A common mechanism for illusory and occluded object completion

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.

Measuring the perceptual grouping of non-adjacent surfaces that are invisibly (amodally) or visibly connected

Classic Gestalt examples of perceptual grouping entail arrays of disconnected surfaces that are grouped on the basis of the surfaces' relative similarity or proximity. However, most natural environments contain multiple objects, each with multiple, connected surfaces. Moreover, an object in a scene is likely to partially occlude other objects in the 2-dimensional retinal projection of the scene. A central question, therefore, is how the visual system forms a 3-dimensional representation of multi-object scenes by determining which surfaces belong to which objects. To this end, a recently developed dynamic grouping methodology determines whether pairs of surfaces are grouped together on the basis of the direction in which motion is perceived across a surface when its luminance is perturbed. It is shown using this method that the visible surfaces of a partially occluded object are perceptually grouped when they are plausibly connected and represented in a depth plane behind the occluding object. Invisible connectivity (amodal completion) as well as connectivity established by a visible surface have a powerful influence on the grouping of surfaces. However, for neither kind of connectivity is grouping affected by the distance between the surfaces. This absence of a distance/proximity effect on grouping is obtained when the space between to-be-grouped surfaces is filled with other surfaces. It contrasts with the strong effect of distance/proximity on the grouping of disconnected surfaces, and on the clarity of illusory contours formed between disconnected contours. It is concluded that distance/proximity is an operative grouping variable only when there is empty space between the to-be-grouped surfaces.

Perceptual mechanisms underlying amodal surface integration of 3-D stereoscopic stimuli

The visual system can represent a partially occluded 3-D surface from images of separated surface segments. The underlying amodal surface integration process accomplishes this by amodally extending each surface segment behind the occluder (amodal surface extension) and integrating the extended surfaces to form a whole surface representation. We conducted five experiments to investigate how depth cues, such as binocular disparity, half-occlusion, and monocular depth cues (T-junctions and L-junctions), contribute to amodal surface extension, and how the geometrical relationship and image similarity among the surface segments affect surface integration. This was achieved by having observers adjust the stereoscopic depth and slant of a comparison stimulus to match those of the tested 3-D stimulus. We found that both binocular disparity and half-occlusion cues are used to determine border-ownership assignment of surface segments and for amodal surface extension. We also found that separated surface segments need to have the same luminance contrast-polarity for them to be integrated as a whole surface. Finally, we found that having the same motion direction, minimum misalignment between boundary contours, and proximity among separated segments facilitate their integration. Overall, our findings reveal a set of "perceptual factors" for amodal surface integration, which arguably reflects our visual system's built-in knowledge of the regularities in natural scenes.

Motion-based super-resolution in the peripheral visual field

Improvements in foveal acuity for moving targets have been interpreted as evidence for the ability of the visual system to combine information over space and time, in order to reconstruct the image at a higher resolution (super-resolution). Here, we directly test whether this occurs in the peripheral visual field and discuss its potential for improving functional capacity in ocular disease. The effect of motion on visual acuity was first compared under conditions in which performance was limited either by natural undersampling in the retinal periphery or by the presence of overlaid masks with opaque elements to simulate retinal loss. To equate the information content of moving and static sequences, we next manipulated the dynamic properties of the masks. Finally, we determined the dependence of motion-related improvements on the object of motion (target or mask) and its trajectory (smooth or jittered). Motion improved visual acuity for masked but not unmasked peripheral targets. Equating the information content of moving and static conditions removed some but not all of this benefit. Residual motion-related improvements were largest in conditions in which the target moved along a consistent and predictable path. Our results show that motion can improve peripheral acuity in situations in which performance is limited by abnormal undersampling. These findings are consistent with the operation of a super-resolution system and could have important implications for any pathology that alters the regular sampling properties of the retinal mosaic.

A perceptually completed whole is less than the sum of its parts

How efficiently do people integrate the disconnected image fragments that fall on their eyes when they view partly occluded objects? In the present study, I used a psychophysical summation-at-threshold technique to address this question by measuring discrimination performance with both isolated and combined features of physically fragmented but perceptually complete objects. If visual completion promotes superior integration efficiency, performance with a visually completed object should exceed what would be expected from performance with the individual object parts shown in isolation. Contrary to this prediction, results showed that discrimination performance with both static and moving versions of physically fragmented but perceptually complete objects was significantly worse than would be expected from performance with their constituent parts. These results present a challenge for future theories of visual completion.

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.

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.

Attentional signatures of perception: multiple object tracking reveals the automaticity of contour interpolation

Multiple object tracking (MOT) is an attentional task wherein observers attempt to track multiple targets among moving distractors. Contour interpolation is a perceptual process that fills-in nonvisible edges on the basis of how surrounding edges (inducers) are spatiotemporally related. In five experiments, we explored the automaticity of interpolation through its influences on tracking. We found that (1) when the edges of targets and distractors jointly formed dynamic illusory or occluded contours, tracking accuracy worsened; (2) when interpolation bound all four targets together, performance improved; (3) when interpolation strength was weakened (by altering the size or relative orientation of inducing edges), tracking effects disappeared; and (4) real and interpolated contours influenced tracking comparably, except that real contours could more effectively shift attention toward distractors. These results suggest that interpolation's characteristics-and, in particular, its automaticity-can be revealed through its attentional influences or "signatures" within tracking. Our results also imply that relatively detailed object representations are formed in parallel, and that such representations can affect tracking when they become relevant to scene segmentation.

Boundary contour-based surface integration affected by color

The visual system represents occluded surfaces by integrating the visible and partially occluded fragments with reliance on surface boundary contours. Does surface integration also depend on color similarity? Using displays with aligned images, we found the visual system has a preference to integrate images with the same color to form occluded surfaces and construct illusory occluding surfaces. This results in enhanced shape discrimination of briefly presented stimuli, and a tendency to perceive global motion of the integrated fragments. The contribution of color to surface integration is observed both in equiluminous setting and in non-equiluminous setting, where achromatic contrast exists.

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.

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.

Transsaccadic representation of layout: what is the time course of boundary extension?

How rapidly does boundary extension occur? Across experiments, trials included a 3-scene sequence (325 ms/picture), masked interval, and repetition of 1 scene. The repetition was the same view or differed (more close-up or wide angle). Observers rated the repetition as same as, closer than, or more wide angle than the original view on a 5-point scale. Masked intervals were 100, 250, 625, or 1,000 ms in Experiment 1 and 42, 100, or 250 ms in Experiments 2 and 3. Boundary extension occurred in all cases: Identical views were rated as too "close-up," and distractor views elicited the rating asymmetry typical of boundary extension (wider angle distractors were rated as being more similar to the original than were closer up distractors). Most important, boundary extension was evident when only a 42-ms mask separated the original and test views. Experiments 1 and 3 included conditions eliciting a gaze shift prior to the rating test; this did not eliminate boundary extension. Results show that boundary extension is available soon enough and is robust enough to play an on-line role in view integration, perhaps supporting incorporation of views within a larger spatial framework.

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.