Symmetry and elongation of objects influence perceived direction of translational motion

Five experiments were conducted to examine how perceived direction of motion is influenced by aspects of shape of a moving object such as symmetry and elongation. Random polygons moving obliquely were presented on a computer screen and perceived direction of motion was measured. Experiments 1 and 2 showed that a symmetric object moving off the axis of symmetry caused motion to be perceived as more aligned with the axis than it actually was. However, Experiment 3 showed that motion did not influence perceived orientation of symmetry axis. Experiment 4 revealed that symmetric shapes resulted in faster judgments on direction of motion than asymmetric shapes only when the motion is along the axis. Experiment 5 showed that elongation causes a bias in perceived direction of motion similar to effects of symmetry. Existence of such biases is consistent with the hypothesis that in the course of evolution, the visual system has been adapted to regularities of motion in the animate world.     

A polarity effect in misoriented object recognition: The role of polar features in the computation of orientation-invariant shape representations

This study investigated the contribution of polar features and internal shape axes to misoriented object recognition. A recognition memory paradigm was used to examine the effects of stimulus orientation on the recognition of previously memorized 2-D novel objects. In contrast to some recent reports, Experiment 1 showed that orientation-invariant performance can be found from the outset of testing with objects containing a salient axis of symmetry. In Experiments 2 and 3 it was found that the removal of a single salient polar feature, while preserving the axis of elongation, was sufficient to increase stimulus orientation time costs. This polarity effect suggests that polar features, and shape axes, play a role in the computation of orientation-invariant shape representations. It is proposed that shape axes facilitate the localization of polar features, which, in turn, are used to resolve the polarity of shape representations during recognition.     

Local and global orientation in visual search

Six experiments investigated the role of global (shape) and local (contour) orientation in visual search for an orientation target. Experiment 1 demonstrated thatsearch for a conjunction of local contours with a distinct global orientation was less efficient than search for a target featurally distinct in terms of both global and local contour orientation. However, Experiments 2 and 4 demonstrated that the presence of a unique line contour was neither sufficient nor necessary to allow efficient search. Experiment 5 found thatsearch for a local orientation difference was strongly impeded by irrelevant variation in global orientation, arguing for a preeminent role for global orientation. Finally, Experiment 6 demonstrated that the orientation search asymmetry holds for the global orientation of stimuli. Taken together, the results are consistent with visual search processes guided predominately by a representation of global orientation.     

THREE-DIMENSIONAL BILATERAL SYMMETRY BIAS IN JUDGMENTS OF FIGURAL IDENTITY AND ORIENTATION

Abstract— The two experiments reported explored a bias toward symmetry in judging identity and orientation of indeterminate two-dimensional shapes. Subjects viewed symmetric and asymmetric filled, random polygons and described, "what each figure looks like" and its orientation. Viewers almost universally interpreted the shapes as silhouettes of bilaterally symmetric three-dimensional (3-D) objects. This assumption of 3-D symmetry tended to constrain perceived vantage of the identified objects such that symmetric shapes were interpreted as straight-on views, and asymmetric shapes as profile or oblique views. Because most salient objects in the world are bilaterally symmetric, these findings are consistent with the view that assuming 3-D symmetry can be a robust heuristic for constraining orientation when identifying objects from indeterminate patterns.     

Visual search for object orientation can be modulated by canonical orientation

The authors studied the influence of canonical orientation on visual search for object orientation. Displays consisted of pictures of animals whose axis of elongation was either vertical or tilted in their canonical orientation. Target orientation could be either congruent or incongruent with the object's canonical orientation. In Experiment 1, vertical canonical targets were detected faster when they were tilted (incongruent) than when they were vertical (congruent). This search asymmetry was reversed for tilted canonical targets. The effect of canonical orientation was partially preserved when objects were high-pass filtered, but it was eliminated when they were low-pass filtered, rendering them as unfamiliar shapes (Experiment 2). The effect of canonical orientation was also eliminated by inverting the objects (Experiment 3) and in a patient with visual agnosia (Experiment 4). These results indicate that orientation search with familiar objects can be modulated by canonical orientation, and they indicate a top-down influence on orientation processing.     

Visual search for simple volumetric shapes

Five experiments measured reaction time (RT) to detect the presence or absence of a simple volumetric shape (target) dependent on the number of display items (distractors) and their depicted three-dimensional (3-D) orientation. Experiments 1–4 examined every pairwise combination of two different simple volumetric shapes in two 3-D orientations. Conditions exhibiting “pop-out” could be predicted by differences in their two-dimensional (2-D) features. Conditions in which search was slower support previously found search asymmetries for particular 2-D features. When the distractors were a mixture of the other shapes in the same 3-D orientation, search was serial, except when the target had a curved principal axis (Experiment 5). The results suggest that these simple volumetric shapes are not processed preattentively.     

Visual search for simple volumetric shapes.

Five experiments measured reaction time (RT) to detect the presence or absence of a simple volumetric shape (target) dependent on the number of display items (distractors) and their depicted three-dimensional (3-D) orientation. Experiments 1-4 examined every pairwise combination of two different simple volumetric shapes in two 3-D orientations. Conditions exhibiting "pop-out" could be predicted by differences in their two-dimensional (2-D) features. Conditions in which search was slower support previously found search asymmetries for particular 2-D features. When the distractors were a mixture of the other shapes in the same 3-D orientation, search was serial, except when the target had a curved principal axis (Experiment 5). The results suggest that these simple volumetric shapes are not processed preattentively.     

Attentional set for axis of symmetry in symmetry-defined visual search

Olivers and van der Helm (1998) showed that symmetry-defined visual search (for both symmetry and asymmetry) requires selective spatial attention. We hypothesize that an attentional set for the orientation of a symmetry axis also is involved in symmetry-defined visual search. We conducted three symmetry-defined visual search experiments with manipulations of the axis of symmetry orientations, and performance was better when the axis orientations within the search array were uniform, rather than a mixture of two orientations, and the attentional set for the axis orientation could be kept. In addition, search performance when the target was defined by the presence of symmetry was equivalent to that when the target was defined by a difference of symmetry axis orientation. These results suggest that attentional set for axis orientation plays a fundamental role in symmetry-defined visual search.     

Local and global mechanisms of one- and two-dimensional orientation illusions

One-dimensional (1-D) orientation illusions induced on a test grating by a tilted and-surrounding 1-D inducing grating have a well-known angular function that exhibits both repulsion and attraction effects. Two-dimensional (2-D) orientation illusions are those induced on a test grating by 2-D image modulation, such as a pair of superimposed inducing gratings at different orientations, usually orthogonal (a plaid). Given the known angular functions induced by the plaid component gratings, two hypotheses were developed that predicted different plaid-induced illusion functions. Hypothesis 1 states that the 1-D component-induced effects simply add linearly; Hypothesis 2 states that there is an additional mechanism that responds to the virtual axes of mirror symmetry of the plaid and adds to the effect. The data of two experiments were consistent with the predictions from the second hypothesis but not the first. Possible neural substrates of mechanisms that extract axes of symmetry are discussed; it is suggested that such global symmetry axes may underlie the perceived orientation of complex shapes.     

Local and global mechanisms of one- and two-dimensional orientation illusions.

One-dimensional (1-D) orientation illusions induced on a test grating by a tilted and surrounding 1-D inducing grating have a well-known angular function that exhibits both repulsion and attraction effects. Two-dimensional (2-D) orientation illusions are those induced on a test grating by 2-D image modulation, such as a pair of superimposed inducing gratings at different orientations, usually orthogonal (a plaid). Given the known angular functions induced by the plaid component gratings, two hypotheses were developed that predicted different plaid-induced illusion functions. Hypothesis 1 states that the 1-D component-induced effects simply add linearly; Hypothesis 2 states that there is an additional mechanism that responds to the virtual axes of mirror symmetry of the plaid and adds to the effect. The data of two experiments were consistent with the predictions from the second hypothesis but not the first. Possible neural substrates of mechanisms that extract axes of symmetry are discussed; it is suggested that such global symmetry axes may underlie the perceived orientation of complex shapes.     

Concavities count for less in symmetry perception

We investigated the relative importance of convexities (protrusions) and concavities (indentations) for the perception of shape. On the one hand, it has been suggested that convexities determine the shape of an object, whereas concavities merely act as “perceptual glue” between the convexities. On the other hand, it has been argued that concavities are more salient than convexities. We show that participants find it easier to detect asymmetry in a 2-D silhouette when there is a mismatch between the shapes of convexities on either side of the axis of symmetry than when there is a mismatch between the shapes of concavities. This is the case even when the concavities are closest to the axis of symmetry, and despite the usual bias toward this axis in symmetry perception. We suggest that the actual shape of concavities is less important in symmetry perception, because the main role of concavities is to act as part boundaries in the representation of the shape of objects.     

Shape parameters explain data from spatial transformations: comment on Pearce et al. (2004) and Tommasi & Polli (2004)

In 2 recent studies on rats (J. M. Pearce, M. A. Good, P. M. Jones, & A. McGregor, see record 2004-12429-006) and chicks (L. Tommasi & C. Polli, see record 2004-15642-007), the animals were trained to search in 1 corner of a rectilinear space. When tested in transformed spaces of different shapes, the animals still showed systematic choices. Both articles rejected the global matching of shape in favor of local matching processes. The present authors show that although matching by shape congruence is unlikely, matching by the shape parameter of the 1st principal axis can explain all the data. Other shape parameters, such as symmetry axes, may do even better. Animals are likely to use some global matching to constrain and guide the use of local cues; such use keeps local matching processes from exploding in complexity.     

How Body Asymmetries Determine Accessibility in Spatial Frameworks

Spatial frameworks are a class of spatial mentalmodel that code locations of objects relative to the body axes. Spatial frameworks predict accessibility of spatial relations from memory primarily on the basis of the relative asymmetry of the body axes, such that highly asymmetric axes lead to faster retrieval of information. The present research examined how bodily asymmetries affect retrieval. Experiment 1 contrasted two theoretical accounts. The Salience Account proposes that relative degrees of asymmetry render axes differentially salient, and hence differentially foregrounded in one's mental model. The Direction Decision Account proposes that an explicit decision process is necessary to access specific locations along body axes. The ease of the decision process presumably depends on the degree of asymmetry that exists to discriminate poles along a body axis. The spatial framework pattern of accessibility was observed both when subjects identified specific directions of objects and when subjects identified just the axis to which objects were associated, supporting the Salience Account. Experiment 2 investigated whether lateralization affects accessibility from spatial frameworks. Performance of highly lateralized individuals did not differ from that of weakly lateralized individuals.     

Intrinsic orientation and study viewpoint in recognizing spatial structure of a shape

Two experiments dissociated the roles of intrinsic orientation of a shape and participants’ study viewpoint in shape recognition. In Experiment 1, participants learned shapes with a rectangular background that was oriented differently from their viewpoint, and then recognized target shapes, which were created by splitting study shapes along different intrinsic axes, at different views. Results showed that recognition was quicker when the study shapes were split along the axis parallel to the orientation of the rectangular background than when they were split along the axis parallel to participants’ viewpoint. In Experiment 2, participants learned shapes without the rectangular background. The results showed that recognition was quicker when the study shape was split along the axis parallel to participants’ viewpoint. In both experiments, recognition was quicker at the study view than at a novel view. An intrinsic model of object representation and recognition was proposed to explain these findings.     

Concept formation and categorization of complex,asymmetric, and impossible figures

Impossible figures are striking examples of inconsistencies between global and local perceptual structures, in which the overall spatial configuration of the depicted image does not yield a coherent three-dimensional object. In order to investigate whether structural “impossibility” is an important perceptual property of depicted objects, we used a category formation task in which subjects were asked to divide pictures of shapes into groups that seemed most natural to them. Category formation is usually unidimensional, such that sorting is dominated by a single perceptual property, so this task can serve as a measure of which dimensions are most salient. In Experiment 1, subjects received sets of 12 line drawings consisting of six possible and six impossible objects. Very few subjects grouped the figures by impossibility on the first try, and only half did so after multiple attempts at sorting. In Experiment 2, we investigated other global properties of figures: symmetry and complexity. Subjects readily sorted objects by complexity, but seldom by symmetry. In Experiment 3, subjects were asked to draw each of the figures before sorting them, which had only a minimal effect on categorization. Finally, in Experiment 4, subjects were explicitly instructed to divide the shapes by symmetry or impossibility. Performance on this task was perfect for symmetry, but not for impossibility. Although global properties of figures seem extremely important to our perception, the results suggest that some of these cues are not immediately obvious or salient for most observers.     

Investigating metrical and structural aspects of visual completion: Priming versus searching

Earlier research on visual occlusion showed some flexibility in the formation of visual completions, as long as the structural aspects (e.g., symmetry) of the visible part of the partly occluded shape were preserved in the completion (de Wit & van Lier, ). In this study, we examined whether changing the overall size of the occluded shape would preserve the overall structure. In Experiment 1, using the primed‐matching paradigm, we found evidence for relative size invariance in the completion process. To investigate whether changes in the structural aspects of shape are generally more salient than those of size, we employed the same stimuli in visual search and change detection paradigms. Experiment 2 demonstrated effects of completion in both paradigms. Experiment 3 showed that the metrical aspects of the shapes used in Experiment 1 are nevertheless detected faster than the structural aspects under search conditions. Because the variation in structural aspects is not more salient than in metrical aspects, we conclude that for these shapes, visual completion is indeed size‐invariant. The relations between performances in the three paradigms are discussed.     

Shape constancy: The effects of changing shape orientation and the effects of changing the position of focal features

Five experiments examined the time taken to judge that two consecutive elongated geometrical shapes had the same structure, irrespective of their orientation. Shape transformations either changed the orientation of the principal axis while maintaining the relative locations of focal features or maintained the orientation of the principal axis while changing the relative locations of focal features, or they changed both. Experiment 1 demonstrated that changes in the orientation of the principal axis were more detrimental to matching than were changes in the locations of the shape’s focal features. Indeed, the time taken to match same-orientation shapes was the same as that taken to match shapes that maintained the same position in the visual field. Further experiments showed that this result was not due to differential apparent motion in the transformation conditions, that it was not due to response bias, and that it generalized across shapes. However, the result was different when subjects could predict the location of the to-be-matched stimulus. In this case, performance was principally affected by the position of the focal feature of the shape and not by the shape’s orientation. It is suggested that the results reflect the efficiency with which subjects can construct matching representations for the stimuli When subjects cannot predict stimulus locations, they generate representations by describing shape structure relative to the shape’s principal axis. When the axis of the to-be-matched shapes is constant, subjects can use the same procedure in generating this representation for both shapes, facilitating matching relative to the case in which the orientation of the axis changes. When subjects can predict the stimulus location, they selectively attend to the focal features of shapes, minimizing the effects of shape orientation.