Search PubMed⌕ Search

Biomedical subjects

C van Leeuwen

Publications and source records attributed to C van Leeuwen.

14 recordsLinked to original sources
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↗