Configural Gestalts remain nothing more than the sum of their parts in visual agnosia

Skeletal representations of shape in the human visual cortex

Shape perception is crucial for object recognition. However, it remains unknown exactly how shape information is represented and used by the visual system. Here, we tested the hypothesis that the visual system represents object shape via a skeletal structure. Using functional magnetic resonance imaging (fMRI) and representational similarity analysis (RSA), we found that a model of skeletal similarity explained significant unique variance in the response profiles of V3 and LO. Moreover, the skeletal model remained predictive in these regions even when controlling for other models of visual similarity that approximate low-to high-level visual features (i.e., Gabor-jet, GIST, HMAX, and AlexNet), and across different surface forms, a manipulation that altered object contours while preserving the underlying skeleton. Together, these findings shed light on shape processing in human vision, as well as the computational properties of V3 and LO. We discuss how these regions may support two putative roles of shape skeletons: namely, perceptual organization and object recognition.

Gestalts at threshold could reveal Gestalts as predictions

We review and revisit the predictive processing inspired “Gestalts as predictions” hypothesis. The study of Gestalt phenomena at and below threshold can help clarify the role of higher-order object selective areas and feedback connections in mid-level vision. In two psychophysical experiments assessing manipulations of contrast and configurality we showed that: (1) Gestalt phenomena are robust against saliency manipulations across the psychometric function even below threshold (with the accuracy gains and higher saliency associated with Gestalts being present even around chance performance); and (2) peak differences between Gestalt and control conditions happened around the time where responses to Gestalts are starting to saturate (mimicking the differential contrast response profile of striate vs. extra-striate visual neurons). In addition, Gestalts are associated with steeper psychometric functions in all experiments. We propose that these results reflect the differential engagement of object-selective areas in Gestalt phenomena and of information- or percept-based processing, as opposed to energy- or stimulus-based processing, more generally. In addition, the presence of nonlinearities in the psychometric functions suggest differential top-down modulation of the early visual cortex. We treat this as a proof of principle study, illustrating that classic psychophysics can help assess possible involvement of hierarchical predictive processing in Gestalt phenomena.

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

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

Individual differences in processing orientation and proximity as emergent features

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.

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.

Weber's law in 2D and 3D grasping

Visually guided grasping movements directed to real, 3D objects are characterized by a distinguishable trajectory pattern that evades the influence of Weber's law, a basic principle of perception. Conversely, grasping trajectories directed to 2D line drawings of objects adhere to Weber's law. It can be argued, therefore, that during 2D grasping, the visuomotor system fails at operating in analytic mode and is intruded by irrelevant perceptual information. Here, we explored the visual and tactile cues that enable such analytic processing during grasping. In Experiment 1, we compared grasping directed to 3D objects with grasping directed to 2D object photos. Grasping directed to photos adhered to Weber's law, suggesting that richness in visual detail does not contribute to analytic processing. In Experiment 2, we tested whether the visual presentation of 3D objects could support analytic processing even when only partial object-specific tactile information is provided. Surprisingly, grasping could be performed in an analytic fashion, violating Weber's law. In Experiment 3, participants were denied of any haptic feedback at the end of the movement and grasping trajectories again showed adherence to Weber's law. Taken together, the findings suggest that the presentation of real objects combined with indirect haptic information at the end of the movement is sufficient to allow analytic processing during grasp.

Brain-decoding fMRI reveals how wholes relate to the sum of parts

The human brain performs many nonlinear operations in order to extract relevant information from local inputs. How can we observe and quantify these effects within and across large patches of cortex? In this paper, we discuss the application of multi-voxel pattern analysis (MVPA) in functional magnetic resonance imaging (fMRI) to address this issue. Specifically, we show how MVPA (i) allows to compare various possibilities of part combinations into wholes, such as taking the mean, weighted mean, or the maximum of responses to the parts; (ii) can be used to quantify the parameters of these combinations; and (iii) can be applied in various experimental paradigms. Through these procedures, fMRI helps to obtain a computational understanding of how local information is integrated into larger wholes in various cortical regions.

Reliability and validity of the Leuven Perceptual Organization Screening Test (L-POST)

Neuropsychological tests of visual perception mostly assess high-level processes like object recognition. Object recognition, however, relies on distinct mid-level processes of perceptual organization that are only implicitly tested in classical tests. Furthermore, the psychometric properties of the existing instruments are limited. To fill this gap, the Leuven perceptual organization screening test (L-POST) was developed, in which a wide range of mid-level phenomena are measured in 15 subtests. In this study, we evaluated reliability and validity of the L-POST. Performance on the test is evaluated relative to a norm sample of more than 1,500 healthy control participants. Cronbach's alpha of the norm sample and test-retest correlations for 20 patients provide evidence for adequate reliability of L-POST performance. The convergent and discriminant validity of the test was assessed in 40 brain-damaged patients, whose performance on the L-POST was compared with standard clinical tests of visual perception and other measures of cognitive function. The L-POST showed high sensitivity to visual dysfunction and decreased performance was specific to visual problems. In conclusion, the L-POST is a reliable and valid screening test for perceptual organization. It offers a useful online tool for researchers and clinicians to get a broader overview of the mid-level processes that are preserved or disrupted in a given patient.