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Biomedical subjects

C van Leeuwen

Publications and source records attributed to C van Leeuwen.

At least 19 recordsLinked to original sources
PubMed1997

Visual search strategy and perceptual organization covary with individual preference and structural complexity.

The pattern of interactions between visual search profile (serial versus nonserial) and perceptual organization type (functionally local versus global) was investigated in three experiments. The task was a matching judgment performed on two separate figures that fit in a jigsaw-puzzle fashion. Reaction times and errors showed that serial search preferably goes together with local organization and nonserial search with global organization. Choice of search profile and organization type depend on task and individual preference. Structural complexity in the target area reinforces the combination of local organization and serial search. This combination was also chosen by subjects preferring accuracy. The combination of global organization and nonserial search was chosen with simpler targets or by subjects preferring speed. The results support an interactive notion of perceptual organization and search. Perceptual organization type is determined by individual preferences in combination with visual search task demands; visual search is guided by the specific organization of the stimulus pattern.

Humans↗
PubMed1995

Seven new mutations in hMSH2, an HNPCC gene, identified by denaturing gradient-gel electrophoresis.

Hereditary nonpolyposis colorectal cancer (HNPCC) is a relatively common autosomal dominant cancer-susceptibility condition. The recent isolation of the DNA mismatch repair genes (hMSH2, hMLH1, hPMS1, and hPMS2) responsible for HNPCC has allowed the search for germ-line mutations in affected individuals. In this study we used denaturing gradient-gel electrophoresis to screen for mutations in the hMSH2 gene. Analysis of all the 16 exons of hMSH2, in 34 unrelated HNPCC kindreds, has revealed seven novel pathogenic germ-line mutations resulting in stop codons either directly or through frameshifts. Additionally, nucleotide substitutions giving rise to one missense, two silent, and one useful polymorphism have been identified. The proportion of families in which hMSH2 mutations were found is 21%. Although the spectrum of mutations spread at the hMSH2 gene among HNPCC patients appears extremely heterogeneous, we were not able to establish any correlation between the site of the individual mutations and the corresponding tumor spectrum. Our results indicate that, given the genomic size and organization of the hMSH2 gene and the heterogeneity of its mutation spectrum, a rapid and efficient mutation detection procedure is necessary for routine molecular diagnosis and presymptomatic detection of the disease in a clinical setup.

Base Sequence↗
PubMed1994

A targeted chain-termination mutation in the mouse Apc gene results in multiple intestinal tumors.

Germ-line mutations in the human adenomatous polyposis coli (APC) gene result in familial adenomatous polyposis, an autosomal dominant disorder characterized by the early onset of multiple adenomatous polyps in the large bowel with a high likelihood of developing colorectal carcinomas. To understand the role of APC in intestinal tumor formation, we have introduced a chain-termination mutation in the 15th exon of the mouse Apc gene and employed it to modify the endogenous gene by homologous recombination in embryonic stem cells. Mice which are heterozygous for the Apc gene modification progressively develop intestinal tumors in a manner that is similar to that observed in patients with familial adenomatous polyposis and in mice which carry a mutation called multiple intestinal neoplasia (Min). Our results indicate that the Apc gene modification is a critical event in the initiation of intestinal tumor formation and results in an autosomal dominant predisposition toward development of spontaneous colonic and intestinal tumors in mice.

Adenomatous Polyposis Coli↗
PubMed1994

Rapid detection of translation-terminating mutations at the adenomatous polyposis coli (APC) gene by direct protein truncation test.

Familial adenomatous polyposis (FAP) is usually associated with protein truncating mutations in the adenomatous polyposis coli (APC) gene. The APC mutations are known to play a major role in colorectal carcinogenesis. For the identification of protein truncating mutations of the APC gene, we developed a rapid, sensitive, and direct screening procedure. The technique is based on the in vitro transcription and translation of the genomic PCR products and is called the protein truncation test. Samples of DNA from individual FAP patients, members of a FAP family, colorectal tumors, and colorectal tumor-derived cell lines were used to show the effectiveness of this method.

Adenomatous Polyposis Coli↗
PubMed1994

Affordances, perceptual complexity, and the development of tool use.

Perceiving the affordance of a tool requires the integration of several complementary relationships among actor, tool, and target. Highers order affordance structures are introduced to deal with these forms of complex action from an ecological-realist point of view. The complexity of the higher order affordance structure was used to predict the difficulty of perceiving the tool function. Predictions were tested in 3 experiments involving children between 9 months and 4 years old. In a classical tool use task dating back to W. Köhler, a desirable target was obtained by using a hook as a tool. The relative positions of the hook and the target were systematically varied to obtain structures differing in complexity. The observed difficulty of the task was found essentially in accordance with the theoretical complexity of the higher order affordance structures involved in perceiving the tool function.

Child, Preschool↗
PubMed1993

Context influence on the perception of figures as conditional upon perceptual organization strategies.

Two experiments tested the effect of context on figure perception. Subjects were shown rapid sequences of three figures: a prime, a whole, and a part. They were asked to decide if the third figure was a part of the second (Experiment 1) or if the second and third figures were the same or different with respect to a particular angle (Experiment 2). The prime served to establish a context for stimuli that followed to be compared. Priming had influence on the part-whole comparison in Experiment 1, but not on the local comparison in Experiment 2. The results of Experiment 1 were interpreted as evidence for a role of prior information in perceptual organization. Experiment 2 showed that the task must require an integrative perceptual organization strategy for the priming effects to occur.

Adult↗
PubMed1991

Testing organization preferences in serial pattern learning.

Subjects studied a series of colored dots for 6 s and then performed a memory reproduction task for which they chose alternative puzzle pieces from a set. Series were presented repeatedly until subjects made a completely correct reproduction. The puzzle pieces contained parts of the series that were expected to interact with the groups identified by spontaneous perceptual organization. By assuming that same grouping in series and puzzle pieces would be preferred, the preferences could be predicted on the basis of an economy principle. The preferences obtained were in accordance with the predictions.

Attention↗
PubMed1991

What has happened to Prägnanz? Coding, stability, or resonance.

Three theoretical measures of Prägnanz were compared with four data sets. The theoretical measures were a stimulus-coding one (structural information load, SIL), a measure related to within memory processes (stability), and one based on the interaction of perception and memory (resonance). The four data sets were obtained in two experiments and involved goodness rating, grouping, and immediate and delayed recall. A complete set of seven-element binary serial patterns was used in each experiment. Both SIL and resonance were shown to correlate reliably with the data sets across tasks. The resonance measure, however, performed best. Prägnanz thus appears to be explained better by resonance than by stimulus coding or memory storage. Resonance explained all systematic variance in the recall tasks, but not in the other tasks. Regarding these, partial-correlation analyses showed that the effect of stability could be fully reduced to resonance. SIL could not be similarly reduced. Therefore, additional perceptual constraints, other than resonance, would be needed for a complete account of goodness in the judging or grouping tasks.

Adult↗
PubMed1990

Indeterminacy of the isomorphism heuristic.

We are less prone today than in Köhler's time to believe in a unified science. The Gestalt program could therefore safely abandon the psychophysical isomorphism heuristic if it wished to. Indeterminacy of the heuristic might be a reason for doing so. The determination of the components involved in the isomorphism of the visual field as well as the electrocortical events would have to occur simultaneously. In the Gestalt program, however, components are determined by their position in the whole. They can therefore not be compared in different contexts, so no independent test of a candidate heuristic is possible.

Arousal↗
PubMed1990

Perceptual-learning systems as conservative structures: is economy an attractor?

It is discussed whether information-processing or connectionist models might explain the law of Prägnanz under constraints resulting from a physical interpretation of these models. An information-processing approach is contrasted with thermodynamics and, finally, an alternative approach more directly inspired by Gestalt is discussed. It is argued that this model could be physically realized with relatively moderate requirements on the substrate in terms of information-processing resources.

Animals↗
PubMed1990

Insertion elements on lactococcal proteinase plasmids.

DNA segments of 809 and 808 nucleotides, with 18-base-pair terminal inverted repeats, are present on the proteinase plasmids pWV05 from Lactococcus lactis subsp. cremoris Wg2 and pSK111 from L. lactis subsp. cremoris SK11, respectively. These DNA segments are highly similar: 77% identical nucleotides and both contain an open reading frame that can encode a protein of 226 amino acids. Furthermore, both DNA segments are located downstream of the proteinase maturation gene prtM, but they differ individually in their orientation with respect to the prtM gene. On the basis of the striking similarity between ISS1, an 808-base-pair insertion sequence (IS) from L. lactis subsp. lactis ML3 lactose plasmid pSK08, and the DNA segments of pWV05 and pSK111, we propose that these DNA segments comprise IS elements. The IS elements from strains Wg2 and SK11 were named ISS1W and ISS1N, respectively. On pWV05, ISS1W is flanked on one side by only part of a second IS element, indicating that pWV05 evolved as a deletion derivative of a precursor plasmid that carried at least two IS elements.

Amino Acid Sequence↗
PubMed1989

Transport of basic amino acids by membrane vesicles of Lactococcus lactis.

The uptake of the basic amino acids arginine, ornithine, and lysine was studied in membrane vesicles derived from cells of Lactococcus lactis which were fused with liposomes in which beef heart mitochondrial cytochrome c oxidase was incorporated as a proton motive force (PMF)-generating system. In the presence of ascorbate N,N,N'N'-tetramethylphenylenediamine-cytochrome c as the electron donor, these fused membranes accumulated lysine but not ornithine or arginine under aerobic conditions. The mechanism of energy coupling to lysine transport was examined in membrane vesicles of L. lactis subsp. cremoris upon imposition of an artificial electrical potential (delta psi) or pH gradient or both and in fused membranes of these vesicles with cytochrome c oxidase liposomes in which the delta psi and delta pH were manipulated with ionophores. Lysine uptake was shown to be coupled to the PMF and especially to the delta psi, suggesting a proton symport mechanism. The lysine carrier appeared to be specific for L and D isomers of amino acids with a guanidine or NH2 group at the C6 position of the side chain. Uptake of lysine was blocked by p-chloromercuribenzene sulfonic acid but not by maleimides. Counterflow of lysine could not be detected in L. lactis subsp. cremoris, but in the arginine-ornithine antiporter-containing L. lactis subsp. lactis, rapid counterflow occurred. Homologous exchange of lysine and heterologous exchange of arginine and lysine were mediated by this antiporter. PMF-driven lysine transport in these membranes was noncompetitively inhibited by arginine, whereas the uptake of arginine was enhanced by lysine. These observations are compatible with a model in which circulation of lysine via the lysine carrier and the arginine-ornithine antiporter leads to accumulation of arginine.

Amino Acids, Diamino↗
PubMed1988

Sequence influence on the organization of meaningless serial stimuli: economy after all.

This article provides evidence for an extension of structural information theory, a theory which describes perceptual organization formally, into a more general theory of representation that takes the role of organizations obtained earlier into account. In eight experiments, subjects study series of colored dots. Each series contained 6-8 dots of different colors, and viewing time was between 400 ms and 60 s. In several experimental procedures, subjects recalled the series presented. The experiments systematically varied the economy of the organizations of the series as described by structural information theory as well as a number of aspects of the context in which the series were presented, like order of presentation of a series, order between the series, viewing span, viewing time, and recall procedure. Recall was influenced both by the economy as well as by the context. It is shown how the context influences the relative strength of the most economical organization as compared with rival, local organizations. A structured network model is presented that accounts for the influence of both the economy and the context.

Adult↗