A common mechanism for illusory and occluded object completion

Does the brain's ventral visual pathway compute object shape?

A rich behavioral literature has shown that human object recognition is supported by a representation of shape that is tolerant to variations in an object's appearance. Such 'global' shape representations are achieved by describing objects via the spatial arrangement of their local features, or structure, rather than by the appearance of the features themselves. However, accumulating evidence suggests that the ventral visual pathway - the primary substrate underlying object recognition - may not represent global shape. Instead, ventral representations may be better described as a basis set of local image features. We suggest that this evidence forces a reevaluation of the role of the ventral pathway in object perception and posits a broader network for shape perception that encompasses contributions from the dorsal pathway.

Interference of Illusory Contour Perception by a Distractor

The visual system is capable of recognizing objects when object information is widely separated in space, as revealed by the Kanizsa-type illusory contours (ICs). Attentional involvement in perception of ICs is an important topic, and the present study examined whether and how the processing of ICs is interfered with by a distractor. Discrimination between thin and short deformations of an illusory circle was investigated in the absence or presence of a central dynamic patch, with difficulty of discrimination varied in three levels (easy, medium, and hard). Reaction time (RT) was significantly shorter in the absence compared to the presence of the distractor in the easy and medium conditions. Correct rate (CR) was significantly higher in the absence compared to the presence of the distractor in the easy condition, and the magnitude of the difference between CRs of distracted and non-distracted responses significantly reduced as task difficulty increased. These results suggested that perception of ICs is more likely to be vulnerable to distraction when more attentional resources remain available. The present finding supports that attention is engaged in perception of ICs and that distraction of IC processing is associated with perceptual load.

Abutting Objects Warp the Three-Dimensional Curvature of Modally Completing Surfaces

Binocular disparity can give rise to the perception of open surfaces or closed curved surfaces (volumes) that appear to vary smoothly across discrete depths. Here I build on my recent papers by providing examples where modally completing surfaces not only fill in from one depth layer's visible contours to another layer's visible contours within virtual contours in an analog manner, but where modally completing surface curvature is altered by the interpolation of an abutting object perceived to be connected to or embedded within that modally completing surface. Seemingly minor changes in such an abutting object can flip the interpretation of distal regions, for example, turning a distant edge (where a surface ends) into rim (where a surface bends to occlude itself) or turning an open surface into a closed one. In general, the interpolated modal surface appears to deform, warp, or bend in three-dimensions to accommodate the abutting object. These demonstrations cannot be easily explained by existing models of visual processing or modal completion and drive home the implausibility of localistic accounts of modal or amodal completion that are based, for example, solely on extending contours in space until they meet behind an occluder or in front of "pacmen." These demonstrations place new constraints on the holistic surface and volume generation processes that construct our experience of a three-dimensional world of surfaces and objects under normal viewing conditions.

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.

Bilateral Symmetry Strengthens the Perceptual Salience of Figure against Ground

Although symmetry has been discussed in terms of a major law of perceptual organization since the early conceptual efforts of the Gestalt school (Wertheimer, Metzger, Koffka and others), the first quantitative measurements testing for effects of symmetry on processes of Gestalt formation have seen the day only recently. In this study, a psychophysical rating study and a “foreground”-“background” choice response time experiment were run with human observers to test for effects of bilateral symmetry on the perceived strength of figure-ground in triangular Kanizsa configurations. Displays with and without bilateral symmetry, identical physically-specified-to-total contour ratio, and constant local contrast intensity within and across conditions, but variable local contrast polarity and variable orientation in the plane, were presented in a random order to human observers. Configurations with bilateral symmetry produced significantly stronger figure-ground percepts reflected by greater subjective magnitudes and consistently higher percentages of “foreground” judgments accompanied by significantly shorter response times. These effects of symmetry depend neither on the orientation of the axis of symmetry, nor on the contrast polarity of the physical inducers. It is concluded that bilateral symmetry, irrespective of orientation, significantly contributes to the, largely sign-invariant, visual mechanisms of figure-ground segregation that determine the salience of figure-ground in perceptually ambiguous configurations.

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.

Developmental trends in interpolation and its spatial constraints: A comparison of subjective and occluded contours

We examined interpolation in 6- and 9-year-old children and in adults, in the two most common forms of fragmentation: subjective and partially occluded contours. Experiment 1 examined the effects on adults' interpolation of contour geometry, specifically, the effect of a scale-dependent factor (i.e., retinal size) and a scale-independent factor (i.e., support ratio). For both subjective and partially occluded contours, interpolation was affected more by support ratio than absolute size. However, subjective contours were less precisely interpolated and their interpolation was affected more by support ratio than was the case for partial occlusion. Experiment 2 used a subset of retinal size and support ratio levels in children and adults. Interpolation of both subjective and occluded contours improved significantly with age, with the two types of contours equally affected by spatial constraints during early childhood. However, while interpolation of occluded contours became more precise with age and less dependent on support ratio by adulthood, interpolation of subjective contours was less improved and became even more tied to support ratio in adulthood. The implications of these differential age-related changes in the spatial constraints on interpolation of the two types of contours for the mechanisms of perceptual completion are discussed.

What can fish brains tell us about visual perception?

Fish are a complex taxonomic group, whose diversity and distance from other vertebrates well suits the comparative investigation of brain and behavior: in fish species we observe substantial differences with respect to the telencephalic organization of other vertebrates and an astonishing variety in the development and complexity of pallial structures. We will concentrate on the contribution of research on fish behavioral biology for the understanding of the evolution of the visual system. We shall review evidence concerning perceptual effects that reflect fundamental principles of the visual system functioning, highlighting the similarities and differences between distant fish groups and with other vertebrates. We will focus on perceptual effects reflecting some of the main tasks that the visual system must attain. In particular, we will deal with subjective contours and optical illusions, invariance effects, second order motion and biological motion and, finally, perceptual binding of object properties in a unified higher level representation.

The brain creates illusions not just for us: sharks (Chiloscyllium griseum) can "see the magic" as well

Bamboo sharks (Chiloscyllium griseum) were tested for their ability to perceive subjective and illusionary contours as well as line length illusions. Individuals were first trained to differentiate between squares, triangles, and rhomboids in a series of two alternative forced-choice experiments. Transfer tests then elucidated whether Kanizsa squares and triangles, grating gaps and phase shifted abutting gratings were also perceived and distinguished. The visual systems of most vertebrates and even invertebrates perceive illusionary contours despite the absence of physical luminance, color or textural differences. Sharks are no exception to the rule; all tasks were successfully mastered within 3-24 training sessions, with sharks discriminating between various sets of Kanizsa figures and alternative stimuli, as well as between subjective contours in >75% of all tests. However, in contrast to Kanizsa figures and subjective contours, sharks were not deceived by Müller-Lyer (ML) illusions. Here, two center lines of equal length are comparatively set between two arrowheads or -tails, in which case the line featuring the two arrow tails appears to be longer to most humans, primates and birds. In preparation for this experiment, lines of varying length, and lines of unequal length randomly featuring either two arrowheads or -tails on their ends, were presented first. Both sets of lines were successfully distinguished by most sharks. However, during presentation of the ML illusions sharks failed to succeed and succumbed either to side preferences or chose according to chance.

Core foundations of abstract geometry

Human adults from diverse cultures share intuitions about the points, lines, and figures of Euclidean geometry. Do children develop these intuitions by drawing on phylogenetically ancient and developmentally precocious geometric representations that guide their navigation and their analysis of object shape? In what way might these early-arising representations support later-developing Euclidean intuitions? To approach these questions, we investigated the relations among young children's use of geometry in tasks assessing: navigation; visual form analysis; and the interpretation of symbolic, purely geometric maps. Children's navigation depended on the distance and directional relations of the surface layout and predicted their use of a symbolic map with targets designated by surface distances. In contrast, children's analysis of visual forms depended on the size-invariant shape relations of objects and predicted their use of the same map but with targets designated by corner angles. Even though the two map tasks used identical instructions and map displays, children's performance on these tasks showed no evidence of integrated representations of distance and angle. Instead, young children flexibly recruited geometric representations of either navigable layouts or objects to interpret the same spatial symbols. These findings reveal a link between the early-arising geometric representations that humans share with diverse animals and the flexible geometric intuitions that give rise to human knowledge at its highest reaches. Although young children do not appear to integrate core geometric representations, children's use of the abstract geometry in spatial symbols such as maps may provide the earliest clues to the later construction of Euclidean geometry.

Illusory contours: a window onto the neurophysiology of constructing perception

Seeing seems effortless, despite the need to segregate and integrate visual information that varies in quality, quantity, and location. The extent to which seeing passively recapitulates the external world is challenged by phenomena such as illusory contours, an example of visual completion whereby borders are perceived despite their physical absence in the image. Instead, visual completion and seeing are increasingly conceived as active processes, dependent on information exchange across neural populations. How this is instantiated in the brain remains controversial. Divergent models emanate from single-unit and population-level electrophysiology, neuroimaging, and neurostimulation studies. We reconcile discrepant findings from different methods and disciplines, and underscore the importance of taking into account spatiotemporal brain dynamics in generating models of brain function and perception.

Fixating picture boundaries does not eliminate boundary extension: implications for scene representation

Observers frequently remember seeing more of a scene than was shown (boundary extension). Does this reflect a lack of eye fixations to the boundary region? Single-object photographs were presented for 14-15 s each. Main objects were either whole or slightly cropped by one boundary, creating a salient marker of boundary placement. All participants expected a memory test, but only half were informed that boundary memory would be tested. Participants in both conditions made multiple fixations to the boundary region and the cropped region during study. Demonstrating the importance of these regions, test-informed participants fixated them sooner, longer, and more frequently. Boundary ratings (Experiment 1) and border adjustment tasks (Experiments 2-4) revealed boundary extension in both conditions. The error was reduced, but not eliminated, in the test-informed condition. Surprisingly, test knowledge and multiple fixations to the salient cropped region, during study and at test, were insufficient to overcome boundary extension on the cropped side. Results are discussed within a traditional visual-centric framework versus a multisource model of scene perception.

When less is more: Line-drawings lead to greater boundary extension than color photographs

Is boundary extension (false memory beyond the edges of the view; Intraub & Richardson, 1989) determined solely by the schematic structure of the view or does the quality of the pictorial information impact this error? To examine this color photograph or line-drawing versions of 12 multi-object scenes (Experiment 1: N=64) and 16 single-object scenes (Experiment 2: N=64) were presented for 14-s each. At test, the same pictures were each rated as being the "same", "closer-up" or "farther away" (5-pt scale). Although the layout, the scope of the view, the distance of the main objects to the edges, the background space and the gist of the scenes were held constant, line-drawings yielded greater boundary extension than did their photographic counterparts for multi-object (Experiment 1) and single-object (Experiment 2) scenes. Results are discussed in the context of the multisource model and its implications for the study of scene perception and memory.

Features of the selectivity for contrast polarity in contour integration revealed by a novel tilt illusion

We studied a novel illusion of tilt inside checkerboards due to the role of contrast polarity in contour integration. The preference for binding of oriented contours having same contrast polarity, over binding of opposite polarity ones (CP rule), has been used to explain several visual illusions. In three experiments we investigated how the binding effect is influenced by luminance contrast value, relatability of contour elements, and distance among them. Experiment 1 showed that the effect was indeed present only when the CP rule was satisfied, and found it to be stronger when the luminance contrast values of the elements are more similar. In experiment 2 the illusion was reported only with relatable edges, and its strength was modulated by the degree of relatability. The CP-rule effectiveness, thus, seems to depend on good continuation. The intensity of contrast polarity signals propagating from an oriented contour might be the less intense, the more its direction deviates from linearity. In experiment 3 we estimated the distance threshold and found it to be smaller than the one found for other illusions, arising with collinear fragments. This seems to show that the reach of the contrast polarity signal inside the association field of a contour unit is shorter along non-collinear orientations than along collinear ones.

Global visual processing in macaques studied using Kanizsa illusory shapes

The ability to extract form information from a visual scene, for object recognition or figure-ground segregation, is a fundamental visual system function. Many studies of nonhuman primates have addressed the neural mechanisms involved in global form processing, but few have sought to demonstrate this ability behaviorally. In this study, we probed global visual processing in macaque monkeys (Macaca nemestrina) using classical Kanizsa illusory shapes as an assay of global form perception. We trained three monkeys on a "similarity match-to-sample" form discrimination task, first with complete forms embedded in fields of noncontour-inducing "pacman" elements. We then tested them with classic Kanizsa illusory shapes embedded in fields of randomly oriented elements. Two of the three subjects reached our criterion performance level of 80% correct or better on four of five illusory test conditions, demonstrating clear evidence of Kanizsa illusory form perception; the third subject mastered three of five conditions. Performance limits for illusory form discrimination were obtained by manipulating support ratio and by measuring threshold for discriminating "fat" and "thin" illusory squares. Our results indicate that macaque monkeys are capable of global form processing similarly to humans and that the perceptual mechanisms for "filling-in" contour gaps exist in macaques as they do in humans.

Perception of subjective contours in fish

The ability of fish to perceive subjective (or illusory) contours, ie contours that lack a physical counterpart in terms of luminance contrast gradients, was investigated. In the first experiment, redtail splitfins (Xenotoca eiseni), family Goodeidae, were trained to discriminate between a geometric figure (a triangle or a square) on various backgrounds and a background without any figure. Thereafter, the fish performed test trials in which illusory squares or triangles were obtained by (i) interruptions of a background of diagonal lines, (ii) phase-shifting of a background of diagonal lines, and (iii) pacmen spatially arranged to induce perception of Kanizsa subjective surfaces. In all three conditions, fish seemed to generalise their responses to stimuli perceived as subjective contours by humans. Fish chose, correctly, squares or triangles made of interrupted or phase-shifted diagonal lines from uniform backgrounds of diagonal lines, as well as illusory square or triangle Kanizsa figures from figures in which the inducing pacmen were scrambled. In the second experiment, fish were trained to discriminate between a vertical and a horizontal bar with luminance contrast gradients, and then tested with vertically and horizontally oriented illusory bars, created either through interruption or spatial phase-shift of inducing diagonal lines. Fish appeared to be able to generalise the orientation discrimination to illusory contours. These results demonstrate that redtail splitfins are capable of perceiving illusory contours.

False memory 1/20th of a second later: what the early onset of boundary extension reveals about perception

Errors of commission are thought to be caused by heavy memory loads, confusing information, lengthy retention intervals, or some combination of these factors. We report false memory beyond the boundaries of a view, boundary extension, after less than 1/20th of a second. Photographs of scenes were interrupted by a 42-ms or 250-ms mask, 250 ms into viewing, before reappearing or being replaced with a different view (Experiment 1). Postinterruption photographs that were unchanged were rated as closer up than the original views; when the photographs were changed, the same pair of closer-up and wider-angle views was rated as more similar when the closer view was first, rather than second. Thus, observers remembered preinterruption views with extended boundaries. Results were replicated when the interruption included a saccade (Experiment 2). The brevity of these interruptions has implications for visual scanning; it also challenges the traditional distinction between perception and memory. We offer an alternative conceptualization that shows how source monitoring can explain false memory after an interruption briefer than an eyeblink.

Looking at scenes while searching for numbers: dividing attention multiplies space

Observers tend to remember seeing a greater expanse of a scene than was shown (boundary extension [BE]). Is undivided visual attention necessary for BE? In Experiment 1, 108 observers viewed photographs with superimposed numerals (2s and 5s). Each appeared for 750 msec, followed by a masked interval and a test picture (same, closer up, or wider angled). Test pictures were rated as the same, closer, or wider angled on a 5-point scale. Visual attention was manipulated with a search task: The observers reported the number of 5s (zero, one, or two). The observers performed search only, picture rating only, or both (giving search priority). Search accuracy was unaffected by condition. BE occurred in both conditions but was greater with divided attention. The results were replicated using incidental BE tests (Experiments 2 and 3). We propose that anticipatory representation of layout occurs automatically during scene perception, with focal attention serving to constrain the boundary error.

Globally consistent depth sorting of overlapping 2D surfaces in a model using local recurrent interactions

The human visual system utilizes depth information as a major cue to group together visual items constituting an object and to segregate them from items belonging to other objects in the visual scene. Depth information can be inferred from a variety of different visual cues, such as disparity, occlusions and perspective. Many of these cues provide only local and relative information about the depth of objects. For example, at occlusions, T-junctions indicate the local relative depth precedence of surface patches. However, in order to obtain a globally consistent interpretation of the depth relations between the surfaces and objects in a visual scene, a mechanism is necessary that globally propagates such local and relative information. We present a computational framework in which depth information derived from T-junctions is propagated along surface contours using local recurrent interactions between neighboring neurons. We demonstrate that within this framework a globally consistent depth sorting of overlapping surfaces can be obtained on the basis of local interactions. Unlike previous approaches in which locally restricted cell interactions could merely distinguish between two depths (figure and ground), our model can also represent several intermediate depth positions. Our approach is an extension of a previous model of recurrent V1-V2 interaction for contour processing and illusory contour formation. Based on the contour representation created by this model, a recursive scheme of local interactions subsequently achieves a globally consistent depth sorting of several overlapping surfaces. Within this framework, the induction of illusory contours by the model of recurrent V1-V2 interaction gives rise to the figure-ground segmentation of illusory figures such as a Kanizsa square.

The Sightless View: Neural Correlates of Occluded Objects

The relationship between neural activity and object perception has received considerable attention using stimulus manipulations such as masking or dichoptic presentation. Here we investigate the same problem by occluding objects with an opaque screen that acts to dissociate the direct perception of the object from the awareness of its presence. We used functional magnetic resonance imaging to measure brain activity when subjects viewed objects (faces and houses) that underwent occlusion and found that the response of the majority of the fusiform face area (FFA) and lateral occipital cortex is the same whether the object is visible or occluded. This suggests that when objects are directly viewed, activity within object-selective regions may reflect the awareness of presence, not the direct perception, of the object. Additionally, we identify a region of premotor cortex that is selectively activated by occlusion of either object type, suggesting its generic involvement with processing occluded objects.

The neural mechanisms of perceptual filling-in

Filling-in is a perceptual phenomenon in which a visual attribute such as colour, brightness, texture or motion is perceived in a region of the visual field even though such an attribute exists only in the surround. Filling-in dramatically reveals the dissociation between the retinal input and the percept, and raises fundamental questions about how these two relate to each other. Filling-in is observed in various situations, and is an essential part of our normal surface perception. Here, I review recent experiments examining brain activities associated with filling-in, and discuss possible neural mechanisms underlying this remarkable perceptual phenomenon. The evidence shows that neuronal activities in early visual cortical areas are involved in filling-in, providing new insights into visual cortical functions.

The visual phantom illusion: a perceptual product of surface completion depending on brightness and contrast

The visual phantom illusion was first discovered by Rosenbach in 1902 and named 'moving phantoms' by Tynan and Sekuler in 1975 because of its strong dependence on motion. It was later revealed that phantoms can be generated by flickering the grating (flickering phantoms) or by low-luminance stationary gratings under dark adaptation (stationary phantoms). Although phantoms are much more visible at scotopic or mesopic adaptation levels (scotopic phantoms) than at photopic levels, we proposed a new phantom illusion which is fully visible in photopic vision (photopic phantoms). In 2001, we revealed that the visual phantom illusion is a higher-order perceptual construct or a Gestalt, which depends on the mechanism of perceptual transparency. Perceptual transparency is known as a perceptual product based upon brightness and contrast. We furthermore manifested the shared mechanisms between visual phantoms and neon color spreading or between visual phantoms and the Petter effect. In our recent study, the visual phantom illusion can also be seen with a stimulus of contrast-modulated gratings. We assume that this effect also depends on perceptual transparency induced by contrast modulation. Moreover, we found that the Craik-O'Brien-Cornsweet effect and other brightness illusions can generate the visual phantom illusion. In any case, we explain the visual phantom illusion in terms of surface completion, which is given by perceptual transparency.

The effect of interpolation and perceptual difficulty on the visual potentials evoked by illusory figures

Completion is the process by which the brain unifies and segregates the parts of an incomplete form. It is qualified as amodal when the form is placed behind an obstacle and modal when the form is at the foreground and closed by illusory contours. The N1, and sometimes the N2, deflections of the visual evoked potentials are known to be larger for modal figures, such as the Kanizsa triangle, than for control figures. This result is generally linked to completion or illusory contours, but it could also be related to a third process: the interpolation of the form by connecting its separate parts. To test the influence of interpolation, a modal triangle, an amodal triangle, a figure with outlined inducers, and a no-triangle figure were randomly presented to 26 subjects. The N1 evoked by the three triangle figures were all larger than the N1 to the no-triangle figure. These results suggest that the N1 amplitude is largely determined by the possibility of interpolating a form in the figure. The greatest N1 to the modal figure further suggests that interpolation may be increased by modal completion and decreased by the features that diminish the saliency of triangle in the amodal figure and the figure with outlined inducers. On the other hand, the largest N2 was evoked by the amodal figure. This effect may index processes activated in response to the great difficulty in perceiving the triangle in the amodal figure, a difficulty that is initially caused by a conflict of perceptions characterizing this figure.

3-d interpolation in object perception: evidence from an objective performance paradigm

Object perception requires interpolation processes that connect visible regions despite spatial gaps. Some research has suggested that interpolation may be a 3-D process, but objective performance data and evidence about the conditions leading to interpolation are needed. The authors developed an objective performance paradigm for testing 3-D interpolation and tested a new theory of 3-D contour interpolation, termed 3-D relatability. The theory indicates for a given edge which orientations and positions of other edges in space may be connected to it by interpolation. Results of 5 experiments showed that processing of orientation relations in 3-D relatable displays was superior to processing in 3-D nonrelatable displays and that these effects depended on object formation. 3-D interpolation and 3-D relatabilty are discussed in terms of their implications for computational and neural models of object perception, which have typically been based on 2-D-orientation-sensitive units.

New indirect measures of “inattentive” visual grouping in a change-detection task

It has often been suggested that Gestalt-like visual grouping processes may operate preattentively, but Mack and Rock (1998) suggested that no visual grouping takes place under "inattention." We introduced a new method to assess this. While participants performed a demanding change-detection task on a small matrix at fixation, task-irrelevant background elements were arranged by color sinilarity into columns, rows, or pseudorandomly. Independent of any change in the target matrix, background grouping could also change or remain the same on each trial. This influenced accuracy of change judgments for the central task, even though background grouping or its change usually could not be explicitly reported when probed with surprise questions as in Mack and Rock. This suggests that visual grouping may arise implicitly under inattention and provides a new method for testing the boundaries of this processing. Here we extended the initial result to changes in background grouping remote from the target and to those occurring across an intervening saccade.

The effect of support ratio on infants' perception of illusory contours

We used a preferential looking technique to investigate the effect of support ratio (a ratio of the physically specified contours to the total edge length) on the perception of Kanizsa illusory contours in infants aged 3-8 months. Previous work has shown that for adult observers the illusory-contour strength increases proportionally with the support ratio. When the support ratio was relatively high (66%), infants preferred illusory contours to non-illusory figures by 3-4 months of age (experiment 1). In contrast, only infants 7-8 months old showed this preference for illusory contours when the support ratio was reduced to 37% (experiment 3). Further, infants showed no preference for an outline version of the illusory-contour figure, which produced no illusory contours (experiment 2). This result confirms that the infants' preference reflects their perception of illusory contours. Our results show that (i) illusory-contour perception emerges at around 3-4 months of age, but (ii) that this ability is very limited until around 7-8 months of age.

Anticipatory spatial representation of 3D regions explored by sighted observers and a deaf-and-blind-observer

Viewers who study photographs of scenes tend to remember having seen beyond the boundaries of the view [boundary extension; J. Exp. Psychol. Learn. Mem. Cogn. 15 (1989) 179]. Is this a fundamental aspect of scene representation? Forty undergraduates explored bounded regions of six common (3D) scenes, visually or haptically (while blindfolded) and then the delimiting borders were removed. Minutes later they reconstructed boundary placement. Boundary extension occurred: mean areas were increased by 53% (vision) and by 17% (haptics). A deaf-and-blind woman (KC) haptically explored the same regions. Although a "haptic expert", she too remembered having explored beyond the boundaries, with performance similar to that of the blindfolded-sighted. Boundary extension appears to be a fundamental aspect of spatial cognition. Possibly constrained by the "scope" of the input modality (vision>haptics), this anticipatory spatial representation may facilitate integration of successively perceived regions of the world irrespective of modality and the perceiver's sensory history.

Object Interpolation in Three Dimensions

Perception of objects in ordinary scenes requires interpolation processes connecting visible areas across spatial gaps. Most research has focused on 2-D displays, and models have been based on 2-D, orientation-sensitive units. The authors present a view of interpolation processes as intrinsically 3-D and producing representations of contours and surfaces spanning all 3 spatial dimensions. The authors propose a theory of 3-D relatability that indicates for a given edge which orientations and positions of other edges in 3 dimensions may be connected to it, and they summarize the empirical evidence for 3-D relatability. The theory unifies and illuminates a number of fundamental issues in object formation, including the identity hypothesis in visual completion, the relations of contour and surface processes, and the separation of local and global processing. The authors suggest that 3-D interpolation and 3-D relatability have major implications for computational and neural models of object perception.

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.

Temporal variations in visual completion: a reflection of spatial limits?

The completion of partly occluded objects appears instantaneous and effortless, but empirically takes measurable time. The current study investigates how amount of occlusion affects the time course and mechanisms of visual completion. Experiment 1 used a primed-matching paradigm to determine completion times for objects occluded by various amounts. Experiments 2 and 3 used a dot-localization paradigm to probe completed contour representations for a qualitative shift above some spatial limit. The results demonstrate that time to completion rises with amount of occlusion. Nonetheless, the visual system can complete highly occluded objects, even when the occlusion renders visible contours nonrelatable. Furthermore, prolonged completion times for highly occluded objects do not result from a breakdown of low-level interpolation processes: The same contour completion mechanism operates on objects occluded by different spatial extents.

Interpolation processes in the visual perception of objects

Visual perception of objects depends on segmentation and grouping processes that act on fragmentary input. This paper gives a brief overview of these processes. A simple geometry accounting for contour interpolation is described, and its applications to 2D, 3D, and spatiotemporal object interpolation processes are considered. A method is described for distinguishing interpolation based on this geometry from more global or top-down influences. Results suggest a separation between interpolation based on relatively local stimulus relations, which give rise to precise boundary representations, and processes involving recognition from partial information, which do not. Aspects of the model-especially the unified treatment of illusory and occluded objects-raise questions about the nature of seeing. Although it is often believed that illusory objects are perceived, while occluded objects are inferred, I suggest that both research and theory converge in supporting a more unified account. Illusory and occluded contours and surfaces do not divide into the real, the perceived, and the inferred, but are all represented, and in key respects, derive from identical perceptual processes.

Surface construal and the mental representation of scenes

What distinguishes scenes from nonscenes? Photographs of objects on both naturalistic and blank backgrounds yielded boundary extension (BE: memory for unseen spatial expanse outside the picture's boundaries). However, line-drawn objects on blank backgrounds did not (Experiment 1). Perhaps the blank background was construed as depicting a real-world surface in the photograph condition but was construed as depicting nothing in the line-drawn condition. To change background construal, the authors used objects cut out of photographs; these were placed on blank backgrounds while viewers watched (Experiments 2 and 3). BE was eliminated. The authors propose that amodal continuation is a fundamental aspect of scene perception. However, not all pictures are scenes--only pictures construed as depicting a truncated view of a continuous world.

Perceptual completion across the vertical meridian and the role of early visual cortex

Perceptual completion can link widely separated contour fragments and interpolate illusory contours (ICs) between them. The mechanisms underlying such long-range linking are not well understood. Here we report that completion is much poorer when ICs cross the vertical meridian than when they reside entirely within the left or right visual hemifield. This deficit reflects limitations in cross-hemispheric integration. We also show that the sensitivity to the interhemispheric divide is unique to perceptual completion: a comparable task which did not require completion showed no across-meridian impairment. We propose that these findings support the existence of specialized completion mechanisms in early visual cortical areas (V1/V2), since those areas are likely to be more sensitive to the interhemispheric divide.

Object-completion effects in the human lateral occipital complex

The ability of the human visual system to recognize partially occluded objects is a striking feat, which has received extensive psychophysical documentation. Here we studied the manifestation of completion effects in the functional magnetic resonance imaging (fMRI) activation of high-order object areas (the lateral occipital complex - LOC). Subjects were presented with three types of images: (i) whole line drawings of animal or unfamiliar shapes ('whole'); (ii) the same shapes, occluded by parallel stripes which occupied roughly half of the surface area of the images ('grid'); and (iii) the same stripes, 'scrambled' so that the relative position of the regions between the stripes was changed while the local feature structure remained intact. Behavioral measurements showed a high degree of object completion in the 'grid' condition, but not in the 'scrambled' condition. The fMRI results show a significantly higher activation to the 'grid' images compared to the 'scrambled' images. This enhanced activation indicates the operation of non-local completion effects, since the local features in both sets of images were the same. The cortical regions showing the highest 'completion' effects co-localized with regions in the LOC which showed the highest activation to the 'whole' images compared to the 'scrambled' images. Activation in early retinotopic areas was similar in both the 'grid' and the 'scrambled' conditions. Our results point to the LOC as a central site in which object completion effects are manifested.