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.

Object maintenance beyond their visible parts in working memory.

Completion of a partially occluded object requires that a representation of the whole is constructed based on the information provided by the physically specified parts of the stimulus. Such processes of amodal completion rely on the generation and maintenance of a mental image that renders the completed object in visual working memory (VWM). The present study examined this relationship between VWM storage and processes of object completion. We recorded event-related potentials to track VWM maintenance by means of the contralateral delay activity (CDA) during a change detection task in which composite objects (notched shapes abutting an occluding shape) to be memorized were primed to induce either a globally completed object or a noncompleted, mosaic representation. The results revealed an effect of completion in VWM despite physically identical visual input: change detection was more accurate for completed compared with mosaic representations when observers were required to memorize two objects, and these differences were reduced with four memorized items. At the electrophysiological level, globally completed (vs. mosaic) objects gave rise to a corresponding increase in CDA amplitudes. These results indicate that although incorporating the occluded portions of the presented shapes requires mnemonic resources, the complete object representations thus formed in VWM improve change detection performance by providing a more simple, regular shape. Overall, these findings demonstrate that mechanisms of object completion modulate VWM, with the memory load being determined by the structured representations of the memorized stimuli. NEW & NOTEWORTHY This study shows that completion of partially occluded objects requires visual working memory (VWM) resources. In the experiment reported, we induced observers to memorize a given visual input either as completed or as noncompleted objects. The results revealed both a behavioral performance advantage for completed vs. noncompleted objects despite physically identical input, and an associated modulation of an electrophysiological component that reflects VWM object retention, thus indicating that constructing an integrated object consumes mnemonic resources.

Perceptual completion of partly occluded contours during childhood.

An early functional onset of perceptual completion has been extensively documented during the first several months after birth. However, there is no indication for the developmental time periods at which these skills become fully developed. We used a version of an object-based attention task in which children and adults performed a same-different size judgment of two features appearing at two of four possible ends of overlapping objects. Single-object over two-object superiority (i.e., faster judgments when the features appeared on the same object than when they appeared on different objects) was observed for a complete object as early as at 4 years of age. However, it is only at 5 years of age that such a single-object advantage was obtained also for an occluded object, and even then the advantage of the single-object and occluded-object conditions over the two-object condition was observed only when the two features in the two-object condition were spatially distant, demonstrating the critical role of spatial proximity in perceptual organization during childhood. The results suggest that perceptual completion during infancy and early childhood demonstrates some rudimentary perceptual skills that become more firmly established with age.

The structure of experience, the nature of the visual, and type 2 blindsight.

Unlike those with type 1 blindsight, people who have type 2 blindsight have some sort of consciousness of the stimuli in their blind field. What is the nature of that consciousness? Is it visual experience? I address these questions by considering whether we can establish the existence of any structural-necessary-features of visual experience. I argue that it is very difficult to establish the existence of any such features. In particular, I investigate whether it is possible to visually, or more generally perceptually, experience form or movement at a distance from our body, without experiencing colour. The traditional answer, advocated by Aristotle, and some other philosophers, up to and including the present day, is that it is not and hence colour is a structural feature of visual experience. I argue that there is no good reason to think that this is impossible, and provide evidence from four cases-sensory substitution, achomatopsia, phantom contours and amodal completion-in favour of the idea that it is possible. If it is possible then one important reason for rejecting the idea that people with type 2 blindsight do not have visual experiences is undermined. I suggest further experiments that could be done to help settle the matter.

Partial modal completion under occlusion: what do modal and amodal percepts represent?

In the occlusion illusion, a partly occluded object is perceived as though it were less occluded than it actually is (Palmer, Brooks, & Lai, 2007). We confirm and extend this finding using a stimulus with a moving occluder. In agreement with Palmer et al.'s (2007) findings and their partial-modal-completion hypothesis, we found that the illusion is indeed related to the sensory evidence for occlusion. Our experiments also confirm their speculation that the occlusion illusion involves an intriguing, seemingly paradoxical percept. In our experiments, subjects viewed an opaque disk with an open sector rotating in front of a background and indicated the perceived angular extent (a) of the occluder and (b) of the part of the background experienced as directly visible through the open sector. While the former was judged quite accurately, the latter was clearly overestimated. Thus, the angular extent of the background experienced as occluded and the extent experienced as directly visible sum to more than 360°, which makes the total percept an impossible figure. We argue that the key to resolving this paradox is to question the seemingly self-evident assumption that occluded and unoccluded portions of a visual scene are represented by amodal and modal percepts, respectively. Instead, we propose that visual percepts are experienced as modal whenever they are based on sufficiently conclusive sensory evidence and are otherwise experienced as amodal. Functionally, this perceptual representation of the conclusiveness of the sensory evidence underlying perceptual inferences might be more useful than estimates about optical visibility.

Bayesian analysis on missing visual information and object complexity on visual search for object orientation and object identity.

Missing visual information, such as a gap between an object or an occluded view, has been shown to disrupt visual search and make amodal completion inefficient. Previous research, using simple black bars as stimuli, failed to show a pop-out effect (flat search slope across increasing visual set sizes) during a feature search when the target was partially occluded, but not in cases where it was fully visible. We wanted to see if this lack of a pop-out effect during feature (orientation) search extended to complex objects (Experiment 1) and identity search (Experiment 2). Participants completed orientation and identity visual search tasks by deciding whether the target was present or not present. Bayesian analyses was conducted to find evidence for observed data to be under the null (pop-out effects) or alternative hypotheses (differences in search slopes). When no occluders or gaps were present, a pop-out effect occurred when searching for a simple objects' orientation or identity. In addition, object complexity affected identity search, with anecdotal evidence suggesting that some complex object may not show a pop-out effect. Furthermore, white occluding bars were more disruptive than having a gap of visual information for feature search but not for identity search. Overall, pop-out effects do occur for simple objects, but when the task is more difficult, search for real-world objects is greatly affected by any type of visual disruption.

What, if anything, can be considered an amodal sensory dimension?

The term 'amodal' is a key topic in several different research fields across experimental psychology and cognitive neuroscience, including in the areas of developmental and perception science. However, despite being regularly used in the literature, the term means something different to the researchers working in the different contexts. Many developmental scientists conceive of the term as referring to those perceptual qualities, such as, for example, the size and shape of an object, that can be picked up by multiple senses (e.g., vision and touch potentially providing information relevant to the same physical stimulus/property). However, the amodal label is also widely used in the case of those qualities that are not directly sensory, such as, for example, numerosity, rhythm, synchrony, etc. Cognitive neuroscientists, by contrast, tend to use the term amodal to refer to those central cognitive processes and brain areas that do not appear to be preferentially responsive to a particular sensory modality or to those symbolic or formal representations that essentially lack any modality and that are assumed to play a role in the higher processing of sensory information. Finally, perception scientists sometimes refer to the phenomenon of 'amodal completion', referring to the spontaneous completion of perceptual information that is missing when occluded objects are presented to observers. In this paper, we review the various different ways in which the term 'amodal' has been used in the literature and the evidence supporting the various uses of the term. Morever, we highlight some of the various properties that have been suggested to be 'amodal' over the years. Then, we try to address some of the questions that arise from the reviewed evidence, such as: Do different uses of the 'term' refer to different domains, for example, sensory information, perceptual processes, or perceptual representations? Are there any commonalities among the different uses of the term? To what extent is research on cross-modal associations (or correspondences) related to, or can shed light on, amodality? And how is the notion of amodal related to multisensory integration? Based on the reviewed evidence, it is argued that there is, as yet, no convincing empirical evidence to support the claim that amodal sensory qualities exist. We thus suggest that use of the term amodal would be more meaningful with respect to abstract cognition rather than necessarily sensory perception, the latter being more adequately explained/understood in terms of highly redundant cross-modal correspondences.

Concavities, negative parts, and the perception that shapes are complete.

When we perceive the shape of an object, we can often make many other inferences about the object, derived from its shape. For example, when we look at a bitten apple, we perceive not only the local curvatures across the surface, but also that the shape of the bitten region was caused by forcefully removing a piece from the original shape (excision), leading to a salient concavity or negative part in the object. However, excision is not the only possible cause of concavities or negative parts in objects-for example, we do not perceive the spaces between the fingers of a hand to have been excised. Thus, in order to infer excision, it is not sufficient to identify concavities in a shape; some additional geometrical conditions must also be satisfied. Here, we studied the geometrical conditions under which subjects perceived objects as been bitten, as opposed to complete shapes. We created 2-D shapes by intersecting pairs of irregular hexagons and discarding the regions of overlap. Subjects rated the extent to which the resulting shapes appeared to be bitten or whole on a 10-point scale. We find that subjects were significantly above chance at identifying whether shapes were bitten or whole. Despite large intersubject differences in overall performance, subjects were surprisingly consistent in their judgments of shapes that had been bitten. We measured the extent to which various geometrical features predict subjects' judgments and find that the impression that an object is bitten is strongly correlated with the relative depth of the negative part. Finally, we discuss the relationship between excision and other perceptual organization processes such modal and amodal completion, and the inference of other attributes of objects, such as the material properties.

Study Replication: Shape Discrimination in a Conditioning Procedure on the Jumping Spider Phidippus regius.

Spiders possess a unique visual system, split into eight different eyes and divided into two fully independent visual pathways. This peculiar organization begs the question of how visual information is processed, and whether the classically recognized Gestalt rules of perception hold true. In a previous experiment, we tested the ability of jumping spiders to associate a geometrical shape with a reward (sucrose solution), and then to generalize the learned association to a partially occluded version of the shape. The occluded shape was presented together with a broken version of the same shape. The former should be perceived as a whole shape only in the case the animals, like humans, are able to amodally complete an object partly hidden by an occluder; otherwise, the two shapes would be perceived as identical. There, the spiders learned the association but failed to generalize. Here, we present a replication of the experiment, with an increased number of subjects, a DeepLabCut-based scoring procedure, and an improved statistical analysis. The results of the experiment follow closely the direction of the effects observed in the previous work but fail to rise to significance. We discuss the importance of study replication, and we especially highlight the use of automated scoring procedures to maximize objectivity in behavioral studies.

Perceptual organization and visual awareness: the case of amodal completion.

We investigated the involvement of visual awareness in amodal completion, and specifically, whether visual awareness plays a differential role in local versus global completion, using a primed shape discrimination paradigm and the color-opponent flicker technique to render the prime invisible. In four experiments, participants discriminated the shape of a target preceded by a partly occluded or a neutral prime. All primes were divergent occlusion patterns in which the local completion is based on good continuation of the contours at the point of occlusion and the global completion is based on maximum symmetry. The target corresponded to the shape that could arise as a result of local or global completion of the occluded prime. For each experiment with an invisible prime we conducted a version with a visible prime. Our results suggest that local completion, but not global completion, of a partly occluded shape can take place in the absence of visual awareness, but apparently only when the visible occluded shape generates a single, local completion. No completion, either local or global, appears to take place in the absence of visual awareness when the visible occluded shape generates multiple completions. The implications of these results to the differential role of visual awareness in local and global completions and to the relationship between multiple completions and unconscious amodal completions are discussed.

The illusion of absence: how a common feature of magic shows can explain a class of road accidents.

The purpose of the present note is to draw attention to the potential role of a recently discovered visual illusion in creating traffic accidents. The illusion consists in a compelling and immediate experience that the space behind an occluding object in the foreground is empty. Although the illusion refers to a region of space, which is invisible due to occlusion (a blind spot), there is evidence to suggest that it is nevertheless driven by visual mechanisms and that it can be just as deceptive and powerful as ordinary visual illusions. We suggest that this novel illusion can make situations involving blind spots in a road user's field of view even more dangerous than one would expect based on the lack of visibility by itself. This could be because it erroneously makes the road user feel that (s)he has actually seen everything there is to see, and thus has verified that the blind spot is empty. This hypothesis requires further testing before definitive conclusions can be drawn, but we wish to make researchers and authorities involved in the analysis of traffic accidents and on-the-spot crash investigations aware of its potential role in order to encourage registration of relevant data and facilitate further research.

How Visual Perception of the Inside of Things Creates the Impossible Dovetail.

Here, we consider a well-known wooden puzzle known as the impossible dovetail. We argue that an intriguing form of amodal completion, dealing with spontaneous interpretations of the inside of objects is the key to understanding why people find it difficult to see how the impossible dovetail is indeed possible.

On the Role of Contrast Polarity: In Response to van der Helm's Comments.

In this work, we discussed and counter-commented van der Helm's comments on our previous paper (Pinna and Conti, Brain Sci., 2019, 9, 149), where we demonstrated unique and relevant visual properties imparted by contrast polarity in eliciting amodal completion. The main question we addressed was: "What is the role of shape formation and perceptual organization in inducing amodal completion?" To answer this question, novel stimuli were studied through Gestalt experimental phenomenology. The results demonstrated the domination of the contrast polarity against good continuation, T-junctions, and regularity. Moreover, the limiting conditions explored revealed a new kind of junction next to the T- and Y-junctions, respectively responsible for amodal completion and tessellation. We called them I-junctions. The results were theoretically discussed in relation to the previous approaches and in the light of the phenomenal salience imparted by contrast polarity. In counter-commenting van der Helm's comments we went into detail of his critiques and rejected all of them point-by-point. We proceeded by summarizing hypotheses and discussion of the previous work, then commenting on each critique through old and new phenomena and clarifying the meaning of our previous conclusions.

The Illusion of Absence in Magic Tricks.

Recently, a curious illusion of absence has been described, where the space behind an occluder is compellingly experienced as empty. This illusion is similar to illusions based on amodal completion in the sense that it refers to occluded portions of a visual scene and informal observations suggest that it may also be largely impervious to conscious knowledge. The aim of the present experiment was to test the hypothesis that the illusion of absence is cognitively impenetrable in the same way as amodal completion. Participants viewed magic tricks based on amodal completion, the illusion of absence, or attentional and reasoning misdirection and tried to infer the secret behind the tricks after one, two, or three presentations. The results show that the tricks based on the illusion of absence are very difficult to debunk, even after repeated presentations. In this regard, they are similar to tricks based on amodal completion but different from tricks based on attentional and reasoning misdirection. The participants also rated how magical they felt the tricks were. Surprisingly, the magic ratings tended to be quite high even in trials where the participants had already discovered the secret behind the trick. This unexpected finding may be taken to suggest that there may be two magical moments in the lifetime of a magic trick: In addition to the magical experience evoked by trick itself, discovering the secret behind the trick may also evoke an experience of impossibility.

Precise Ensemble Face Representation Given Incomplete Visual Input.

Humans can recognize faces in the presence of environmental noise. Here, we explore whether ensemble perception of faces is similarly robust. Is summary statistical information available from crowds of faces that are visually incomplete? Observers viewed sets of faces varying in identity or expression and adjusted a test face to match the perceived average. In one condition, faces amodally completed behind horizontal bars. In another condition, identical facial information was presented, but in the foreground (i.e., face parts appeared on fragmented strips in front of a background). Baseline performance was determined by performance on sets of fully visible faces. The results revealed that the ensemble representation of amodally completing sets was significantly better than the fragmented sets and marginally worse than in the fully visible condition. These results suggest that some ensemble information is available given limited visual input and supports a growing body of work suggesting that ensembles may be represented in the absence of complete visual information.

From Michotte Until Today: Why the Dichotomous Classification of Modal and Amodal Completions Is Inadequate.

The distinction between modal and amodal completion is ubiquitous in the perception literature. It goes back to the seminal publication "Les compléments amodaux des structures perceptives" by A. Michotte, G. Thinès, and G. Crabbé (Publications Universitaires de Louvain: Louvain) in 1964. We review and discuss this work in this article and show commonalities and differences to today's view. We then argue that the dichotomous distinction between modal and amodal completions is problematic in phenomenological, empirical, logical, and theoretical terms. Finally, we propose alternative criteria allowing for a more differentiated classification scheme for completion phenomena. This scheme seems to be consistent with all known empirical findings and can also be generalized to nonvisual domains of perception.

The Limiting Case of Amodal Completion: The Phenomenal Salience and the Role of Contrast Polarity.

In this work, we demonstrated unique and relevant visual properties imparted by contrast polarity in perceptual organization and in eliciting amodal completion, which is the vivid completion of a single continuous object of the visible parts of an occluded shape despite portions of its boundary contours not actually being seen. T-junction, good continuation, and closure are considered the main principles involved according to relevant explanations of amodal completion based on the simplicity-Prägnanz principle, Helmholtz's likelihood, and Bayesian inference. The main interest of these approaches is to explain how the occluded object is completed, what is the amodal shape, and how contours of partially visible fragments are relatable behind an occluder. Different from these perspectives, amodal completion was considered here as a visual phenomenon and not as a process, i.e., the final outcome of perceptual processes and grouping principles. Therefore, the main question we addressed through our stimuli was "What is the role of shape formation and perceptual organization in inducing amodal completion?" To answer this question, novel stimuli, similar to limiting cases and instantiae crucis, were studied through Gestalt experimental phenomenology. The results demonstrated the domination of the contrast polarity against good continuation, T-junctions, and regularity. Moreover, the limiting conditions explored revealed a new kind of junction next to the T- and Y-junctions, respectively responsible for amodal completion and tessellation. We called them I-junctions. The results were theoretically discussed in relation to the previous approaches and in the light of the phenomenal salience imparted by contrast polarity.

Neuroimaging Findings on Amodal Completion: A Review.

Amodal completion is the phenomenon of perceiving completed objects even though physically they are partially occluded. In this review, we provide an extensive overview of the results obtained from a variety of neuroimaging studies on the neural correlates of amodal completion. We discuss whether low-level and high-level cortical areas are implicated in amodal completion; provide an overview of how amodal completion unfolds over time while dissociating feedforward, recurrent, and feedback processes; and discuss how amodal completion is represented at the neuronal level. The involvement of low-level visual areas such as V1 and V2 is not yet clear, while several high-level structures such as the lateral occipital complex and fusiform face area seem invariant to occlusion of objects and faces, respectively, and several motor areas seem to code for object permanence. The variety of results on the timing of amodal completion hints to a mixture of feedforward, recurrent, and feedback processes. We discuss whether the invisible parts of the occluded object are represented as if they were visible, contrary to a high-level representation. While plenty of questions on amodal completion remain, this review presents an overview of the neuroimaging findings reported to date, summarizes several insights from computational models, and connects research of other perceptual completion processes such as modal completion. In all, it is suggested that amodal completion is the solution to deal with various types of incomplete retinal information, and highly depends on stimulus complexity and saliency, and therefore also give rise to a variety of observed neural patterns.

Mid-level Priming by Completion vs. Mosaic Solutions.

We report two experiments on the role of mid-level processes in image segmentation and completion. In the primed matching task of Experiment 1, a cue→prime sequence was presented before the imperative stimulus consisting of target shapes with positive versus negative contour curvature polarity and one versus two axes of mirror symmetry. Priming shapes were included in two composite occlusion displays with the same T-junction information and different geometric features supporting a distinct balance between completion and mosaic solutions. A cue, either congruent or incongruent with targets, preceded the presentation of the composite priming display. Matching performance was affected by primes in the expected direction, while cue congruency participated only in a marginally significant three-way interaction, and prime duration had no effect. In Experiment 2, the cue→prime sequence was replaced by a fixation cross to control for the priming effect obtained in Experiment 1. The study confirmed that contour connectability and curvature polarity are effective structural factors capable of competing with symmetry in mid-level image segmentation and completion processes.

A Remarkable Depth Confusion in Images of the Incomplete Statues of Bruno Catalano.

Images of Bruno Catalano's sculptures of incomplete bodies give rise to a remarkable depth confusion in which the background is partly pushed to the front. We argue that this confusion is related to what happens in the Kanizsa square, although the effect in the images of Catalano's sculpture appears to be driven by knowledge-based processing.