very different, and suggests that their conclusions are unwarranted. The author also outlines a model consistent with the identity hypothesis and with recent physiological evidence, including a quantitative proposal for contour interpolation strength. The model suggests that V1 and V2 receive from higher visual areas feedback that modulates their responses to stimuli eliciting modal completion, amodal completion, and collinear contour facilitation. The model qualitatively explains the unstable percepts evoked by various recently devised stimuli.     

The amodal brain and the offloading hypothesis

In this article, I argue that a growing body of evidence shows that concepts are amodal and I provide a novel interpretation of the body of evidence that was taken to support neo-empiricist theories of concepts: the offloading hypothesis in the 1990s and 2000s.     

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.     

Perception and imagination: amodal perception as mental imagery

When we see an object, we also represent those parts of it that are not visible. The question is how we represent them: this is the problem of amodal perception. I will consider three possible accounts: (a) we see them, (b) we have non-perceptual beliefs about them and (c) we have immediate perceptual access to them, and point out that all of these views face both empirical and conceptual objections. I suggest and defend a fourth account, according to which we represent the occluded parts of perceived objects by means of mental imagery. This conclusion could be thought of as a (weak) version of the Strawsonian dictum, according to which “imagination is a necessary ingredient of perception itself”.     

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.     

Modality-specific and amodal sources of interference in the attentional blink

When two masked targets (T1 and T2) are visually or auditorily presented in rapid succession, processing of T1 produces an attentional blink (AB)--that is, a transient impairment of T2 identification. The present study was conducted to compare the relative impact of masking T1 and T2 between vision and audition. Within a rapidly presented sequence, each of the two verbal targets, discriminated by their offset (Experiment 1) or their onset (Experiment 2), could be followed by either a single item, acting as a mask, or a blank gap. Masking of T2 appeared to be necessary for the occurrence of the AB for both the visual and the auditory modality. However, whereas masking of T1 affected the expression of the visual AB in both experiments, the same effect was observed in the auditory modality only when the targets varied at the onset. These results provide further evidence that processing auditory and visual information is restricted by similar attentional limitations but also suggest that these limits are constrained by properties specific to each sensory system.