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Numerous examples of meaningful inter-individual differences in visual processing have been documented in low- and high-level vision. For mid-level vision or perceptual organization, vision scientists have only recently started to study the inter-individual differences structure. In this study, we focus on orientation and proximity as emergent features and combine a quantitative information processing approach with an individual differences approach. We first replicated the results reported in Hawkins, Houpt, Eidels, and Townsend (2016) in a set of 52 observers. That is, observers showed higher processing capacity for detecting a change in a stimulus configuration when the emergent features orientation or proximity were changed. Next, we asked whether individual differences processing capacities were similar across emergent features. The capacity to detect any type of change correlated moderately across individuals, whereas the capacity to detect changes in either emergent feature alone was not strongly correlated. This indicates that there is no general sensitivity to emergent features and that observers can be good at detecting orientation changes whilst being poor at detecting proximity changes (and vice versa). An additional exploratory multivariate analysis of the data revealed that response times and accuracies correlated strongly within each emergent feature. Moreover, specific factors related to change detection and inward displacements were observed, revealing consistent individual differences in our data. We discuss the results in the context of the literature on individual differences in vision where both specific, fragmented factors as well as broad, general factors have been reported., (Copyright © 2020 Elsevier Ltd. All rights reserved.)

Shape perception is intrinsically holistic: combinations of features give rise to configurations with emergent properties that are different from the sum of the parts. The current study investigated neural markers of holistic shape representations learned by means of categorization training. We used the EEG frequency tagging technique, where two parts of a shape stimulus were 'tagged' by modifying their contrast at different temporal frequencies. Signals from both parts are integrated and, as a result, emergent frequency components (so-called, intermodulation responses, IMs), caused by nonlinear interaction of two frequency signals, are observed in the EEG spectrum. First, participants were trained in 4 sessions to discriminate highly similar, unfamiliar shapes into two categories, defined based on the combination of features. After training, EEG was recorded while frequency-tagged shapes from either the trained or the untrained shape family were presented. For all IMs combined, no learning effects were detected, but post hoc analyses of higher-order IMs revealed stronger occipital and occipito-temporal IMs for both trained and untrained exemplars of the trained shape family as compared to the untrained shape family. In line with recent findings, we suggest that the higher-order IMs may reflect high-level visual computations, like holistic shape categorization, resulting from a cascade of non-linear operations. Higher order frequency responses are relatively low in power, hence results should be interpreted cautiously and future research is needed to confirm these effects. In general, these findings are, to our knowledge, the first to show IMs as a neural correlate of perceptual learning., (Copyright © 2018 Elsevier Ltd. All rights reserved.)

A large body of research reports individual differences in local and global visual processing in relation to expertise, culture and psychopathology. However, recent research has suggested that various different measures of local-global processing are not strongly associated with one another, calling its construct validity into question. The current study sought to further explore the validity of local-global processing biases in perception by developing three tasks based on two existing paradigms: the Embedded Figures Test (EFT) and the Navon hierarchical letters task. The newly developed tasks aimed to control for stimulus and response factors that may have impacted upon the reliability of previous research. They were administered to a large sample of undergraduate students (N>100). The results of two new versions of the EFT indicated that disembedding performance is influenced by the structure of the embedding context. In addition, global precedence and interference in the Navon task remained present even when local attentional approaches to global hierarchical stimuli were restricted. Inter-task correlations within the EFT were high but low between the EFT and the Navon task, lending support to the notion that local-global processing is not a monolithic construct, but representative of a number of distinct perceptual abilities and biases. Future research may use these task distinctions to pinpoint more precisely which aspects of perceptual processing characterise specific (clinical) participant populations., (Copyright © 2017 Elsevier Ltd. All rights reserved.)

We asked whether "good composition" or "visual rightness" of artworks manifest themselves in a particular arrangement of basic image features, such as oriented luminance edges. Specifically, we analysed the layout of edge orientations in images from a collection of >1600 paintings of Western provenance by comparing pairwise the orientation of each edge in an image with the orientations of all other edges in the same image. From the resulting orientation histograms, we calculated Shannon entropy and parallelism (i.e., the degree to which lines are parallel in the image). For comparison, we analysed the same second-order image properties in photographs of diverse natural patterns and man-made objects and scenes. Results showed that Shannon entropy of relative orientations of edge pairs was high and parallelism was low for the paintings and some of the natural patterns, but differed from other sets of photographs, including other man-made stimuli. The differences were also observed when images were matched for image content. Moreover, high entropy of edge orientations was found in traditional artworks produced by different techniques, in artworks that represented different content matter and art genres, as well as in artworks from other cultural backgrounds (East Asian and Islamic). In conclusion, we found that high entropy of edge orientations characterizes diverse sets of traditional artworks from various cultural backgrounds., (Copyright © 2017 Elsevier Ltd. All rights reserved.)

Perceptual grouping processes are typically studied using sparse displays of spatially separated elements. Unless the grouping cue of interest is a proximity cue, researchers will want to ascertain that such a cue is absent from the display. Various solutions to this problem have been employed in the literature; however, no validation of these methods exists. Here, we test a number of local density metrics both through their performance as constrained ideal observer models, and through a comparison with a large dataset of human detection trials. We conclude that for the selection of stimuli without a density cue, the Voronoi density metric is preferable, especially if combined with a measurement of the distance to each element's nearest neighbor. We offer the entirety of the dataset as a benchmark for the evaluation of future, possibly improved, metrics. With regard to human processes of grouping by proximity, we found observers to be insensitive to target groupings that are more sparse than the surrounding distractor elements, and less sensitive to regularity cues in element positioning than to local clusterings of target elements., (Copyright © 2015 Elsevier Ltd. All rights reserved.)

Segmentation of a visual scene in "figure" and "ground" is essential for perception of the three-dimensional layout of a scene. In cases of bi-stable perception, two distinct figure-ground interpretations alternate over time. We were interested in the temporal dynamics of these alternations, in particular when the same image is presented repeatedly, with short blank periods in-between. Surprisingly, we found that the intermittent presentation of Rubin's classical "face-or-vase" figure, which is frequently taken as a standard case of bi-stable figure-ground perception, often evoked perceptual switches during the short presentations and stabilization was not prominent. Interestingly, bi-stable perception of Kanizsa's anomalous transparency figure did strongly stabilize across blanks. We also found stabilization for the Necker cube, which we used for comparison. The degree of stabilization (and the lack of it) varied across stimuli and across individuals. Our results indicate, against common expectation, that the stabilization phenomenon cannot be generally evoked by intermittent presentation. We argue that top-down feedback factors such as familiarity, semantics, expectation, and perceptual bias contribute to the complex processes underlying the temporal dynamics of bi-stable figure-ground perception., (Copyright © 2014 Elsevier Ltd. All rights reserved.)

The perception of an illusory surface, a subjectively perceived surface that is not given in the image, is one of the most intriguing phenomena in vision. It strongly influences the perception of some fundamental properties, namely, depth, lightness and contours. Recently, we suggested (1) that the context-sensitive mechanism of depth computation plays a key role in creating the illusion, (2) that the illusory lightness perception can be explained by an influence of depth perception on the lightness computation, and (3) that the perception of variations of the Kanizsa figure can be well-reproduced by implementing these principles in a model (Kogo, Strecha, et al., 2010). However, depth perception, lightness perception, contour perception, and their interactions can be influenced by various factors. It is essential to measure the differences between the variation figures in these aspects separately to further understand the mechanisms. As a first step, we report here the results of a new experimental paradigm to compare the depth perception of the Kanizsa figure and its variations. One of the illusory figures was presented side-by-side with a non-illusory variation whose stereo disparities were varied. Participants had to decide in which of these two figures the central region appeared closer. The results indicate that the depth perception of the illusory surface was indeed different in the variation figures. Furthermore, there was a non-linear interaction between the occlusion cues and stereo disparity cues. Implications of the results for the neuro-computational mechanisms are discussed., (Copyright © 2014 Elsevier Ltd. All rights reserved.)

In contour integration, a relevant question is whether snakes and ladders are processed similarly. Higher presentation time thresholds for ladders in detection tasks indicate this is not the case. However, in a detection task only processing differences at the level of element linking and possibly contour localization might be picked up, while differences at the shape encoding level cannot be noticed. In this study, we make a direct comparison of detection and shape discrimination tasks to investigate if processing differences in the visual system between snakes and ladders are limited to contour detection or extend to higher level contour processing, like shape encoding. Stimuli consisted of elements that were oriented collinearly (snakes) or orthogonally (ladders) to the contour path and were surrounded by randomly oriented background elements. In two tasks, six experienced subjects either detected the contour when presented with a contour and a completely random stimulus or performed a shape discrimination task when presented with two contours with different curvature. Presentation time was varied in 9 steps between 8 and 492 ms. By applying a generalized linear mixed model we found that differences in snake and ladder processing are not limited to a detection stage but are also apparent at a shape encoding stage., (Copyright © 2013 Elsevier B.V. All rights reserved.)

A model that computes the border-ownership (BOWN) and the depth map of the image (DISC model, Kogo et al., 2010) is applied to investigate the stochastic nature of perception in "face or vase" stimuli. The 2-D integration of BOWN signals determines the figure-ground relationships in the image. In the present application of the model, BOWN signals are made stochastic and feedback connections are established between the higher level depth perception and the lower level BOWN signals. This feedback modifies the BOWN signals to further enhance the figure-ground segregation. Adaptation results in a decay of the response which leads to alternation. This triggers recovery of the adaptation. The alternation rate decreases in response to the increased levels of disambiguation. A detailed examination of the model parameters shows a prolongation of the alternation when the stimulus is presented intermittently in some conditions. In this framework, the adaptation and the recovery processes need to be perception dependent (not physical input dependent) to reproduce human perception., (Copyright © 2011 Elsevier Ltd. All rights reserved.)

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., (Copyright © 2010 Elsevier Ltd. All rights reserved.)

In studies of shape perception, the detection of contours and the segregation of regions enclosed by these contours have mostly been treated in isolation. However, contours and surfaces somehow need to be combined to create a stable perception of shape. In this study, we used a 2AFC task with arrays of oriented Gabor elements to determine whether and to what extent human observers integrate information from the contour and from the interior surface of a shape embedded in this array. The saliency of the shapes depended on the alignment of Gabors along the shape outline and on the isolinearity of Gabors inside the shape. In two experiments we measured detectability of shapes defined by the contour cue, by the surface cue, and by the combination of both cues. As a first step, we matched performance in the two single-cue conditions. We then compared shape detectability in the double-cue condition with the two equally detectable single-cue conditions. Our results show a clear double-cue benefit: Participants used both cues to detect the shapes. Next, we compared performance in the double-cue condition with the performance predicted by two models of sensory cue combination: a minimum rule (probability summation) and an integration rule (information summation). Results from Experiment 2 indicate that participants applied a combination rule that was better than mere probability summation. We found no evidence against the integration rule., (Copyright © 2010 Elsevier Ltd. All rights reserved.)

Recently we developed a model that reproduces the Kanizsa square illusion based on two principles: (1) a spatial 2-D integration of luminance ratio and differentiated depth signals creates a "primary" lightness map and a depth map, respectively, which is then followed by (2) a modification of the primary lightness values under influence of the perceived depth (Kogo, Strecha, Van Gool, & Wagemans, 2010). Within this model, the process of the spatial integration inevitably introduced an arbitrary offset. In order to obtain absolute values of depth and lightness, the offset values needed to be determined by other constraints. This is the anchoring problem of the depth and lightness measurements. Here we report the anchoring rules that were established by investigating the model's responses to the Kanizsa square and its wide range of variations. For the primary lightness map, the highest value rule was applied, while the area rule appeared most plausible for the depth map. By applying the same principles to simple figures consisting of black and white areas of different size ratios, the model succeeded in reproducing published empirical results on lightness anchoring (Li & Gilchrist, 1999)., (Copyright 2010 Elsevier Ltd. All rights reserved.)

Through saccadic eye movements, the retinal projection of an extrafoveally glimpsed object can be brought into foveal vision quickly. We investigated what influence visual detail collected before the saccade exerts on the postsaccadic percept. Participants were instructed to saccade towards a peripheral stimulus, and to indicate on a continuum of ellipses with varying aspect ratios which exact shape they had perceived to be present after saccade landing. Compared to both an identical ellipse preview and a qualitatively different square preview, a quantitatively different ellipse preview was observed to shift the mean postsaccadic percept towards the presaccadic aspect ratio parameter value. This integration of subtly different form information was accompanied by an integration of the identity of both stimuli presented: In the great majority of these trials, subjects indicated that they had not noticed the occurrence of a change to the stimulus. When a blank screen preceded the postsaccadic stimulus onset the influence of presaccadic stimulus information on postsaccadic perception was weaker. An immediate postsaccadic mask on the other hand abolished the effect entirely. We conclude that integration of parametric visual form information occurs across saccades, but that it relies on a quickly decaying and maskable visual memory., (2010 Elsevier Ltd. All rights reserved.)

One of the ultimate goals of vision research is to understand how some elements are grouped together and differentiated from others to form object representations in a complex visual scene. There exists an extensive literature on this grouping/segmentation problem, but most of the studies have used un-recognizable stimuli that have little to do with object recognition per se. We used Gabor-rendered outlines of real-world objects to study some relationships between bottom-up and top-down processes in both spatial- and motion form perception. We manipulated low-level properties, such as element orientation and local motion, while incorporating higher-level properties, such as object complexity and identity, and found that adding local motion improved overall performance in both object detection and object identification tasks. Adding orientation jitter effectively decreased object detection performance in both static and motion conditions, and increased reaction time for identification in the static condition. Orientation jitter had much less effect on reaction times for identification in the local motion condition than in the static condition. Both contour properties ("good continuation") and object properties (identifiability) had a positive effect on detection and reaction time for identification.

The aim of this study was to evaluate the influence of complexity and symmetry on shape recognition, by measuring the recognition of unfamiliar shapes (created using Fourier Boundary Descriptors, FBDs) through a delayed matching task. Between complexity levels the shapes differed in the frequency of the FBDs and within complexity levels in their phase. Shapes were calibrated to be physically equally similar for the different complexity levels. Matching two sequentially presented shapes was slower and less accurate when complexity increased and for asymmetrical compared to symmetrical versions of the shapes. Thus, we show that simplicity in general and symmetry in particular enhance the short-term recognition of unfamiliar shapes.

Using kinetic contours derived from everyday objects, we investigated how motion affects object identification. In order not to be distinguishable when static, kinetic contours were made from random dot displays consisting of two regions, inside and outside the object contour. In Experiment 1, the dots were moving in only one of two regions. The objects were identified nearly equally well as soon as the dots either in the figure or in the background started to move. RTs decreased with increasing motion coherence levels and were shorter for complex, less compact objects than for simple, more compact objects. In Experiment 2, objects could be identified when the dots were moving both in the figure and in the background with speed and direction differences between the two. A linear increase in either the speed difference or the direction difference caused a linear decrease in RT for correct identification. In addition, the combination of speed and motion differences appeared to be super-additive.

Categorization models often assume an intermediate stimulus representation by units implementing "distance functions", that is, units that are activated according to the distance or similarity among stimuli. Here we show that a popular example of these models, ALCOVE, is able to account for the performance of monkeys during category learning when it takes the perceived similarity among stimuli into account. Similar results were obtained with a slightly different model (ITCOVE) that included experimentally measured tuning curves of neurons in inferior temporal (IT) cortex. These results show the intimate link between category learning and perceived similarity as represented in IT cortex.

The Pure Distance Law predicts grouping by proximity in dot lattices that can be organised in four ways by grouping dots along parallel lines. It specifies a quantitative relationship between the relative probability of perceiving an organisation and the relative distance between the grouped dots. The current study was set up to investigate whether this principle holds both for centrally and for eccentrically displayed dot lattices. To this end, dot lattices were displayed either in central vision, or to the right of fixation with their closest border at 3 degrees or 15 degrees. We found that the Pure Distance Law adequately predicted grouping of centrally displayed dot lattices but did not capture the eccentric data well, even when the eccentric dot lattices were scaled. Specifically, a better fit was obtained when we included the possibility in the model that in some trials participants could not report an organisation and consequently responded randomly. A plausible interpretation for the occurrence of random responses in the eccentric conditions is that under these circumstances an attention shift is required from the locus of fixation towards the dot lattice, which occasionally fails to take place. When grouping could be reported, scale and eccentricity appeared to interact. The effect of the relative interdot distances on the perceptual organisation of the dot lattices was estimated to be stronger in peripheral vision than in central vision at the two largest scales, but this difference disappeared when the smallest scale was applied.

We measure the performance of five subjects in a two-alternative-forced-choice slant-discrimination task for differently textured planes. As textures we used uniform lattices, randomly displaced lattices, circles (polka dots), Voronoi tessellations, plaids, 1/f noise, "coherent" noise and a leopard skin-like texture. Our results show: (1) Improving performance with larger slants for all textures, (2) and some cases of "non-symmetrical" performance around a particular orientation. (3) For orientations sufficiently slanted, the different textures do not elicit major differences in performance, (4) while for orientations closer to the vertical plane there are marked differences among them. (5) These differences allow a rank-order of textures to be formed according to their "helpfulness"--that is, how easy the discrimination task is when a particular texture is mapped on the plane. Polka dots tend to allow the best slant discrimination performance, noise patterns the worst. Two additional experiments were conducted to test the generality of the obtained rank-order. First, the tilt of the planes was rotated by 90 degrees. Second, the task was changed to a slant report task via probe adjustment. The results of both control experiments confirmed the texture rank-order previously obtained. We then test a number of spatial-frequency-based slant-from-texture models and discuss their shortcomings in explaining our rank-order. Finally, we comment on the importance of these results for depth-perception research in general, and in particular the implications our results have for studies of cue combination (sensor fusion) using texture as one of the cues involved.

Recent research has established the detrimental effect of lorazepam, a benzodiazepine, on both implicit and explicit memory. Furthermore, lorazepam is known to affect perceptual integration. Diazepam, on the other hand, though being a benzodiazepine too, only impairs explicit memory, leaving implicit memory fairly intact. Little is known about the effect of diazepam on perceptual integration. The present study aimed at filling in this gap, by comparing the effects of lorazepam and diazepam on the detection of discontinuities in random-shaped outlines. In line with previous findings, the results in a lorazepam-treated group were quite different from the results in a placebo-treated group. The results in a diazepam-treated group were analogous to the results in the placebo-treated group and different from the results in the lorazepam-treated group. This shows that lorazepam and diazepam differ, not only with respect to their effect on implicit memory, but also with respect to their effect on perceptual integration. It is argued that this bears important consequences for memory research that makes use of a pharmacological dissociation rationale.

Human observers were shown projected angles, embedded in solid cross-like figures and were asked whether these projected angles could be the projection of an orthogonal angle in 3-D space (i.e. whether the two legs of the cross were orthogonal to each other). We found that performance depended on the viewpoint at which the angle was viewed: Both slant (i.e. the angle between the normal of the target angle relative to the plane of projection) and roll (i.e. the rotation around the normal of the target angle) had a systematic effect on the proportion of errors when observers were shown non-orthogonal angles. With orthogonal angles, however, this effect was absent (i.e. very low error rate with no systematic effect of slant and roll). Instead of assuming a viewpoint-dependent bias towards orthogonality, a computational analysis of the task, using a Bayesian approach, and a computer simulation showed that the viewpoint-dependency can be modelled by a fixed set of biases in order to constrain the set of possible scenes that could give rise to the projection.

In three experiments a simple Euclidean transformation (reflection, translation, rotation) was applied to collections of twelve dots in such a way that they contained equal lower-order structure, defined on the pairwise grouping of elements with their partner following transformation (e.g. parallel virtual lines), but differed in the presence vs absence of higher-order structure, defined on pairs of pairwise groupings (e.g. virtual quadrangles with correlated angles). Based on the much better performance levels (d') in the case of additional higher-order structure, we conclude that global regularities are easier to detect when the local correspondences are supported by higher-order ones formed between them. These enable the lower-order groupings to spread out across the whole pattern very rapidly (called bootstrapping). As a preliminary attempt to specify these principles, we proposed a working model with two basic components: first, a function expressing the cost of a perceptual grouping or the lack of regularity, and, secondly, an algorithm based on simulated annealing to minimize the cost function. The simulation results obtained with our current implementation of these principles showed satisfactory qualitative agreement with human regularity detection performance. Finally, the theory was shown to capture the essence of a large number of grouping phenomena taken from diverse domains such as detection of symmetry in dot patterns, global structure in Glass and vector patterns, correspondence in stereoscopic transparency and apparent motion. Therefore, we are convinced that, in principle, the mechanism used by the human visual system to detect regularity incorporates something like bootstrapping based on higher-order structure. We regard this as a promising step towards unraveling the intriguing mechanisms of classic Gestalt phenomena.