Biomedical subjects

P van der Helm

Publications and source records attributed to P van der Helm.

5 recordsLinked to original sources

PubMed2000

A code-theoretical note on object handedness.

This study is a theoretical exercise dealing with discrimination between images and mirror-images. It focuses on the way codes of shapes represent their handedness. We compare two code systems with different reference frames. These frames determine the specific sensitivity of each system. One system uses an asymmetric reference frame. It is called the H-system and was inspired by an idea of Corballis (1988 Psychological Review 95 115-123). The other system, being our proposal, uses a symmetric reference frame and we have named it the M-system. We demonstrate the following. A code of the H-system provides a cue for the handedness of a shape, but not for rotation, i.e. no cue for the appropriate kind of code rotation which should be tested in case images and mirror-images are discerned by mental rotation. The M-system is the converse in both respects. A code of this system does not provide a handedness cue but, instead, a rotation cue. Thus, for handedness discrimination, the H-system neither needs nor guides mental rotation, whereas the M-system does both. This M-system generates object-centred structural codes enriched with viewpoint information. Various visual experiments reported in the literature favour the M-system over the H-system, implying that perception does not make use of an asymmetric but of a symmetric reference frame.

Form Perception↗

PubMed1995

Overexpression of a tryptophan decarboxylase cDNA in Catharanthus roseus crown gall calluses results in increased tryptamine levels but not in increased terpenoid indole alkaloid production.

The enzyme tryptophan decarboxylase (TDC) (EC 4.1.1.28) catalyses a key step in the biosynthesis of terpenoid indole alkaloids in C. roseus by converting tryptophan into tryptamine. Hardly any tdc mRNA could be detected in hormone-independent callus and cell suspension cultures transformed by the oncogenic T-DNA of Agrobacterium tumefaciens. Supply of tryptamine may therefore represent a limiting factor in the biosynthesis of alkaloids by such cultures. To investigate this possibility, chimaeric gene constructs, in which a tdc cDNA is linked in the sense or antisense orientation to the cauliflower mosaic virus 35S promoter and terminator, were introduced in C. roseus cells by infecting seedlings with an oncogenic A. tumefaciens strain. In the resulting crown gall tumour calluses harbouring the tdc sense construct, an increased TDC protein level, TDC activity and tryptamine content but no significant increase in terpenoid indole alkaloid production were observed compared to empty-vector-transformed tumour calluses. In tumour calluses containing the tdc antisense construct, decreased levels of TDC activity were measured. Factors which might be responsible for the lack in increased terpenoid indole alkaloid production in the tdc cDNA overexpressing crown gall calluses are discussed.

Alkaloids↗

PubMed1994

Integrating global and local aspects of visual occlusion.

The phenomenon of visual occlusion has frequently been studied by means of two-dimensional line drawings. These drawings may elicit various interpretations. Sometimes a mosaic of shapes is seen, sometimes a shape that partly occludes another shape. In the latter case, observers often have a clear idea about the form of the partly occluded shape. Local and global pattern aspects both seem to be decisive with respect to the preferred interpretation. An attempt is made to integrate these aspects by applying the global-minimum principle to the perceptual complexity of three distinct components of those pattern interpretations: (i) The internal structure, dealing with each of the shapes separately, (ii) the external structure, dealing with the positional relation between these shapes, and (iii) the virtual structure, dealing with the occluded parts of the shapes. The perceptual complexity of each of these three components can be expressed in terms of structural information. The hypothesis that the perceptually preferred interpretation is the one for which the total information load is minimal is tested on many patterns stemming from different studies on pattern completion.

Form Perception↗

PubMed1993

Metrical information load of lines and angles in line patterns.

Acute angles and long lines are more complex than obtuse angles and short lines. This source of complexity is called metrical information, for which a measure was proposed by Leeuwenberg (1982). According to this measure, the metrical information of a static pattern is specified as the sum of the mechanical impulses necessary to move an object according to this pattern. The perceptual relevance of this metrical load was tested in an experiment in which judged complexities were gathered for a number of simple line patterns.

Adult↗

PubMed1991

Unity and variety in visual form.

Some stimuli are perceived as unitary patterns, and others as dual or plural patterns. Such 'unity-and-variety' phenomena are explained by various process approaches of perception, such as the global precedence hypothesis, the preattentive orientation detection assumption, and the recognition-by-components model. However, these three approaches, which will be discussed in this paper, each explain a different subset of these phenomena. It will be argued that not only these three subsets but also other unity-and-variety phenomena can be explained from just one point of view by adopting the descriptive minimum principle. This principle states that the preferred interpretation of a pattern is reflected by the simplest of all possible representations of that pattern. The highest hierarchical level in the simplest pattern-representation will be called the 'superstructure' of the pattern. The superstructure of a pattern neither refers necessarily to the largest or global pattern component, nor is assessed necessarily in a primary stage in the perception process. Yet, it will be argued that the superstructure is decisive in determining whether a shape is perceived either as unitary or as dual.

Attention↗