Visual search for a circular region perceived as a figure versus as a hole: Evidence of the importance of part structure

Where there is no object formation, there is no perceptual organization: Evidence from the configural superiority effect

Object formation is considered the aim of perceptual organization, but such a proposition has been neglected in empirical studies. In the current study, we investigated the role of object formation in configural superiority. Essentially, discrimination on bar orientations was enhanced by adding a right angle to each of the bars. Such facilitation is due to the emergent feature (EF) of closure formed by combining the bars with right angles. To study object formation, visual stimuli were generated by random dot stereograms to form objects or holes in 3D. Behaviorally, we found that the EF of closure facilitated oddball discrimination on objects, as demonstrated by previous studies, but did not facilitate oddball discrimination on holes with the same shape as objects. Multivariate pattern analysis of functional magnetic resonance imaging (fMRI) data showed that the EF of closure increased the object classification accuracy compared to the holes in the lateral occipital cortex (LOC), where object information is encoded, but not in the early visual cortex (EVC). The neural representations of objects and holes with and without EFs were further investigated using representational similarity analysis. The results demonstrate that in the LOC, the neural representations of objects with EFs showed a greater difference than those of the other three, that is, objects without EFs and holes with or without EFs. However, the uniqueness of objects with EFs was not observed in the EVC. Thus, our results suggest that the EF of closure, which leads to the configural superiority effect, only emerges for objects but not for holes, and only in the LOC but not the EVC. Our study provides the first empirical evidence suggesting that object formation plays an indispensable role in perceptual organization.

Bouba and Kiki inside objects: Sound-shape correspondence for objects with a hole

Visual holes (cutouts in a surface) have recently intrigued vision scientists as interesting and useful stimuli in the studies of shape perception and as a perceptual conundrum regarding figure/ground organization. Adopting the Bouba/Kiki paradigm, this study addressed a controversial issue of whether the perceived shape of a closed region alters when the region changes from a solid object to an empty hole, in a more direct manner than previous studies did. Observers were presented with two doughnut-like cardboard cutouts, one with a flower-shaped hole and the other with a star-shaped hole, and then matched them with two nonsense words. The curvature profile of the hole boundary was manipulated so that the shape of the interior region (i.e., a hole) and that of the exterior region (i.e., material edges) give rise to opposite shape impressions (i.e., one rounded and the other spiky). The results of Experiment 1 revealed that shape-name matching for holed objects is based on the interior shapes of holes, but not those of materially defined inner edges. The following three experiments replicated the same pattern of results even when holes appeared like oral apertures in animal character faces (Experiments 2-3) and when they were irregular, non-symmetric, and low in semantic association with familiar real-world objects (Experiment 4). Lastly, Experiment 5 showed that shape-name matching for "C"-shaped, negative-part stimuli is also interior-shape-based if the opening of the interior region is relatively small. These findings suggest that the interior shapes of holes are automatically accessible. I conclude with a discussion of my hypothesis that the only global-level, unitary shape representation of a bounded region of a single connected surface is that of the interior region for both objects and holes, imposing an important constraint in visual shape processing.

The dissociations of visual processing of "hole" and "no-hole" stimuli: An functional magnetic resonance imaging study

INTRODUCTION

"Where to begin" is a fundamental question of vision. A "Global-first" topological approach proposed that the first step in object representation was to extract topological properties, especially whether the object had a hole or not. Numerous psychophysical studies found that the hole (closure) could be rapidly recognized by visual system as a primitive property. However, neuroimaging studies showed that the temporal lobe (IT), which lied at a late stage of ventral pathway, was involved as a dedicated region. It appeared paradoxical that IT served as a key region for processing the early component of visual information. Did there exist a distinct fast route to transit hole information to IT? We hypothesized that a fast noncortical pathway might participate in processing holes.

METHODS

To address this issue, a backward masking paradigm combined with functional magnetic resonance imaging (fMRI) was applied to measure neural responses to hole and no-hole stimuli in anatomically defined cortical and subcortical regions of interest (ROIs) under different visual awareness levels by modulating masking delays.

RESULTS

For no-hole stimuli, the neural activation of cortical sites was greatly attenuated when the no-hole perception was impaired by strong masking, whereas an enhanced neural response to hole stimuli in non-cortical sites was obtained when the stimulus was rendered more invisible.

CONCLUSIONS

The results suggested that whereas the cortical route was required to drive a perceptual response for no-hole stimuli, a subcortical route might be involved in coding the hole feature, resulting in a rapid hole perception in primitive vision.

Processing convexity and concavity along a 2-D contour: figure-ground, structural shape, and attention

Interest in convexity has a long history in vision science. For smooth contours in an image, it is possible to code regions of positive (convex) and negative (concave) curvature, and this provides useful information about solid shape. We review a large body of evidence on the role of this information in perception of shape and in attention. This includes evidence from behavioral, neurophysiological, imaging, and developmental studies. A review is necessary to analyze the evidence on how convexity affects (1) separation between figure and ground, (2) part structure, and (3) attention allocation. Despite some broad agreement on the importance of convexity in these areas, there is a lack of consensus on the interpretation of specific claims--for example, on the contribution of convexity to metric depth and on the automatic directing of attention to convexities or to concavities. The focus is on convexity and concavity along a 2-D contour, not convexity and concavity in 3-D, but the important link between the two is discussed. We conclude that there is good evidence for the role of convexity information in figure-ground organization and in parsing, but other, more specific claims are not (yet) well supported.

The visual system prioritizes locations near corners of surfaces (not just locations near a corner)

When a new visual object appears, attention is directed toward it. However, some locations along the outline of the new object may receive more resources, perhaps as a consequence of their relative importance in describing its shape. Evidence suggests that corners receive enhanced processing, relative to the straight edges of an outline (corner enhancement effect). Using a technique similar to that in an original study in which observers had to respond to a probe presented near a contour (Cole et al. in Journal of Experimental Psychology: Human Perception and Performance 27:1356-1368, 2001), we confirmed this effect. When figure-ground relations were manipulated using shaded surfaces (Exps. 1 and 2) and stereograms (Exps. 3 and 4), two novel aspects of the phenomenon emerged: We found no difference between corners perceived as being convex or concave, and we found that the enhancement was stronger when the probe was perceived as being a feature of the surface that the corner belonged to. Therefore, the enhancement is not based on spatial aspects of the regions in the image, but critically depends on figure-ground stratification, supporting the link between the prioritization of corners and the representation of surface layout.

Time-course of perceptual processing of "hole" and "no-hole" figures: an ERP study

Closure or the presence of a "hole" is an emergent perceptual feature that can be extracted by the visual system early on. This feature has been shown to have perceptual advantages over openness or "no-hole". in this study, we investigated when and how the human brain differentiates between "hole" and "no-hole" figures. Event-related potentials (ERPs) were recorded during a passive observation paradigm. Two pairs of simple figures (Experiment 1) and two sets of Greek letters (Experiment 2) were used as stimuli. The ERPs of "hole" and "no-hole" figures differed ∼90 ms after stimulus onset: "hole" figures elicited smaller P1 and N1 amplitudes than "no-hole" figures. These suggest that both P1 and N1 components are sensitive to the difference between "hole" and "no-hole" figures; perception of "hole" and "no-hole" figures might be differentiated early during visual processing.

Advantage of hole stimulus in rivalry competition

Mounting psychophysical evidence suggests that early visual computations are sensitive to the topological properties of stimuli, such as the determination of whether the object has a hole or not. Previous studies have demonstrated that the hole feature took some advantages during conscious perception. In this study, we investigate whether there exists a privileged processing for hole stimuli during unconscious perception. By applying a continuous flash suppression paradigm, the target was gradually introduced to one eye to compete against a flashed full contrast Mondrian pattern which was presented to the other eye. This method ensured that the target image was suppressed during the initial perceptual period. We compared the initial suppressed duration between the stimuli with and without the hole feature and found that hole stimuli required less time than no-hole stimuli to gain dominance against the identical suppression noise. These results suggest the hole feature could be processed in the absence of awareness, and there exists a privileged detection of hole stimuli during suppressed phase in the interocular rivalry.

Can we track holes?

The evidence is mixed as to whether the visual system treats objects and holes differently. We used a multiple object tracking task to test the hypothesis that figural objects are easier to track than holes. Observers tracked four of eight items (holes or objects). We used an adaptive algorithm to estimate the speed allowing 75% tracking accuracy. In Experiments 1-5, the distinction between holes and figures was accomplished by pictorial cues, while red-cyan anaglyphs were used to provide the illusion of depth in Experiment 6. We variously used Gaussian pixel noise, photographic scenes, or synthetic textures as backgrounds. Tracking was more difficult when a complex background was visible, as opposed to a blank background. Tracking was easier when disks carried fixed, unique markings. When these factors were controlled for, tracking holes was no more difficult than tracking figures, suggesting that they are equivalent stimuli for tracking purposes.

Perceiving parts and shapes from concave surfaces

"A hole is nothing at all, but it can break your neck." In a similar fashion to the danger illustrated by this folk paradox, concave regions pose difficulties to theories of visual shape perception. We can readily identify their shapes, but according to principles of how observers determine part boundaries, concavities in a planar surface should have very different figural shapes from the ones that we perceive. In three experiments, we tested the hypothesis that observers perceive local image features differently in simulated 3-D concave and convex regions but use them to arrive at similar shape percepts. Stimuli were shape-from-shading images containing regions that appeared either concave or convex in depth, depending on their orientation in the picture plane. The results show that concavities did not benefit from the same global object-based attention or holistic shape encoding as convexities and that the participants relied on separable spatial dimensions to judge figural shape in concavities. Concavities may exploit a secondary process for shape perception that allows regions composed of perceptually independent features to ultimately be perceived as gestalts.

Rapid Figure – Ground Responses to Stereograms Reveal an Advantage for a Convex Foreground

Convexity has long been recognised as a factor that affects figure – ground segmentation, even when pitted against other factors such as symmetry [Kanizsa and Gerbino, 1976 Art and Artefacts Ed.M Henle (New York: Springer) pp 25–32], It is accepted in the literature that the difference between concave and convex contours is important for the visual system, and that there is a prior expectation favouring convexities as figure. We used bipartite stimuli and a simple task in which observers had to report whether the foreground was on the left or the right. We report objective evidence that supports the idea that convexity affects figure – ground assignment, even though our stimuli were not pictorial in that depth order was specified unambiguously by binocular disparity.

Shape representation by a network of V4-like cells

Cells in extrastriate visual cortex have been reported to be selective for various configurations of local contour shape [Pasupathy, A., & Connor, C. E. (2001). Shape representation in area V4: Position-specific tuning for boundary conformation. The Journal of Neurophysiology, 86 (5), 2505-2519; Hegdé, J., & Van Essen, D. C. (2003). Strategies of shape representation in macaque visual area V2. Visual Neuroscience, 20 (3), 313-328]. Specifically, Pasupathy and Connor found that in area V4 most cells are strongly responsive to a particular local contour conformation located at a specific position on the object's boundary. We used a population of "V4-like cells"-units sensitive to multiple shape features modeled after V4 cell behavior-to generate representations of different shapes. Standard classification algorithms (earth mover's distance, support vector machines) applied to this population representation demonstrate high recognition accuracies classifying handwritten digits in the MNIST database and objects in the MPEG-7 Shape Silhouette database. We compare the performance of the V4-like unit representation to the "shape context" representation of Belongie et al. [Belongie, S., Malik, J., & Puzicha, J. (2002). Shape matching and object recognition using shape contexts. IEEE Transactions on Pattern Analysis and Machine Intelligence, 24 (24), 509-522]. Results show roughly comparable recognition accuracies using the two representations when tested on portions of the MNIST database. We analyze the relative contributions of various V4-like feature sensitivities to recognition accuracy and robustness to noise - feature sensitivities include curvature magnitude, direction of curvature, global orientation of the contour segment, distance of the contour segment from object center, and modulatory effect of adjacent contour regions. Among these, local curvature appears to be the most informative variable for shape recognition. Our results support the hypothesis that V4 cells function as robust shape descriptors in the early stages of object recognition.

Amodal completion and visual holes (static and moving)

Occlusion is a frequent occurrence in a cluttered world of opaque objects. Often information about the shape of partly occluded objects can be gathered from the visible portion of the object and in particular its contours. Here we address the case where a region of a surface is visible exclusively through an aperture (visual hole). We make several observations about the grouping of surface regions visible through holes, and the appearance of moving objects and holes. These observations support the view that holes are shape properties of the object-with-hole.