Biomedical subjects

P A van der Helm

Publications and source records attributed to P A van der Helm.

8 recordsLinked to original sources

PubMed2000

Simplicity versus likelihood in visual perception: from surprisals to precisals.

The likelihood principle states that the visual system prefers the most likely interpretation of a stimulus, whereas the simplicity principle states that it prefers the most simple interpretation. This study investigates how close these seemingly very different principles are by combining findings from classical, algorithmic, and structural information theory. It is argued that, in visual perception, the two principles are perhaps very different with respect to the viewpoint-independent aspects of perception but probably very close with respect to the viewpoint-dependent aspects which, moreover, seem decisive in everyday perception. This implies that either principle may have guided the evolution of visual systems and that the simplicity paradigm may provide perception models with the necessary quantitative specifications of the often plausible but also intuitive ideas provided by the likelihood paradigm.

Gestalt Theory↗

PubMed1998

Symmetry and selective attention: a dissociation between effortless perception and serial search.

It is widely assumed that symmetry is an important visual primitive, probably encoded without the need for attention. Julesz's (1981) definition of effortless perception, which states that any stimulus property perceived for exposure durations of 160 msec or less is detected preattentively, contributed greatly to this belief. Single pattern studies confirm that symmetry is detected within this limit. In the present study, however, Julesz's operationalization is compared with the multiple pattern visual search task, to see whether symmetry as a wholistic property is detected in parallel. The results show that symmetry detection times are highly dependent on the number of distractor patterns. The findings are similar for dot patterns, wire polygons, solid block shapes, and simple parentheses. We conclude that symmetry detection per se requires selective attention, but that some related grouping or segmentation mechanism may operate preattentively.

Adult↗

PubMed1997

In support of hierarchy in object representations.

The descriptive minimum principle states that the preferred interpretation of a pattern is reflected by the simplest representation of that pattern. Such a simplest representation generally has a hierarchical structure. The pattern component represented at the highest hierarchical level is said to constitute the "superstructure" of the pattern, and pattern components represented at lower levels are said to constitute the "subordinate" structure. The primed-matching paradigm has been employed in two experiments to test whether superstructures of three-dimensional objects are perceptually more dominant than subordinate structures. In the first experiment, the test pairs consisted of two-dimensional line drawings of three-dimensional objects; each prime was a two-dimensional face of such an object, corresponding to either the superstructure or the subordinate structure. Two priming conditions were employed. In the "literal" condition, the object face was presented as it appeared in the drawing of the object (physical similarity). In the "frontal" condition, the object face was presented in the frontal-parallel plane (representational similarity). Object matching was found to be facilitated more by priming superstructures than by priming subordinate structures. In the second experiment, the order was reversed; the test pairs were composed of the object faces and the object drawings wee taken as primes. Again, there were facilitating effects for both superstructures and subordinate structures, but this time without differentiation between superstructures and subordinate structures.

Adult↗

PubMed1996

Goodness of visual regularities: a nontransformational approach.

Until recently, the transformational approach provided the only available formal analysis of visual regularities like repetition and mirror symmetry. This theoretical study presents a new analysis, based on the recently developed concept of holographic regularity. This concept applies to the intrinsic character of regularity and specifies the unique formal status of perceptually relevant regularities. The crucial point is that the two analyses imply the same structure for repetition but a different structure for mirror symmetry. Transformationally, mirror symmetry is an all-or-nothing property, whereas holographically, it is a graded property. This difference pervades the understanding of both perfect regularities and perturbed regularities. Whereas the transformational approach explains hardly any goodness phenomenon, the holographic approach explains a wide variety of goodness phenomena in a coherent way that is ecologically plausible as well.

Attention↗

PubMed1995

Competing global and local completions in visual occlusion.

In visual occlusion 2 amodal-completion tendencies occur frequently. One tendency leads toward the simplest completed shape (a global completion) and the other to a shape for which the completion itself is as simple as possible (a local completion). Two experimental paradigms were used to test the strengths of these completion tendencies: a drawing task and a simultaneous matching task. The experimental results support the notion that the preference for either a global or a local completion is the consequence of a competition between interpretations. Finally, the authors discuss how the preference for a completion can be predicted by a model that is based on a quantification of both global and local aspects.

Adult↗

PubMed1995

Multiple completions primed by occlusion patterns.

There is a strong tendency to complete a partly occluded shape. Two types of pattern completion, global and local, are frequently reported. By means of the primed-matching paradigm, it has previously been shown that global completions are prevalent for stimuli in which regularity is abundantly present. In our study the primed-matching paradigm is applied to such stimuli in order to find out whether the rival local completion is generated as well. Therefore anomalous completions are added to the experimental design. Priming effects both on global and on local completions are compared with priming effects on those anomalous completions. The data indeed suggest that the occlusion patterns evoked not only a global but also a local completion.

Fixation, Ocular↗

PubMed1994

The dynamics of Prägnanz.

There is quite wide-spread agreement about the relevance of pattern Prägnanz (Koffka, 1935) with respect to the human interpretation of visual patterns. There is less agreement about whether pattern Prägnanz is based solely on pattern information (static) or also on the history of the perceiver (dynamic). In Van Leeuwen and Van den Hof (1991), experimental data concerning serial patterns are presented within the framework of the dynamic-network approach initiated by Buffart (1986, 1987). These experimental data are claimed to give evidence against the static-coding approach initiated by Leeuwenberg (1969, 1971). In the present paper, however, I show first that Buffart's theoretical basis is incorrect, and that in fact Leeuwenberg's static-coding approach is the basis for the dynamic-network approach. Second, I show that those experimental data rather give evidence in favor of the static-coding approach, by using those same data for a test of the most recent static-coding model (Van der Helm & Leeuwenberg, 1991; Van der Helm, Van Lier, & Leeuwenberg, 1992). Finally, I propose a reconciliation between the two approaches, in the sense that the dynamic-network model could be shaped in such a way that it yields a simulation, and maybe even an enrichment, of the static-coding model.

Humans↗

PubMed1992

Serial pattern complexity: irregularity and hierarchy.

In perception research, various models have been designed for the encoding of, for example, visual patterns, in order to predict the human interpretation of such patterns. Each of these encoding models provides a few coding rules to obtain codes for a pattern, each code expressing regularity and hierarchy in that pattern. Some of these models employ the minimum principle which states that the human interpretation of a pattern is reflected by the simplest code for that pattern, ie the simplest code according to a given complexity metric. In this paper a new complexity metric is proposed. This metric is based on a formal analysis of the concept of regularity. Some conclusions of this analysis are sketched. The new metric does not depend on artifacts of the coding rules. It accounts for the amounts of irregularity and hierarchy as represented in a code of a pattern, such that these two amounts can be added to determine the complexity of a code. An experiment is discussed that shows that the new metric performs significantly better than the metrics used previously. In particular, the new metric predicts more local pattern organizations than the old metrics. This implies that various local pattern organizations do not falsify the minimum principle anymore.

Attention↗