Biomedical subjects

J Wagemans

Publications and source records attributed to J Wagemans.

13 recordsLinked to original sources

PubMed1997

Higher-level cortical processing of colour.

Wavelength information serves both to organise shapes and to provide the appearance of surfaces. These two roles are considered within a model that outlines the role of colour in picture naming (Davidoff and de Bleser, 1993). An introduction to the papers in this issue is given within the framework of the model. The topics in this issue cover the role of colour in scene segmentation, colour constancy and the memory for the colours of objects.

Cerebral Cortex↗

PubMed1997

Perceptual grouping measured by color assimilation: regularity versus proximity.

Two powerful grouping principles in visual perception have been studied rather frequently, namely, regularity and proximity. In many cases, however, the impact of either regularity or proximity was examined. The goal of the present research is to provide and test an experimental paradigm on the basis of which the relative strengths of these grouping factors can be studied. The proposed paradigm is based on early Gestalt observations that colors of local elements within a perceptual whole tend to look more like each other than they actually are. Three experiments were set up to test whether this phenomenon of color assimilation can provide a means to compare perceptual groupings induced by regularity and proximity. In all experiments, the stimuli were linear configurations of colored elements with variable distances between the elements. The task of the participants was to compare the color of a single test element with the colors of two different groups of elements. In Experiment 1, for all colors, the color of the test element was judged to be more similar to the color of the elements with which it can be grouped by means of regularity. In Experiment 2, for specific colors, the color of the test element tended to be more similar to the elements with which it can be grouped by means of proximity. Finally, in Experiment 3, configurations with a greater variety of positions were tested, including those yielding conflicting groupings, that is, on the basis of regularity and proximity different groupings were to be expected. The color judgements for these stimuli clearly reflected the competition between the two factors.

Color Perception↗

PubMed1996

Qualitative cues in the discrimination of affine-transformed minimal patterns.

An important factor in judging whether two retinal images arise from the same object viewed from different positions may be the presence of certain properties or cues that are 'qualitative invariants' with respect to the natural transformations, particularly affine transformations, associated with changes in viewpoint. To test whether observers use certain affine qualitative cues such as concavity, convexity, collinearity, and parallelism of the image elements, a 'same-different' discrimination experiment was carried out with planar patterns that were defined by four points either connected by straight line segments (line patterns) or marked by dots (dot patterns). The first three points of each pattern were generated randomly; the fourth point fell on their diagonal bisector. According to the position of that point, the patterns were concave, triangular (three points being collinear), convex, or parallel sided. In a 'same' trial, an affine transformation was applied to one of two identical patterns; in a 'different' trial, the affine transformation was applied after the point lying on the diagonal bisector was perturbed a short, fixed distance along the bisector, inwards for one pattern and outwards for the other. Observers' ability to discriminate 'same' from 'different' pairs of patterns depended strongly on the position of the fourth, displaced, point: performance varied rapidly when the position of the displaced point was such that the patterns were nearly triangular or nearly parallel sided, consistent with observers using the hypothesised qualitative cues. The experimental data were fitted with a simple probabilistic model of discrimination performance that used a combination of these qualitative cues and a single quantitative cue.

Adult↗

PubMed1995

Detection of visual symmetries.

This paper reviews empirical evidence for the detection of visual symmetries and explanatory theories and models of symmetry detection. First, mirror symmetry is compared to other types of symmetry. The idea that symmetry detection is preattentive is then discussed and other roles that attention might play in symmetry detection are considered. The major part of the article consists of a critical examination of the extensive literature about the effects on symmetry detection of several major factors such as the orientation of the symmetry axis, the location of the stimulus in the visual field, grouping, and perturbations. Constraints on plausible models of symmetry detection are derived from this rich database and several proposals are evaluated against it. As a result of bringing this research together, open questions and remaining gaps to be filled by future research are identified.

Animals↗

PubMed1994

Perceptual organisation and object recognition--POOR is the acronym, rich the notion.

Instead of studying perceptual organisation and object recognition in relative isolation, they can be viewed as two highly related sets of processes performed by the visual system to achieve its goal of acquiring information about the world. Fifteen papers devoted to specific subproblems within this active area of research have been brought together in two successive issues of Perception. Collectively they demonstrate that focusing on the functional interrelationships between perceptual organisation and object recognition will enrich our understanding of each of the subprocesses involved. The editorial provides an overview of the papers together with a discussion on how they relate to one another. If a general message is to be extracted from this set of papers, it is that the reported findings and the speculations offered to explain them suggest that the visual system's processes cannot be characterised in general by simple dichotomies such as analytic versus wholistic, bottom-up versus top-down, local versus global, low-level versus high-level, parallel versus serial, etc. Instead, it appears that a wide variety of mechanisms is available to the visual system. Therefore, a complete understanding of its functioning will require careful examination of the circumstances within which one processing mechanism seems to be selected over another, depending on the available information, the task demands, and perhaps even the observer's individual characteristics.

Humans↗

PubMed1994

Invariance from the Euclidean geometer's perspective.

It is remarkable how well the human visual system can cope with changing viewpoints when it comes to recognising shapes. The state of the art in machine vision is still quite remote from solving such tasks. Nevertheless, a surge in invariance-based research has led to the development of methods for solving recognition problems still considered hard until recently. A nonmathematical account explains the basic philosophy and trade-offs underlying this strand of research. The principles are explained for the relatively simple case of planar-object recognition under arbitrary viewpoints. Well-known Euclidean concepts form the basis of invariance in this case. Introducing constraints in addition to that of planarity may further simplify the invariants. On the other hand, there are problems for which no invariants exist.

Attention↗

PubMed1993

Higher-order structure in regularity detection.

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.

Cues↗

PubMed1993

Skewed symmetry: a nonaccidental property used to perceive visual forms.

Mathematically, skewed symmetry is a nonaccidental property because it can be interpreted as bilateral symmetry in depth viewed from a nonorthogonal angle. To find out whether this is a useful property in the perception of visual forms, 4 experiments were designed in which the Ss had to determine whether 2 symmetric or random patterns were the same regardless of possible affine transformations between them. The results provided mixed evidence: Although there was always a large symmetry advantage, skewed symmetry was only perceived as bilateral symmetry in depth for dot patterns with higher order types of symmetry (Experiment 1), when the dots were connected to form closed polygons (Experiments 2 and 4), or when they were surrounded by a frame to enhance their planarity (Experiment 3). In other cases, Ss relied on local groupings on the basis of proximity or curvilinearity, which are qualitatively affine invariant.

Attention↗

PubMed1993

Effects of element type and spatial grouping on symmetry detection.

The influence of local and global attributes of symmetric patterns on the perceptual salience of symmetry was investigated. After tachistoscopic viewing, subjects discriminated between symmetric and either random patterns (experiment 1) or their perturbed counterparts (experiment 2) created by replacing one third of the mirror element-pairs of symmetric stimuli with 'random' elements. In general, it was found that perceptibility of symmetry, measured by response time and detection accuracy, was not influenced in a consistent way by type of pattern element (dots or line segments oriented vertically, horizontally, obliquely, or in all three orientations about the symmetry axis). Nor did axis orientation (vertical, horizontal, oblique), advance knowledge of axis orientation, practice effects, or subject sophistication differentially affect detection. A highly salient global percept of symmetry emerged, on the other hand, when elements were clustered together within a pattern, or grouped in symmetric pairs along a single symmetry axis or two orthogonal axes. Results suggest that mirror symmetry is detected preattentively, presumably by some kind of integral code which emerges from the interaction between display elements and the way they are organized spatially. It is proposed that symmetry is coded and signalled by the same spatial grouping processes as those responsible for construction of the full primal sketch.

Attention↗

PubMed1992

Perceptual use of nonaccidental properties.

Under the assumption of a general viewpoint, particular image properties, such as cotermination, straightness, and parallelism, can be used to infer, more or less reliably, the corresponding characteristics in the world. In this paper, the literature about these nonaccidental properties (NAPs) is reviewed to trace its historical roots, to list the properties that function as NAPs, and to discuss the psychological evidence for their detection and use. Against this background, four experiments are reviewed and four are fully described that were designed to test the perceptual use of skewed symmetry (SS), which results from orthographic projection of planar bilateral or mirror symmetry (BS). Despite the large symmetry advantage obtained in all experiments, SS is only perceived as BS-in-depth in cases of closed polygons or dot patterns with higher-order types of symmetry. In all random dot patterns and in some symmetric patterns with low "Gestalt", subjects relied on more local groupings which are qualitatively affine invariant, such as clusters based on proximity or curvilinearity. Based on previous approaches in the literature and these new findings, I suggest some distinctions between different ways of using NAPs, which might foster further research.

Female↗

PubMed1991

Detection of symmetry in tachistoscopically presented dot patterns: effects of multiple axes and skewing.

We examined the effects of multiple axes and skewing on the detectability of symmetry in tachistoscopically presented (100-msec) dot patterns to test the role of normal grouping processes based on higher order regularities in element positions. In addition to the first-order regularities of orientational uniformity and midpoint collinearity (Jenkins, 1983), bilateral symmetry (BS) gives rise to second-order relations between two pairs of symmetric elements (represented by correlation quadrangles). We suggest that they allow the regularity (i.e., BS) to emerge simply as a result of the position-based grouping that takes place normally, so that no special symmetry-detection mechanism has to be postulated. In combination with previously investigated variables--number and orientation of axes--we introduced skewing (resulting from orthographic projection of BS) to manipulate the kind and number of higher order regularities. In agreement with our predictions, the data show that the effect of skewing angle (varied at three 15 degrees steps, clockwise and counterclockwise) on the preattentive detectability of symmetry (measured with d') increases as the number of axes decreases. On the basis of some more specific findings, we suggest that it is not as much the number of correlation quadrangles that determines the saliency of a regularity as it is the degree to which they facilitate or "bootstrap" each other.

Attention↗

PubMed1990

"Smart" mechanisms emerging from cooperation and competition between modules.

Three sets of experiments are described. The first set concerns the detection of the 3-D possibility of shaded 2-D block patterns. The data indicate that the human perceptual system is able to do this above chance level, but in a specific and restricted way, which suggests the possibility of a module devoted to it. The second set concerns the determination of the 3-D orientation of coloured 2-D patterns. The data show that the human perceptual system uses physical constraints of colour mixing in doing this. Again, the mechanism seems to have characteristics suggesting modularity. The third set is about the detection of skewed symmetry in dot patterns. Skewed symmetry can be used as a source of information about the slant and tilt of a surface with bilateral symmetry present on it. Therefore, a module for recovering this information might exist. All these empirical data show that Fodor's criteria of modularity can be used to test hypotheses about visual mechanisms recovering 3-D information from 2-D inputs. But this does not imply that the results prove modularity and cannot be interpreted otherwise. As an exercise the same story about these experiments is told with the use of jargon from ecological realism and connectionist approaches. It is concluded that some of the approaches to some perceptual mechanisms are not as divergent as they might seem. The main theme underlying modular, connectionist, and ecological approaches is the avoidance of central intelligence agencies by the incorporation of physical constraints. Within this broad framework different questions can be asked and answers attempted that may depend on personal taste.

Adult↗

PubMed1989

The effects of kinetic occlusion and categorization on amodal completion. A comment on Gerbino and Salmaso (1987).

The experimental study of Gerbino and Salmaso (1987) focused on the process of amodal completion, i.e. the process that leads to the impression of a total pattern in the absence of information about all parts of the pattern. One of the questions that they have attempted to answer with respect to this process is whether or not it depends on the categorization of the modal part (i.e. the part about which visual information is present) as a modification of a complete prototype. The major conclusion of their experiments is that amodal completion does not depend upon categorization. Furthermore, they interpret their results as indicating that neither the truncated form of the occluded part nor the unification/segregation of contour segments are phenomenally real. We designed some variants of the tunnel effect to demonstrate (1) that the explicit categorization of some parts of the input can have an influence on amodal completion and (2) that the truncated form of a pattern can be phenomenally real. Our results corroborate these hypotheses and can, therefore, interpreted as refuting two rather central claims of Gerbino and Salmaso (1987). The conclusion of our experiment must be that amodal completion is not as primary as they have supposed but can be influenced by kinetic occlusion and categorization stored in visual memory.

Adult↗