Perceptual grouping in Gabor lattices: Proximity and alignment

Exploring perceptual grouping by proximity principle in multistable dot lattices: Dissociation between vision-for-perception and vision-for-action

Perceptual grouping, a fundamental mechanism in our visual system, significantly influences our interpretation of and interaction with the surrounding world. This study explores the impact of the proximity principle from the perspective of the Two Visual Systems (TVS) model. The TVS model argues that the visual system comprises two distinct streams: the ventral stream, which forms the neural basis for "vision-for-perception," and the dorsal stream, which underlies "vision-for-action." We designed a perceptual grouping task using dot lattices as well as a line-orientation discrimination task. Data were collected using vocal and mouse methods for the vision-for-perception mode, and joystick and pen-paper methods for the vision-for-action mode. Each method, except for vocal, included separate blocks for right and left hands. The proximity data were fitted using exponential and power models. Linear mixed-effects models were used for the statistical analyses. The results revealed similar line-orientation discrimination accuracy across all conditions. The exponential model emerged as the best fit, demonstrating adherence to the Pure Distance Law in both perceptual modes. Sensitivity to the proximity principle was higher in the vision-for-action mode compared to the vision-for-perception. In terms of orientation biases, a strong preference for vertical orientation was observed in the vision-for-perception mode, whereas a noticeable preference toward either of the oblique orientations was detected in the vision-for-action mode. Analysis of free-drawn lines demonstrated an affordance bias in the vision-for-action mode. This suggests a remarkable tendency to perceive organizations within specific orientations that offer more affordances due to the interaction between the body postures and tools.

Additivity of grouping by proximity and luminance similarity is dependent on relative grouping strength: An analysis of individual differences in grouping sensitivity

It has previously been shown that grouping by proximity is well described by a linear function relating the perceived orientation of a dot lattice to the ratio of the distances between the dots in the different orientations. Similarly, luminance influences how observers perceptually group stimuli. Using the dot lattice paradigm, it has been shown that proximity and luminance similarity interact additively, which means that their effects can be summed to predict an observers' percept. In this study, we revisit the additive interplay between proximity and luminance similarity and we ask whether this pattern might be the result of inappropriately averaging different types of observers or the imbalance between the strength of proximity grouping and luminance similarity grouping. To address these questions, we first ran a replication of the original study reporting the additive interplay between proximity and luminance similarity. Our results showed a convincing replication at the aggregate and individual level. However, at the individual level, all observers showed grouping by proximity whereas some observers did not show grouping by luminance similarity. In response, we ran a second experiment with enlarged luminance differences to reinforce the strength of grouping by luminance similarity and balance the strength of the two grouping cues. Interestingly, in this second experiment, additivity was not observed but instead a significant interaction was obtained. This disparity suggests that the additivity or interaction between two grouping cues in a visual stimulus is not a general rule of perceptual grouping but a consequence of relative grouping strength.

Multidimensional feature interactions in visual crowding: When configural cues eliminate the polarity advantage

Crowding occurs when surrounding objects (flankers) impair target perception. A key property of crowding is the weaker interference when target and flankers strongly differ on a given dimension. For instance, identification of a target letter is usually superior with flankers of opposite versus the same contrast polarity as the target (the "polarity advantage"). High performance when target-flanker similarity is low has been attributed to the ungrouping of target and flankers. Here, we show that configural cues can override the usual advantage of low target-flanker similarity, and strong target-flanker grouping can reduce - instead of exacerbate - crowding. In Experiment 1, observers were presented with line triplets in the periphery and reported the tilt (left or right) of the central line. Target and flankers had the same (uniform condition) or opposite contrast polarity (alternating condition). Flanker configurations were either upright (||), unidirectionally tilted (\\ or //), or bidirectionally tilted (\/ or /\). Upright flankers yielded stronger crowding than unidirectional flankers, and weaker crowding than bidirectional flankers. Importantly, our results revealed a clear interaction between contrast polarity and flanker configuration. Triplets with upright and bidirectional flankers, but not unidirectional flankers, showed the polarity advantage. In Experiments 2 and 3, we showed that emergent features and redundancy masking (i.e. the reduction of the number of perceived items in repeating configurations) made it easier to discriminate between uniform triplets when flanker tilts were unidirectional (but not when bidirectional). We propose that the spatial configurations of uniform triplets with unidirectional flankers provided sufficient task-relevant information to enable a similar performance as with alternating triplets: strong-target flanker grouping alleviated crowding. We suggest that features which modulate crowding strength can interact non-additively, limiting the validity of typical crowding rules to contexts where only single, independent dimensions determine the effects of target-flanker similarity.

Depth from blur and grouping under inattention

Previous studies provided evidence in support of attention operating in three-dimensional space, and the iterative and multistage nature of organizational processes in relation to attention and depth. We investigated depth perception and attentional demands in grouping organizations that contain blur as a depth cue. Contrary to previous studies, in our displays, no depth from occlusion could be implied from a shared border between groups or surfaces. To evaluate depth perception, subjective reports were collected where participants indicated which elements, blurry or sharp, they perceived as closer. To examine whether depth perception from blur can alleviate attentional demands, we used an inattention paradigm. We presented displays of grouping organizations by collinearity or color similarity that were previously found to require attention and added blur to the figure or the background elements to generate depth perception. In addition, we presented similar displays containing grouping by blur similarity as a single cue. We hypothesized that adding blur would facilitate the segmentation of element groups due to their perceived depth, which might lead to a diminished demand for attention. Our results confirmed that blur led to depth perception, and that sharp elements were perceived as closer more frequently than blurry elements. Thus, these results provide novel evidence for depth from blur in grouping where no inference of occlusion can be derived from a border. However, although the results suggest that blur information was processed under inattention, little evidence was found for decreased attentional demands for grouping processes in the presence of blur.

How do humans group non-rigid objects in multiple object tracking?: Evidence from grouping by self-rotation

Previous studies on perceptual grouping found that people can use spatiotemporal and featural information to group spatially separated rigid objects into a unit while tracking moving objects. However, few studies have tested the role of objects' self-motion information in perceptual grouping, although it is of great significance to the motion perception in the three-dimensional space. In natural environments, objects always move in translation and rotation at the same time. The self-rotation of the objects seriously destroys objects' rigidity and topology, creates conflicting movement signals and results in crowding effects. Thus, this study sought to examine the specific role played by self-rotation information on grouping spatially separated non-rigid objects through a modified multiple object tracking (MOT) paradigm with self-rotating objects. Experiment 1 found that people could use self-rotation information to group spatially separated non-rigid objects, even though this information was deleterious for attentive tracking and irrelevant to the task requirements, and people seemed to use it strategically rather than automatically. Experiment 2 provided stronger evidence that this grouping advantage did come from the self-rotation per se rather than surface-level cues arising from self-rotation (e.g. similar 2D motion signals and common shapes). Experiment 3 changed the stimuli to more natural 3D cubes to strengthen the impression of self-rotation and again found that self-rotation improved grouping. Finally, Experiment 4 demonstrated that grouping by self-rotation and grouping by changing shape were statistically comparable but additive, suggesting that they were two different sources of the object information. Thus, grouping by self-rotation mainly benefited from the perceptual differences in motion flow fields rather than in deformation. Overall, this study is the first attempt to identify self-motion as a new feature that people can use to group objects in dynamic scenes and shed light on debates about what entities/units we group and what kinds of information about a target we process while tracking objects.

Competition between perceptual grouping cues in an indirect objective task

The integration between Gestalt grouping cues has been a relatively unexplored issue in vision science. The present work introduces an objective indirect method based on the repetition discrimination task to determine the rules that govern the dominance dynamics of the competition between both intrinsic (Experiment 1: proximity vs luminance similarity) and extrinsic grouping cues (Experiment 2: common region vs connectedness) by means of objective measures of grouping (reaction times and accuracy). Prior to the main task, a novel objective equating task was introduced with the aim of equating the grouping strength of the cues for the visuomotor system. The main task included two single conditions with the grouping cues acting alone as well as two competing conditions displaying the grouping factors pitted against one another. Conventional aggregated analyses were combined with individual analysis and both revealed a consistent pattern of processing dominance of: (1) luminance similarity over proximity and (2) common region over connectedness. Interestingly, the individual analyses showed that, despite the heterogeneous responses to the single conditions, the pattern of dominance between cues was robustly homogeneous among the participants in the competing conditions.

Illusory Form Perception and Perceptual Grouping Operations under Conditions of Restricted Visual Awareness

In the present study, we conducted two experiments (Experiment 1: 35 participants, M = 29; SD = 8.4; Experiment 2: 36 participants, M = 25; SD = 6.1) with the intention to explore whether underlying perceptual grouping operations and illusory form perception generate dissociable priming effects when Kanizsa-like figures are presented as primes and the rotated inducers as controls under conditions of restricted awareness. Using five different stimulus onset asynchronies (SOA conditions, Experiment 1: 27, 40 and 53 ms; Experiment 2: 27, 80 and 227 ms), we displayed masked illusory and groping primes that could be congruent or incongruent in their orientation with subsequent probe stimuli (vertical vs. horizontal). We found significant priming effects in both Experiment 1 and 2 (p < .001, η2p = .31 and p = .016, η2p = .16, respectively), but, crucially, no significant priming differences between illusory and grouping primes across SOA conditions. Overall, our results are important in showing that a dissociation of the percept generated by the grouping of the inducers from that generated by the illusory form is crucial in the study of illusory form perception under conditions of restricted awareness. In addition, they provide further evidence of perceptual organization operations occurring under very restrictive awareness conditions.

Neural Signatures of the Configural Superiority Effect and Fundamental Emergent Features in Human Vision

The concepts of grouping, emergence, and superadditivity (when a whole is qualitatively different from the sum of its parts) are critical in Gestalt psychology and essential to properly understand the information processing mechanisms underlying visual perception. However, very little is known about the neural processes behind these phenomena (particularly in terms of their generality vs. specificity and their time-course). Here, we used the configural superiority effect as a way to define "emergence" and "emergent features" operationally, employing an approach that can isolate different emergent features and compare them on a common scale. By assessing well-established event related potentials in a HD-EEG system, we found that the critical processes behind configural superiority and superadditive Gestalt phenomena are present in the window between 100 and 200 ms after stimulus onset and that these effects seem to be driven by specific attentional selection mechanisms. Also, some emergent features seem to be differentially processed in different brain hemispheres. These results shed new light on the issues of the generality vs. specificity of the neural correlates of different Gestalt principles, the hemispheric asymmetries in the processing of hierarchical image structure and the role of the N1 ERP component in reflecting feature selective mechanisms.

Extrinsic grouping factors in motion-induced blindness

We investigated how various grouping factors altered subjective disappearances of the individual targets in the motion-induced blindness display. The latter relies on a moving mask to render highly salient static targets temporarily subjectively invisible. Specifically, we employed two extrinsic grouping factors, the connectedness and the common region, and examined whether their presence would make targets more resilient against the suppression. In addition, we investigated whether the presence of an illusory Kanizsa triangle would affect the suppression of the inducing Pac-Man elements. We quantified the perceptual dynamics using the proportion of the disappearance time (this indicates whether targets became more resilient against the suppression), and the proportion of simultaneous disappearance and reappearance events (characterizes the tendency for the targets to disappear or reappear as a group). We report that a single mask that encompassed all targets (a common region grouping) significantly increased the proportion of simultaneous disappearance and reappearance events, but had no effect on the proportion of the disappearance time. In contrast, a line that connected two targets significantly decreased the total invisibility time, but had no impact on the simultaneity of the disappearance and reappearance events. We found no statistically significant effect of the presence of the illusory Kanizsa triangle on either measure. Finally, we found no interaction either between the common region and the connectedness or between the common region and the presence of the illusory Kanizsa triangle. Our results indicate that extrinsic grouping factors might influence the perception differently than the intrinsic ones and highlight the importance of using several measures to characterize the perceptual dynamics, as various grouping factors might affect it differentially.

Objective measurement of gestalts: Quantifying grouping effect by tilt aftereffect

The importance of holistic perception is described by Gestalt psychology and its principles. Gestalt psychologists have promoted a formal and testable framework for these principles. Quantitative measurements have been introduced from Gestalt psychology in order to complement traditional phenomenal descriptions. Here we demonstrated a new method of measuring grouping effects objectively and quantitatively, by means of tilt aftereffect (TAE) from visual adaptation. Experiment 1 validated the method by measuring grouping based on either proximity or color similarity. Experiments 2 and 3 verified that this paradigm is also effective with dimotif lattices in which different perceptual organizations compete. The novel TAE-based paradigm is an objective and effective method for studying perceptual organization, especially the relationship between different Gestalt principles.

Dominance dynamics of competition between intrinsic and extrinsic grouping cues

In the present study we examined the dominance dynamics of perceptual grouping cues. We used a paradigm in which participants selectively attended to perceptual groups based on several grouping cues in different blocks of trials. In each block, single and competing grouping cues were presented under different exposure durations (50, 150 or 350ms). Using this procedure, intrinsic vs. intrinsic cues (i.e. proximity and shape similarity) were compared in Experiment 1; extrinsic vs. extrinsic cues (i.e. common region and connectedness) in Experiment 2; and intrinsic vs. extrinsic cues (i.e. common region and shape similarity) in Experiment 3. The results showed that in Experiment 1, no dominance of any grouping cue was found: shape similarity and proximity grouping cues showed similar reaction times (RTs) and interference effects. In contrast, in Experiments 2 and 3, common region dominated processing: (i) RTs to common region were shorter than those to connectedness (Exp. 2) or shape similarity (Exp. 3); and (ii) when the grouping cues competed, common region interfered with connectedness (Exp. 2) and shape similarity (Exp. 3) more than vice versa. The results showed that the exposure duration of stimuli only affected the connectedness grouping cue. An important result of our experiments indicates that when two grouping cues compete, both the non-attended intrinsic cue in Experiment 1, and the non-dominant extrinsic cue in Experiments 2 and 3, are still perceived and they are not completely lost.

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

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

Does the Relative Strength of Grouping Principles Modulate the Interactions between them?

This study examines the influence of the relative strength of grouping principles on interactions between the intrinsic principle of proximity and the extrinsic principle of common region in the process of perceptual organization. Cooperation and competition between intrinsic and extrinsic principles were examined by presenting the principle either alone or conjoined with another principle. The relative grouping strength of the principles operating alone was varied in two different groups of participants so that it was similar for one group and very different for the other group. Results showed that, when principles acting alone had different strengths, the grouping effect of the strongest principle was similar to that of the cooperation condition, and the effect of the weakest principle was similar to that of competing conjoined principles. In contrast, when the strength of principles acting alone was similar, the effect of conjoined cooperating principles was greater than that of either principle acting alone. Moreover, the effect of conjoined competing principles was smaller than that of either principle operating alone. Results show that cooperation and competition between intrinsic and extrinsic principles are modulated by the relative grouping strength of principles acting alone. Furthermore, performance in these conditions could be predicted on the basis of performance in single-principle conditions.

Ensemble Empirical Mode Decomposition Analysis of EEG Data Collected during a Contour Integration Task

We discuss a data-driven analysis of EEG data recorded during a combined EEG/fMRI study of visual processing during a contour integration task. The analysis is based on an ensemble empirical mode decomposition (EEMD) and discusses characteristic features of event related modes (ERMs) resulting from the decomposition. We identify clear differences in certain ERMs in response to contour vs noncontour Gabor stimuli mainly for response amplitudes peaking around 100 [ms] (called P100) and 200 [ms] (called N200) after stimulus onset, respectively. We observe early P100 and N200 responses at electrodes located in the occipital area of the brain, while late P100 and N200 responses appear at electrodes located in frontal brain areas. Signals at electrodes in central brain areas show bimodal early/late response signatures in certain ERMs. Head topographies clearly localize statistically significant response differences to both stimulus conditions. Our findings provide an independent proof of recent models which suggest that contour integration depends on distributed network activity within the brain.

Perceptual grouping across eccentricity

Across the visual field, progressive differences exist in neural processing as well as perceptual abilities. Expansion of stimulus scale across eccentricity compensates for some basic visual capacities, but not for high-order functions. It was hypothesized that as with many higher-order functions, perceptual grouping ability should decline across eccentricity. To test this prediction, psychophysical measurements of grouping were made across eccentricity. Participants indicated the dominant grouping of dot grids in which grouping was based upon luminance, motion, orientation, or proximity. Across trials, the organization of stimuli was systematically decreased until perceived grouping became ambiguous. For all stimulus features, grouping ability remained relatively stable until 40°, beyond which thresholds significantly elevated. The pattern of change across eccentricity varied across stimulus feature, in which stimulus scale, dot size, or stimulus size interacted with eccentricity effects. These results demonstrate that perceptual grouping of such stimuli is not reliant upon foveal viewing, and suggest that selection of dominant grouping patterns from ambiguous displays operates similarly across much of the visual field.

Spatial arrangement in texture discrimination and texture segregation

We investigated the role of spatial arrangement of texture elements in three psychophysical experiments on texture discrimination and texture segregation. In our stimuli, oriented Gabor elements formed an iso-oriented and a randomly oriented texture region. We manipulated (1) the orientation similarity in the iso-oriented region by adding orientation jitter to the orientation of each Gabor; (2) the spatial arrangement of the Gabors: quasi-random or regular; and (3) the shape of the edge between the two texture regions: straight or curved. In Experiment 1, participants discriminated an iso-oriented stimulus from a stimulus with only randomly oriented elements. Experiment 2 required texture segregation to judge the shape of the texture edge. Experiment 3 replicated Experiment 2 with Gabors of a smaller spatial extent in a denser arrangement. We found comparable performance levels with regular and quasi-random Gabor positions in the discrimination task but not in the segregation tasks. We conclude that spatial arrangement plays a role in a texture segregation task requiring shape discrimination of the texture edge but not in a texture discrimination task in which it is sufficient to discriminate an iso-oriented region from a completely random region.

A century of Gestalt psychology in visual perception: II. Conceptual and theoretical foundations

Our first review article (Wagemans et al., 2012) on the occasion of the centennial anniversary of Gestalt psychology focused on perceptual grouping and figure-ground organization. It concluded that further progress requires a reconsideration of the conceptual and theoretical foundations of the Gestalt approach, which is provided here. In particular, we review contemporary formulations of holism within an information-processing framework, allowing for operational definitions (e.g., integral dimensions, emergent features, configural superiority, global precedence, primacy of holistic/configural properties) and a refined understanding of its psychological implications (e.g., at the level of attention, perception, and decision). We also review 4 lines of theoretical progress regarding the law of Prägnanz-the brain's tendency of being attracted towards states corresponding to the simplest possible organization, given the available stimulation. The first considers the brain as a complex adaptive system and explains how self-organization solves the conundrum of trading between robustness and flexibility of perceptual states. The second specifies the economy principle in terms of optimization of neural resources, showing that elementary sensors working independently to minimize uncertainty can respond optimally at the system level. The third considers how Gestalt percepts (e.g., groups, objects) are optimal given the available stimulation, with optimality specified in Bayesian terms. Fourth, structural information theory explains how a Gestaltist visual system that focuses on internal coding efficiency yields external veridicality as a side effect. To answer the fundamental question of why things look as they do, a further synthesis of these complementary perspectives is required.

Production and perception rules underlying visual patterns: effects of symmetry and hierarchy

Formal language theory has been extended to two-dimensional patterns, but little is known about two-dimensional pattern perception. We first examined spontaneous two-dimensional visual pattern production by humans, gathered using a novel touch screen approach. Both spontaneous creative production and subsequent aesthetic ratings show that humans prefer ordered, symmetrical patterns over random patterns. We then further explored pattern-parsing abilities in different human groups, and compared them with pigeons. We generated visual plane patterns based on rules varying in complexity. All human groups tested, including children and individuals diagnosed with autism spectrum disorder (ASD), were able to detect violations of all production rules tested. Our ASD participants detected pattern violations with the same speed and accuracy as matched controls. Children's ability to detect violations of a relatively complex rotational rule correlated with age, whereas their ability to detect violations of a simple translational rule did not. By contrast, even with extensive training, pigeons were unable to detect orientation-based structural violations, suggesting that, unlike humans, they did not learn the underlying structural rules. Visual two-dimensional patterns offer a promising new formally-grounded way to investigate pattern production and perception in general, widely applicable across species and age groups.

A century of Gestalt psychology in visual perception: I. Perceptual grouping and figure-ground organization

In 1912, Max Wertheimer published his paper on phi motion, widely recognized as the start of Gestalt psychology. Because of its continued relevance in modern psychology, this centennial anniversary is an excellent opportunity to take stock of what Gestalt psychology has offered and how it has changed since its inception. We first introduce the key findings and ideas in the Berlin school of Gestalt psychology, and then briefly sketch its development, rise, and fall. Next, we discuss its empirical and conceptual problems, and indicate how they are addressed in contemporary research on perceptual grouping and figure-ground organization. In particular, we review the principles of grouping, both classical (e.g., proximity, similarity, common fate, good continuation, closure, symmetry, parallelism) and new (e.g., synchrony, common region, element and uniform connectedness), and their role in contour integration and completion. We then review classic and new image-based principles of figure-ground organization, how it is influenced by past experience and attention, and how it relates to shape and depth perception. After an integrated review of the neural mechanisms involved in contour grouping, border ownership, and figure-ground perception, we conclude by evaluating what modern vision science has offered compared to traditional Gestalt psychology, whether we can speak of a Gestalt revival, and where the remaining limitations and challenges lie. A better integration of this research tradition with the rest of vision science requires further progress regarding the conceptual and theoretical foundations of the Gestalt approach, which is the focus of a second review article.

Towards a new kind of experimental psycho-aesthetics? Reflections on the Parallellepipeda project

Experimental psycho-aesthetics-the science aimed at understanding the factors that determine aesthetic experience-is reviewed briefly as background to describe the Parallellepipeda project, a cross-over project between artists and scientists in Leuven. In particular, I sketch how it started and developed further, with close interactions between the participating artists and scientists. A few examples of specific research projects are mentioned to illustrate the kind of research questions we address and the methodological approach we have taken. We often found an effect of providing participants with additional information, a difference between novice and expert participants, and a shift with increasing experience with an artwork, in the direction of tolerating more complexity and acquiring more order from it. By establishing more connections between parts of an artwork and more associations to the artwork, it becomes a stronger Gestalt, which is more easily mastered by the viewer and leads to increased appreciation. In the final part of the paper, I extract some general lessons from the project regarding a possible new way of doing psycho-aesthetics research, which is able to solve some of the problems of traditional experimental psycho-aesthetics (eg, trade-off between experimental control and ecological validity).

Integrating biological motion: the role of grouping in the perception of point-light actions

The human visual system is highly sensitive to biological motion and manages to organize even a highly reduced point-light stimulus into a vivid percept of human action. The current study investigated to what extent the origin of this saliency of point-light displays is related to its intrinsic Gestalt qualities. In particular, we studied whether biological motion perception is facilitated when the elements can be grouped according to good continuation and similarity as Gestalt principles of perceptual organization. We found that both grouping principles enhanced biological motion perception but their effects differed when stimuli were inverted. These results provide evidence that Gestalt principles of good continuity and similarity also apply to more complex and dynamic meaningful stimuli.

Context modulates the ERP signature of contour integration

We investigated how the electrophysiological signature of contour integration is changed by the context in which a contour is embedded. Specifically, we manipulated the orientations of Gabor elements surrounding an embedded shape outline. The amplitudes of early visual components over posterior scalp regions were changed by the presence of a contour, and by the orientation of elements surrounding the contour. Differences in context type had an effect on the early P1 and N1 components, but not on the later P2 component. The presence of a contour had an effect on the N1 and P2 components, but not on the earlier P1 component. A modulatory effect of context on contour integration was observed on the N1 component. These results highlight the importance of the context in which contour integration takes place.

Interactions between Intrinsic Principles of Similarity and Proximity and Extrinsic Principle of Common Region in Visual Perception

The interactions between intrinsic grouping principles (by proximity or by similarity in shape or luminance) and extrinsic grouping by common region were examined by presenting both principles acting alone or conjoined with another principle in visual patterns. The procedure used in our study closely mirrored that of Quinlan and Wilton (1998 Perception27 417 – 430). However, in the present experiment, the intrinsic and extrinsic grouping principles involved had similar relative salience in order to avoid their possible effects on interactions. The results showed that the grouping effect of conjoined cooperating principles was greater than that of either principle acting alone, and the grouping effect of conjoined competing principles was lower than that of either principle operating alone. Compatibility of data with additive effects of grouping principles has been examined.

The influence of orientation and contrast flicker on contour saliency of outlines of everyday objects

One of the most important tasks of the visual system is the extraction of edges and object contours, and the integration of discrete elements to form a coherent global percept. A great deal is known about the spatial properties of contour extraction, but less is known about the dynamics and spatio-temporal aspects. We used Gabor-rendered outlines of real-world objects, where we could manipulate low-level properties, such as element orientation and phase, while incorporating higher-level properties, such as object complexity and identity, to study dynamic relationships in object detection. First we manipulated the time available for integration by changing back and forth between coherent and non-coherent orientations of the contour elements. We then manipulated contrast flicker by reversing the spatial phase of the Gabor elements at various frequencies. We found similar results to earlier studies on contour detection: detection was better for contrast flicker than for orientation flicker, and detection performance was curvature-dependent for orientation flicker but not for contrast flicker. Our results support the existence of at least two temporal frequency channels in the visual system, one low-pass and one band-pass peaking around 10-12 Hz. In addition, we found that object properties, such as identity and complexity, affected detection performance.

The utility of image descriptions in the initial stages of vision: a case study of printed text

Vision research has made very substantial progress towards understanding how we see. It is one area of psychology where the three-way thrust of behavioural measurements (psychophysics), brain imaging, and computational studies have been combined quite routinely for some years. The purpose of this paper is to demonstrate a relatively unusual form of computational modelling that we characterise as involving image descriptions. Image descriptions are statements about structures in images and relationships between structures. Most modelling in vision is either conceived in fairly abstract terms, or is done at the level of images. Neither is entirely satisfactory, and image descriptions are a simple formulation of age-old ideas about a Vocabulary of image features that are detected and parameterized from actual digital images. For our example, we use the domain of the visual perception of printed text. This is an area that has been characterized by thorough, robust psychophysical experiments. The fundamental requirements of visual processing in this domain are: grouping of some parts if the image into words; at the same time segmenting words from each other. We show how these are readily understood in terms of our model of image descriptions, and show quantitatively that typographical practice, refined over centuries, is about optimum for the visual system at least as represented by our model. In addition, we show that the same notion of image descriptions could, in principle, support word recognition in certain circumstances.

Perceptual multistability in figure-ground segregation using motion stimuli

In a series of experiments using ambiguous stimuli, we investigate the effects of displaying ordered, discrete series of images on the dynamics of figure-ground segregation. For low frame presentation speeds, the series were perceived as a sequence of discontinuous, static images, while for high speeds they were perceived as continuous. We conclude that using stimuli varying continuously along one parameter results in stronger hysteresis and reduces spontaneous switching compared to matched static stimuli with discontinuous parameter changes. The additional evidence that the size of the hysteresis effects depended on trial duration is consistent with the stochastic nature of the dynamics governing figure-ground segregation. The results showed that for continuously changing stimuli, alternative figure-ground organizations are resolved via low-level, dynamical competition. A second series of experiments confirmed these results with an ambiguous stimulus based on Petter's effect.

Identification of everyday objects on the basis of fragmented outline versions

Although Attneave (1954 Psychological Review 61 183 193) and Biederman (1987 Psychological Review 94 115-147) have argued that curved contour segments are most important in shape perception, Kennedy and Domander (1985 Perception 14 367-370) showed that fragmented object contours are better identifiable when straight segments are shown. We used the set of line drawings published by Snodgrass and Vanderwart (1980 Journal of Experimental Psychology: Human Learning and Memory 6 174-215), to make outline versions that could be used to investigate this issue with a larger and more heterogeneous stimulus set. Fragments were placed either around the 'salient' points or around the midpoints (points midway between two salient points), creating curved versus relatively straight fragments when the original outline was fragmented (experiment 1), or angular and straight fragments when straight-line versions were fragmented (experiment 2). We manipulated fragment length in each experiment except the last one, in which we presented only selected points (experiment 3). While fragmented versions were on average more identifiable when straight fragments were shown, certain objects were more identifiable when the curved segments or the angles were shown. A tentative explanation of these results is presented in terms of an advantage for straight segments during grouping processes for outlines with high part salience, and an advantage for curved segments during matching processes for outlines with low part salience.

Spatial organisation in passive tactile perception: is there a tactile field?

The perceptual field is a cardinal concept of sensory psychology. 'Field' refers to a representation in which perceptual contents have spatial properties and relations which derive from the spatial properties and relations of corresponding stimuli. It is a matter of debate whether a perceptual field exists in touch analogous to the visual field. To study this issue, we investigated whether tactile stimuli on the palm can be perceived as complex stimulus patterns, according to basic spatial principles. Subjects judged the intensity of a target stimulus to the palm, ignoring two brief preceding touches at nearby flanker locations. We found that the judgements of the target intensity were boosted by flankers when the target lay on the line joining the flankers in comparison to when the target lay away from this line. Therefore, we suggest that a tactile spatial organisation, i.e. a tactile field, exists; the field supports the relation of collinearity; it is automatically and implicitly activated by touch, and it groups spatially coherent perceptual contents.

Young infants readily use proximity to organize visual pattern information

Four experiments relying on novelty and spontaneous preference procedures were performed to determine whether 3-4-month-old infants utilize the Gestalt principle of proximity to organize visual pattern information. In Experiment 1, infants familiarized with arrays of elements that could be organized into either columns or rows were tested for their preference between vertical and horizontal bars. The infants preferred the novel organization of bars. Experiments 2 and 3 showed that the novelty preference could not be attributed to an a priori preference or an inability to discriminate between the elements comprising the patterns. Experiment 4 replicated the results of Experiment 1 in a bars --> elements version of the task, indicating that extended exposure is not necessary for infants to organize based on proximity. The results suggest that infants readily organize visual pattern information in accord with proximity. Implications of this finding for models of the ontogenesis and microgenesis of object perception in infants and adults are discussed.

Formation of visual "objects" in the early computation of spatial relations

Perceptual grouping is the process by which elements in the visual image are aggregated into larger and more complex structures, i.e., "objects." This paper reports a study of the spatial factors and time-course of the development of objects over the course of the first few hundred milliseconds of visual processing. The methodology uses the now well-established idea of an "object benefit" for certain kinds of tasks (here, faster within-object than between-objects probe comparisons) to test what the visual system in fact treats as an object at each point during processing. The study tested line segment pairs in a wide variety of spatial configurations at a range of exposure times, in each case measuring the strength of perceptual grouping as reflected in the magnitude of the object benefit. Factors tested included nonaccidental properties such as collinearity, cotermination, and parallelism; contour relatability; Gestalt factors such as symmetry and skew symmetry, and several others, all tested at fine (25 msec) time-slices over the course of processing. The data provide detailed information about the comparative strength of these factors in inducing grouping at each point in processing. The result is a vivid picture of the chronology of object formation, as objects progressively coalesce, with fully bound visual objects completed by about 200 msec of processing.

Segmentation of object outlines into parts: a large-scale integrative study

In this study, a large number of observers (N=201) were asked to segment a collection of outlines derived from line drawings of everyday objects (N=88). This data set was then used as a benchmark to evaluate current models of object segmentation. All of the previously proposed rules of segmentation were found supported in our results. For example, minima of curvature (i.e. locations along the contour where negative curvature takes an extreme value) were often used as segmentation points. The second point of a pair connected by a segmentation line often depended on more global shape characteristics such as proximity, collinearity, symmetry, and elongation. Based on these results, a framework is presented in which all of the previously proposed (and now empirically validated) segmentation rules or rules for part formation are integrated.