From perceptual organization to visual illusions and back.

In modern vision science, illusions are compelling phenomena useful as tools to explore vision under limiting psychophysical conditions. Illusions manifest at least two issues that challenge scientists. The first issue is related to the definition of illusion and to the complexity of the mismatch between the geometrical/physical and the phenomenal domains. The second issue concerns two different meanings of the term "illusion," respectively related to the demonstration of the illusion through the mismatch between domains and to the phenomenal illusoriness, i.e., the perception of something having the nature of an illusion, unreal, ambiguous, fallacious, and deceptive. In this work, we explored the notion of illusion starting from the principles of perceptual organization as described by Gestalt psychologists. On the basis of several phenomenal conditions, step by step, we suggested some new hypotheses, whose purpose was to answer the following questions: what is physical and what is phenomenal? Is there and, if any, what is the dividing line between illusions and non-illusions? Is it true that illusions are rare phenomena? Why do illusions exist? What is their perceptual and evolutionist role? These questions and the related issues were phenomenally discussed by deepening and extending the notion of perceptual organization and by exploring the biological implications of both illusions and illusoriness. On the basis of our results, the perception of illusion and illusoriness can be considered as a further challenge for vision scientists useful to shed new insights within the biological meanings of visual perception and within the no-man land between sensory and cognitive processes that elicit visual consciousness not fully explored yet.

Similarity and Dissimilarity in Perceptual Organization: On the Complexity of the Gestalt Principle of Similarity.

The main purpose of this work is to explore the Gestalt principle of similarity and to demonstrate that the use of this term alone is not sufficient to understand the dynamics of grouping fully and correctly. More generally, this work aims to show that the Gestalt principle of similarity alone is not sufficient for a full understanding of perceptual organization occurring both in the classical and mostly in the new phenomena here presented. Limits and incompleteness of the similarity principle have suggested the basic, more general and stronger role of dissimilarity in perceptual grouping under a large variety of conditions. Dissimilarity was shown as a basic principle of figure-ground segregation, as a tool useful to create at will new groups and visual objects within patterns where they are totally invisible, as an attribute that is able to accentuate different shape components within the same object, as a way to distort shapes and create visual illusions, but also to reduce or annul them and, finally, to decompose, ungroup and reshape single objects. The results demonstrated the necessity to introduce a principle of dissimilarity that is complementary to similarity as already studied by Gestalt psychologists.

Rectangularity Is Stronger Than Symmetry in Interpreting 2D Pictures as 3D Objects.

It is known that the human brain has a strong preference for rectangularity in interpreting pictures as 3D shapes. Symmetry is also considered to be a factor that the human vision system places high priority on when perceiving 3D objects. Thus, a question is raised: which is more basic, the rectangularity preference or the symmetry preference? To answer this question, we carried out experiments using pictures that have at least two interpretations as 3D objects, one of which was rectangular but not symmetric, and the other of which was symmetric but not rectangular. We found that the preference for rectangularity is stronger than that for symmetry. This observation will help us to understand various 3D optical illusions, including the room-size illusion and the ambiguous object illusion.

Gaze Parameters in the Analysis of Ambiguous Geometric Shapes.

This study explores perceptual organisation and shape perception when viewing a tetragon and an additional element (a dot) that is located at varying positions and distances next to the tetragon. The aim of the study is to determine the factors that can alter the interpretation of object configuration and impact whether the presented tetragon is perceived as a diamond or a square. Methods used in this study are a forced-choice task as a subjective measurement and eye tracking as an objective measurement of perceptual processes. Overall, 31 stimuli were presented to the participants: a tetragon in two different sizes with an additional element (a dot) located inside or outside the object at three different positions at three distances. The results indicate significant changes in shape perception, depending on the location of the additional element. The results are complemented with eye movement analysis indicating that as the distance between the elements increases, there is a higher probability of either of the two shape interpretations and the gaze is less likely to be directed to the area between the stimuli. Furthermore, the subjective perception of shape is codetermined by the shape perception when the tetragon is presented without the additional element.

On a lightness phenomenon.

This work demonstrates a lightness phenomenon useful to extend the notion of "belongingness," which is crucial to explain a class of illusions that include simultaneous lightness contrast, the Koffka-Benussi ring, the Benary cross, and the White effect. These phenomena manifest some kind of dissimilarity, difference, or change responsible for the perceived contrast. The dissimilarity is related to the "belongingness" of the crucial gray elements (i) to a unique or separated/divided object, as in the Koffka-Benussi ring, or (ii) to the figure or to the background, as in the Benary and White effects. If we plausibly assume that differences and changes are biologically important to be detected and if necessary highlighted, then any visible difference might induce a contrast effect. This is the main hypothesis demonstrated by the lightness phenomenon based on checks grouped vertically, split in two upper and lower halves, and segregated from the homogeneous gray background. The checks are alternated and vertically/horizontally reversed in the upper and lower halves of the pattern. Despite the constant visual organization and in spite of the identical local contrast within each check, the inner area of the elements of the upper group appears darker than the one of the lower group. The visible dissimilarity, although not related to the notion of belongingness, is sufficient to elicit a clear lightness difference.

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 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.

When the whole is equal to the sum of its parts: A new approach to study face and body perception and representation.

In this paper, a new approach and a novel method to study face perception is proposed and tested using several qualitative experiments. This method is based on three main tasks: a description task (subjects were asked to freely describe the target stimulus), a free pictorial task (free drawing/painting of what subjects were asked), and a pictorial reproduction task (making a copy of what subjects perceived). These tasks were carried out with children and adults and extended to conditions related to visual arts. The starting points of this work were the canonical perspective and the holistic processes involved in face perception. The aim of this work was to answer the two following basic questions: Are canonical perspective and holistic processes really effective for face perception? Is face perception other than the sum of its parts? The outcomes of the experiments clearly refuted the role of canonical perspective and weaken the holistic approach to face and body perception. The whole human body has been shown instead to appear as if built starting from every single component, therefore body and faces are like wholes, decomposable in a mosaic of juxtaposed independent components reduced to a reference image. In short, the whole is equal to the sum of its parts. Finally, our results also show evidence supporting the introduction of the notion of icon.

A new principle of figure-ground segregation: The accentuation.

The problem of perceptual organization was studied by Gestalt psychologists in terms of figure-ground segregation. In this paper we explore a new principle of figure-ground segregation: accentuation. We demonstrate the effectiveness of accentuation relative to other Gestalt principles, and also consider it autonomous as it can agree with or oppose them. We consider three dynamic aspects of the principle, namely: attraction, accentuation and assignment. Each creature needs to attract, fascinate, seduce, draw attention (e.g., a mate or a prey animal) or distract, refuse, dissuade, discourage, repulse (e.g., a predator). Similarly, each organism needs to accentuate, highlight, stress, underline, emphasize or distract from another. Thus, accentuation assigns meaning to a visual pattern such as a coat, a plumage or a flower. False eyes (ocelli) and dots (diematic patterns) demonstrate "deceiving camouflage by accentuation" that confuses predators/preys and hides or highlights vital body parts (butterflies/flowers). They also display the deceiving appearance and exhibition of biological fitness. The same accents may serve different or even opposite goals. We conclude that accentuation improves the adaptive fitness of organisms in multifarious ways.

On the Role of Color in Reading and Comprehension Tasks in Dyslexic Children and Adults.

We investigated the effect of chromatic variations on the reading process with normal and dyslexic readers. We demonstrate that color can induce wholeness, parts-whole organization and phenomenal fragmentation during reading and comprehension tasks within written texts made up of words and non-words in the following conditions: monochromatic (the whole text colored with only one color), word (each word colored in different color), half word (half word colored with a color different from the one of the second half), syllable (every syllable colored with a different color) and letter (each letter with a different color). The dependent variables considered were reading time, reading errors and incorrect answers to a comprehension test. The main results demonstrated that these parameters of reading performance are all influenced by the five aforementioned chromatic conditions. These outcomes manifest similar trends in four groups of readers: children and adults combined with normal or dyslexic readers. Possible applied research and clinical applications are discussed together with some basic questions related to color vision suggesting that the main purposes of color for living beings is to generate wholeness, parts-whole organization and perceptual segmentation.

Facing the Spectator.

We investigated the familiar phenomenon of the uncanny feeling that represented people in frontal pose invariably appear to "face you" from wherever you stand. We deploy two different methods. The stimuli include the conventional one-a flat portrait rocking back and forth about a vertical axis-augmented with two novel variations. In one alternative, the portrait frame rotates whereas the actual portrait stays motionless and fronto-parallel; in the other, we replace the (flat!) portrait with a volumetric object. These variations yield exactly the same optical stimulation in frontal view, but become grossly different in very oblique views. We also let participants sample their momentary awareness through "gauge object" settings in static displays. From our results, we conclude that the psychogenesis of visual awareness maintains a number-at least two, but most likely more-of distinct spatial frameworks simultaneously involving "cue-scission." Cues may be effective in one of these spatial frameworks but ineffective or functionally different in other ones.

From Grouping to Coupling: A New Perceptual Organization in Vision, Psychology, and Biology.

In this work, perceptual organization has been studied with the same spirit and phenomenological methods used by Gestalt psychologists. This was accomplished through new conditions that cannot be explained in terms of the classical principles of grouping. Perceptual grouping represents the way through which our visual system builds integrated elements on the basis of the maximal homogeneity among the components of the stimulus pattern. Our results demonstrated the inconsistency of organization by grouping, and more importantly, the inconsistency of the principle of similarity. On the contrary, they suggested the unique role played by the principle of dissimilarity among elements that behaves like an accent or a visual emphasis within a whole. The principle of accentuation was here considered as imparting a directional structure to the elements and to the whole object thus creating new phenomena. The salience of the resulting phenomena reveals the supremacy of dissimilarity in relation to similarity and the fact that it belongs to a further organization dynamics that we called "coupling." In biology, coupling and its principle of accentuation are very strongly related to disruptive camouflage. Moreover, they are source of sexual attraction. They advertise the presence and elicit species identification/communication. In human beings accentuation is needed to show ourselves to others, to understand the way we dress, choose, and create clothes or invent fashion, the way we change our body accentuating several parts and hiding some others, the way we use maquillage. The existence of maquillage itself is derived from the need to accentuate something with the purpose to increase sexual attraction, to exhibit physical strength and beauty, to show or hide social status (e.g., being the king, a warrior, a priest, etc.). Last but not least, accentuation plays a basic role also in making it easier or difficult to read and understand written words.

The accentuation principle of figure-ground segregation and the downbeat illusion.

Pinna and Sirigu (2011) demonstrated a new principle of grouping, called the accentuation principle, stating that, all else being equal, elements tend to group in the same oriented direction of the discontinuous element placed within a whole set of continuous/homogeneous components. The discontinuous element behaves like an accent, i.e. a visual emphasis within the wholeness of components as shown in the next section. In this work, the accentuation principle has been extended to new visual domains. In particular, it is shown how this principle affects shape perception. Moreover several visual object attributes are also highlighted, among which orientation, spatial position, inner dynamics and apparent motion that determine the so-called organic segmentation and furthermore tend to induce figure-ground segregation. On the basis of the results of experimental phenomenology, the accentuation can be considered as a complex principle ruling grouping, figure-ground segregation, shape and meaning formation. Through a new musical illusion of downbeat, it is also demonstrated that this principle influences perceptual organization not only in space but also in time and, thus, in both visual and musical domains. This illusion can be heard in eight measures of Pagodes, a solo piano music by Claude Debussy (1862-1918), where a strong physical-perceptual discrepancy in terms of upbeats and downbeats inversion is strongly perceived in both staves.

Material properties from contours: New insights on object perception.

In this work we explored phenomenologically the visual complexity of the material attributes on the basis of the contours that define the boundaries of a visual object. The starting point is the rich and pioneering work done by Gestalt psychologists and, more in detail, by Rubin, who first demonstrated that contours contain most of the information related to object perception, like the shape, the color and the depth. In fact, by investigating simple conditions like those used by Gestalt psychologists, mostly consisting of contours only, we demonstrated that the phenomenal complexity of the material attributes emerges through appropriate manipulation of the contours. A phenomenological approach, analogous to the one used by Gestalt psychologists, was used to answer the following questions. What are contours? Which attributes can be phenomenally defined by contours? Are material properties determined only by contours? What is the visual syntactic organization of object attributes? The results of this work support the idea of a visual syntactic organization as a new kind of object formation process useful to understand the language of vision that creates well-formed attribute organizations. The syntax of visual attributes can be considered as a new way to investigate the modular coding and, more generally, the binding among attributes, i.e., the issue of how the brain represents the pairing of shape and material properties.

Directional organization and shape formation: new illusions and Helmholtz's Square.

According to Helmholtz's Square illusion, a square appears wider when it is filled with vertical lines and higher when filled with horizontal lines (Helmholtz von, 1866). Recently, Pinna (2010a) demonstrated that the grouping of small squares on the basis of the similarity principle influences also perception of their shape and of the whole emerging shapes. The direction imparted by grouping is the main attribute that influences the shape by polarizing it in the same direction both globally and locally. The rectangle illusion is opposite to what expected on the basis of Helmholtz's Square illusion. Aim of this work is to solve the antinomy between the two sets of illusions and to demonstrate a common explanation based on the interaction between different sources of directional organization. This was accomplished by introducing some new phenomena and through phenomenological experiments proving the role played by the directional shape organization in shape formation. According to our results, Helmholtz's square illusion shows at least two synergistic sources of directional organization: the direction of the grouping of the lines due to their similarity of the luminance contrast and the direction of the grouping of the lines due to the good continuation.

On the purposes of color for living beings: toward a theory of color organization.

Phylogenetic and paleontological evidence indicates that in the animal kingdom the ability to perceive colors evolved independently several times over the course of millennia. This implies a high evolutionary neural investment and suggests that color vision provides some fundamental biological benefits. What are these benefits? Why are some animals so colorful? What are the adaptive and perceptual meanings of polychromatism? We suggest that in addition to the discrimination of light and surface chromaticity, sensitivity to color contributes to the whole, the parts and the fragments of perceptual organization. New versions of neon color spreading and the watercolor illusion indicate that the visual purpose of color in humans is threefold: to inter-relate each chromatic component of an object, thus favoring the emergence of the whole; to support a part-whole organization in which components reciprocally enhance each other by amodal completion; and, paradoxically, to reveal fragments and hide the whole-that is, there is a chromatic parceling-out process of separation, division, and fragmentation of the whole. The evolution of these contributions of color to organization needs to be established, but traces of it can be found in Harlequin camouflage by animals and in the coloration of flowers.

The microgenesis of the watercolor effect.

The "watercolor effect" is the wash of illusory color that fills in between two enclosing bichromatic contours. We studied the microgenesis of this illusion by varying the duration of the eliciting stimulus (a yellow/purple contour outlining the Mediterranean Sea) and by varying the duration of a blank interval from stimulus offset to an after-coming mask (the ISI). The illusory wash was rated in strength and also matched to a comparison disk of adjustable color but similar luminance. Results indicate that the watercolor effect grows rapidly as stimulus duration is increased to 100 ms and then grows much more slowly. Increasing the ISI beyond 10 ms had no effect, suggesting that the wash arises only during stimulation. Participants who recognized that the bounding contour depicted the Mediterranean reported twice as strong an illusory effect as those who did not, indicating that visual long-term memory modulates the watercolor effect despite the rapidity of its generation.

Perceptual organization of shape, color, shade, and lighting in visual and pictorial objects.

THE MAIN QUESTIONS WE ASKED IN THIS WORK ARE THE FOLLOWING: Where are representations of shape, color, depth, and lighting mostly located? Does their formation take time to develop? How do they contribute to determining and defining a visual object, and how do they differ? How do visual artists use them to create objects and scenes? Is the way artists use them related to the way we perceive them? To answer these questions, we studied the microgenetic development of the object perception and formation. Our hypothesis is that the main object properties are extracted in sequential order and in the same order that these roles are also used by artists and children of different age to paint objects. The results supported the microgenesis of object formation according to the following sequence: contours, color, shading, and lighting.

The role of amodal completion in shape formation: some new shape phenomena.

Amodal completion occurs when a portion of an object is hidden as a result of its occlusion behind another object. Under these conditions, the object perceived as occluded is seen as a unitary shape, whose boundary contours amodally complete behind the overlapping modal object. Kanizsa (1972, Studia Psychologica 14 208-210) and his collaborators demonstrated some effects related to the amodal completion: shrinkage of the whole figure partially occluded; expansion of the modally visible portions of the same figure; shape deformations against the Gestalt principles of regularity, simplicity, symmetry, and past experience; global increasing of colour quantity of the partially occluded figure. The aim of this work is to demonstrate that the amodal completion is not a necessary factor in inducing the previous effects. This was accomplished through phenomenological experiments whose stimuli were crucial instances (counterexamples) disproving the amodal completion hypothesis and proving the role played by thedirectional symmetry of the element components of each stimulus pattern. Some new phenomena demonstrated the main role of the directional shape organisation, considered as a principle of shape formation.