Acts of perceptual inquiry: problems for any stimulus-based simplicity theory

Prägnanz in visual perception

How do we perceptually and cognitively organize incoming stimulation? A century ago, Gestalt psychologists posited the law of Prägnanz: psychological organization will always be as 'good' as possible given the prevailing conditions. To make the Prägnanz law a useful statement, it needs to be specified further (a) what a 'good' psychological organization entails, (b) how the Prägnanz tendency can be realized, and (c) which conditions need to be taken into account. Although the Gestalt school did provide answers to these questions, modern-day mentions of Prägnanz or good Gestalt often lack these clarifications. The concept of Prägnanz has been (mis)understood in many different ways, and by looking back on the rich history of the concept, we will attempt to present a more fine-grained view and promote a renewed understanding of the central role of Prägnanz in visual perception and beyond. We review Gestalt psychology's answers to the questions listed above, and also discuss the four main uses of the Prägnanz concept in more detail: (a) a Prägnanz tendency in each organizational process, (b) Prägnanz as a property of a Gestalt, (c) Prägnanz steps as internal reference points, and (d) Prägnanz in relation to aesthetic appreciation. As a key takeaway, Prägnanz is a multifaceted Gestalt psychological concept indicating the "goodness" of an experienced organization. Both the removal of unnecessary details and the emphasis on characteristic features of the overall organization compared to a reference organization can contribute to the emergence of a 'better' Gestalt. The stimulus constellation is not the only factor in determining the goodness of an organization, also the stimulus' interaction with an individual in a specific spatial and temporal context plays a role. Taking the ideas on Prägnanz as a generative framework and keeping the original Gestalt psychological context in mind, future research on perceptual organization can improve our understanding of the principles underlying psychological organization by further specifying how different organizational principles interact in concrete situations. Public significance statement: This paper reviews what a 'good' psychological organization entails, and how the incoming stimulation is clarified in human perception to achieve the best possible psychological organization. The review debunks common misconceptions on the meaning of "goodness" and synthesizes the most important perspectives and developments on "goodness" from its conception until now.

Electrophysiological evidence of the amodal representation of symmetry in extrastriate areas

Extrastriate visual areas are strongly activated by image symmetry. Less is known about symmetry representation at object-level rather than image-level. Here we investigated electrophysiological responses to symmetry, generated by amodal completion of partially-occluded polygon shapes. We used a similar paradigm in four experiments (N = 112). A fully-visible abstract shape (either symmetric or asymmetric) was presented for 250 ms (t0). A large rectangle covered it entirely for 250 ms (t1) and then moved to one side to reveal one half of the shape hidden behind (t2, 1000 ms). Note that at t2 no symmetry could be extracted from retinal image information. In half of the trials the shape was the same as previously presented, in the other trials it was replaced by a novel shape. Participants matched shapes similarity (Exp. 1 and Exp. 2), or their colour (Exp. 3) or the orientation of a triangle superimposed to the shapes (Exp. 4). The fully-visible shapes (t0-t1) elicited automatic symmetry-specific ERP responses in all experiments. Importantly, there was an exposure-dependent symmetry-response to the occluded shapes that were recognised as previously seen (t2). Exp. 2 and Exp.4 confirmed this second ERP (t2) did not reflect a reinforcement of a residual carry-over response from t0. We conclude that the extrastriate symmetry-network can achieve amodal representation of symmetry from occluded objects that have been previously experienced as wholes.

Neural responses to reflection symmetry for shapes defined by binocular disparity, and for shapes perceived as regions of background

Human perception of symmetry is associated with activation in an extended network of extrastriate visual areas. This activation generates an ERP called the Sustained Posterior Negativity (SPN). In most studies so far, the stimuli have been defined by luminance. We tested whether the SPN is present when stimuli are defined by stereoscopic disparity using random dot stereograms (RDS). In Experiment 1, we compared the SPN signal for contours specified by binocular disparity and contours specified by monocular cues. The SPN was equivalent, suggesting that the type of contour does not alter the SPN signal. In Experiment 2 we exploited the unique property of RDS to provide unambiguous figure-ground arrangements. Psychophysical work has shown that symmetry is more easily detected when it is a property of a single object (i.e., within a figure), compared to a property of a gap between two objects (i.e., the ground). Therefore, the target regions in this experiment could either be foreground or background. The SPN onset was delayed when the symmetry was in a ground region. This may be because object formation interferes with the processing of shape information in the ground region.

An early history of the Gestalt factors of organisation

Wertheimer's (1923, Psychologische Forschung 4 301 - 350) idea that the perceptual world is articulated according to factors of organisation is widely acknowledged as one of the most original contributions of Gestalt psychology and stands as a milestone in the history of vision research. An inquiry focused on the forerunners of some of Wertheimer's factors of perceptual organisation is documented here. In fact, in 1900 Schumann described grouping by proximity and by vertical symmetry, and in 1903 G E Müller identified the factors of sameness/similarity and contour. Other authors contributed to the early description of these factors, such as Rubin, who in 1922 originally illustrated grouping by similarity. Even though Wertheimer himself granted these authors due recognition, later psychologists have paid little attention to their contributions. Some possible reasons for this negligence are briefly discussed.

Illusions of perception of 3-D house models

In five experiments some edgelines on different polyhedrons (house models in 3-D) were, from a certain vantage point, optically confluent, ie in optical prolongation of each other in 2-D on the retinal image and on photos. Other edgelines on the same polyhedrons were non-confluent, ie optically separate in 2-D. These conditions were found to lead to five different illusory shapes in 3-D. Five spatiofigural illusions were perceived. From these findings an edgeline principle is formulated that: "a straight edgeline in 2-D, whether confluent or separate, is perceived as a unitary and continuously straight edgeline in 3-D". To this is added a supplementary perceptual principle, an amodal completion principle. In the experiments reported here, the illusory perception of shapes in 3-D with confluent edgelines as well as the veridical perception of other shapes in 3-D with only separate or non-confluent edgelines could all be explained by the edgeline principle and the amodal completion principle. By applying the concepts of edgeline confluence and the edgeline principle, a new explanation of the Kopfermann (1930 Psychologische Forschung 13 293- 364) cube phenomena is proposed together with one example of how to test this explanation experimentally.

Asymmetric priming effects in visual processing of occlusion patterns

In three experiments, we examined the effect of temporal context in amodal completion of partly occluded nontarget figures. In a primed same-different task, test pairs were preceded by a sequence of two primes, one of which was a single, the other a composite figure. Single figures reappeared in the composite ones, which also contained a square that could be viewed, alternatively, as an occluder or as yielding a mosaic fit to the other shape. To measure context influences between single and composite figures, both of which were nontargets, we studied their combined effect as primes on the test pairs of the same-different task, expecting that congruency between both primes should lead to a superadditive priming effect on the task. We found that single figures presented first provided facilitatory context for local and global occlusion as well as for mosaic interpretations of subsequently presented composite figures. These effects occurred only when the composite figure was presented briefly (50 msec). No superadditive facilitation occurred when composite figures were presented first and single figures followed them. The restriction of the effect to short presentations and its temporal asymmetry were taken as evidence that prior context biases possible occlusion interpretations during the process of completion, rather than afterward.

Contour grouping inside and outside of facial contexts

We examine how contours group in isolation compared with when they are embedded in face-like contexts. As previously shown, contours that seem to group by phenomenological observation also show powerful effects on task performance: with contours that group, selective attention to one while ignoring another is poor (as indexed by Garner Interference (GI), but not Stroop Interference), whereas divided attention across contours is good. With contours that do not group, however, the reverse happens. Here we test pairs of curved lines (parentheses) displayed either in isolation or within contexts including cartoon faces, where these curves may serve as mouths or eyebrows. The results with isolated contours replicate previous findings of poor selective attention, but within face-like contexts the same contours showed nearly perfect selective attention (i.e., zero GI). Thus, contour grouping was weaker inside than outside of faces, a finding that contrasts with the widely-held belief that faces are processed configurally, not by local features.

What does a compound letter tell the psychologist's mind?

The paradigm based on using compound stimuli for studying global and local processing is revisited. Noting that not all researchers employ compound stimuli for the same purpose, the issue of its purpose is discussed. It is argued that the paradigm is pertinent for examining at least three notions--formation preference, global addressability, and within-object global precedence. It is suggested that findings in the paradigm are accommodated well by a disjunction of those three perceptual dispositions. A number of further issues associated with the interpretation of findings obtained with it are examined as well. An experimental study is reported that is meant to examine one such issue--a possible artifact putatively introduced by the special attribute of element homogeneity characteristic of compound stimuli. Seven experiments were used to examine to what extent, if at all, global advantage observed in compound stimulus paradigms depends on element heterogeneity. Across those experiments, heterogeneity did not have any effect that could be interpreted as suggesting that the paradigm is biased in favor of the global structure due to element homogeneity.

Miracles of perception

This paper draws a bird's eye view of various counter-intuitive characteristics of perception. Peculiar is that perception is a both tool and topic of its study. As a consequence, its output is easily mistaken for its input. Furthermore, its output is characterized by remarkable Gestalt features, such as mutual dependence of stimulus elements and detour solutions. Detour solutions require a complex perception process of testing countless optional pattern interpretations against a criterion. Likelihood is a plausible criterion for reasoning. For perception, however, the simplicity criterion is more appropriate. The consideration is that reasoning aims at establishing properties of distal objects whereas perception aims at establishing objects from proximal properties. The role of knowledge in perception seems plausible but often leads to conflicts. For instance, the assumption that knowledge about handedness is present in pattern representations conflicts with image mirror-image discrimination data. Moreover, knowledge does not provide an anchor for subjective time direction, but a Gestalt quality does.