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PubMedDate not supplied

[Dynamics of electrodermal activity during changes in the hydration of the epidermis: theoretical model].

The presented theoretical model explains three ways of epidermal hydration changes. These ways are; water movement from both dermal layer and sweat gland ducts to the epidermis, and then, from the epidermis to the air. Epidermal hydration changes influenced electrodermal activity. It was revealed, that epidermal hydration changes impact both amplitudes and latencies of the electrodermal activity.

Body Water↗
PubMed1994

[Surfactant interactions in model and biological membranes. IX. Theoretical model of biological activity in relation to the length of hydrocarbon chains].

A theoretical model is proposed according to which the quasi-parabolic dependence of the biological activity of amphiphilic compounds on their hydrophobic substituent chain length (the "cut-off" effect) is caused by a combination of partition equilibria and geometrical parameters of these compounds interacting with the bilayer of phospholipid membranes. The most important parameter seems to be a change in the thickness of the phospholipid bilayer. The model is quantified for the case of normal alcohols.

Cell Membrane↗
PubMed2005

The effects of hypoxia on the theoretical modelling of tumour control probability.

Theoretical modelling of tumour response is increasingly used for the prediction of treatment result and has even been proposed as ranking criteria in some algorithms for treatment planning. Tumour response to radiation is greatly influenced by the details of tumour microenvironment, especially hypoxia, that unfortunately are not always taken into consideration for these simulations. This paper intends to investigate the effects of various assumptions regarding hypoxia distribution in tumours on the predictions of treatment outcome. A previously developed model for simulating theoretically the oxygenation in biologically relevant tissues, including results from oxygen diffusion, consumption and perfusion limitations in tumours, was used to investigate the effects of the different aspects of hypoxia on the predictions of treatment outcome. Thus, both the continuous distribution of values and the temporal variation of hypoxia patterns were taken into consideration and were compared with a 'black-and-white' simplification with a fully hypoxic compartment and a fully oxic one. It was found that the full distribution of oxygenation in the tissue is needed for accurate results. The 'black-and-white' simplification, while showing the same general trends for the predictions of radiation response, could lead to serious over-estimations of the tumour control probability. It was also found that the presence of some hypoxia for every treatment fraction leads to a decrease in the predicted local control, regardless of the change of the hypoxic pattern throughout the duration of the whole treatment. The results thus suggest that the assumptions regarding tumour hypoxia influence very much the predictions of treatment outcome and therefore they have to be very carefully incorporated into the theoretical modelling.

Cell Hypoxia↗
PubMed1997

[Model development in nursing science; the construction of a theoretical model].

The aim of the paper is to describe the development of nursing theory. As an example, a theoretical model of the compliance of young diabetics and related factors is built. The content of the theoretical model is not described but the process of developing the model is presented. In the first phase, a hypothetical model of young diabetics' compliance was developed inductively. The data were collected by interviewing 51 young diabetics aged 13-17 years, by observing the behaviour of 18 of these young people during an adaptation course and by analysing their drawings (N = 17). The data were analysed by using continuous comparative analyses and content analyses. In the second phase, an instrument for testing the model was developed. For this purpose, data were collected with a questionnaire from young diabetics aged 12-17 years (N = 91). The content validity, construct validity and reliability of the instrument turned out to be quite good. In the third phase, the hypothetical model was tested and developed further by using LISREL (linear structural relations) analyses. The data were collected with a questionnaire from young diabetics aged 13-17 years (N = 346). In the fourth phase, the model was expanded using the qualitative data (N = 51). The categories which had been discovered by continuous comparative analyses were quantified. The data were analysed by cross-tabulation, the chi square test and discriminant analyses. A summary of the results has been presented by building a theoretical model of young diabetics' compliance and related factors.

Adaptation, Psychological↗
PubMed1987

Schizophrenic thought disorder: a model-theoretic perspective.

Model theory is a branch of mathematics that treats such abstruse questions as "is there another number system, different from 0,1,2, ... that satisfies all the axioms of arithmetic?" (yes), and "can every mathematical hypothesis be proved true or false?" (no). It would not seem likely that any contribution could be made to our understanding of though disorder in schizophrenia from such a remote quarter. The detailed results of model theory obviously will not apply; but I believe certain ways of thinking common in that field, but not in psychology, may help lead us toward explanations.

Humans↗
PubMed1990

Contraceptive and sexual behavior of black female adolescents. A test of a social-psychological theoretical model.

To test a previous theoretical model, we studied the relationships between social, attitude, and psychological factors and adolescent contraceptive behavior over a 6-month period. A pretested questionnaire was administered to a random sample of 115 black females aged 12 to 18 years from a low socioeconomic population. Six months after the initial interview 113 of the subjects were administered a second questionnaire assessing their sexual and contraceptive behavior. Based on regression analysis, previous pregnancy and coital frequency explained 12.7% (p less than or equal to 0.0001) of the variation in engaging in unprotected coitus. When controlling for sexual activity, birth control knowledge and previous broken appointments explained an additional 5.5% of variation in the regression model (total R2 = 0.185). Coital frequency of the subjects was positively associated with Tanner stage, length of the relationship, effectiveness of previous contraceptive method(s), and degree of birth control knowledge. Coital frequency was inversely associated with a negative attitude toward pregnancy and negative parental attitudes toward adolescent pregnancy. The perceived risk of pregnancy was positively correlated with coital frequency and previous pregnancy. These findings support our model, but suggest that among low socioeconomic black female adolescents the interpretation of several model components depends on the subjects' attitude toward pregnancy and her perception of her parent's attitude toward adolescent pregnancy.

Adolescent↗
PubMedDate not supplied

Analysis of data fusion methods in virtual screening: theoretical model.

This paper presents a theoretical model of how data fusion can be used to combine the results of multiple similarity searches of chemical databases. The model is based on frequency distributions of similarity values that are fused using a multiple integration over regions defined by the particular fusion rule that is being applied. For pairwise fusion, the resulting double integrals are straightforward to evaluate for simple model distributions. Similarity values for recovered-active and recovered-nonactive frequency distributions are independently modeled using a constant background, linearly biased terms, and a first-order correlated term. The model shows that two standard fusion rules can give performance enhancements in some cases but that the results of fusion are dependent on many factors that, taken together, can lead to seemingly inconsistent levels of enhancement.

Algorithms↗
PubMed1981

Interpretation of particle growth and aggregation patterns obtained from the Coulter counter: a simple theoretical model.

We used a simple theoretical model to stimulate the changes in particle size distribution that occur when crystals grow, or disaggregate to a defined extent. The change in total particle volume was used as an index of growth, whereas aggregation and disaggregation were expressed as the change in total particle number. Indices of inhibition of growth and of aggregation in hypothetical test cases were calculated in relation to a control and compared with the inhibition index derived by the standard method of comparing the distribution of particle volume around a given arbitrary diameter on the particle distribution curve. We concluded that the expression of particle growth and aggregation in terms of total volume and number respectively is a preferable alternative to the standard technique.

Calcium Oxalate↗
PubMed1996

How prayer heals: a theoretical model.

This article presents a theoretical model that outlines various possible explanations for the healing effects of prayer. Four classes of mechanisms are defined on the basis of whether healing has naturalistic or supernatural origins and whether it operates locally or nonlocally. Through this framework, most of the currently proposed hypotheses for understanding absent healing and other related phenomena-hypotheses that invoke such concepts as subtle energy, psi, consciousness, morphic fields, and extended mind-are shown to be no less naturalistic than the Newtonian, mechanistic forces of allopathic biomedicine so often derided for their materialism. In proposing that prayer may heal through nonlocal means according to mechanisms and theories proposed by the new physics, Dossey is almost alone among medical scholars in suggesting the possible limitations and inadequacies of hypotheses based on energies, forces, and fields. Yet even such nonlocal effects can be conceived of as naturalistic; that is, they are explained by physical laws that may be unbelievable or unfamiliar to most physicians but that are nonetheless becoming recognized as operant laws of the natural universe. The concept of the supernatural, however, is something altogether different, and is, by definition, outside of or beyond nature. Herein may reside an either wholly or partly transcendent Creator-God who is believed by many to heal through means that transcend the laws of the created universe, both its local and nonlocal elements, and that are thus inherently inaccessible to and unknowable by science. Such an explanation for the effects of prayer merits consideration and, despite its unprovability by medical science, should not be dismissed out of hand.

Humans↗
PubMed2005

[Uncertainties about the self and the issue of the proper theoretical model in immunology].

Theoretical immunology constitutes a critical basis of all medical discoveries. Immunology has been dominated since the 1940s by the self/nonself model. Here we try to shed light on the origins of this theoretical model and to show how and why this model has been called into question during the last thirty years. This paper has three aims. Firstly, we explore the sources of the immune self, going upstream from immunology to ecology-biology, psychology and eventually philosophy. Here the key questions : is the immune self really analogous with the philosophical and psychological selves in which it originates? What is the signification and adequacy of such a conceptual borrowing? We suggest that the << self >> vocabulary in immunology is not clear and precise. Secondly, we present the experimental inadequacies of the self/non-self model. We show then how both the vagueness of the term << self >> and these experimental flaws casted doubt on theories of immunology. Among the several models that have been proposed recently, none has attracted a consensus. Some immunologists have even suggested that immunology should rid itself of theorical concerns and concentrate on molecular aspects. This suggestion, however, is unacceptable\; hence it is still necessary to find a theoretical framework for immunology. Finally, we try to suggest a way to escape this uncomfortable situation of doubt. The immune << self >> and the immune << system >> (<< network >>) are rooted in strong metaphysical conceptions of identity, the main characteristic of which is to consider the organism as an enclosed and self-constructing entity. By contrast, based on experimental data about immune tolerance and host-pathogen interactions, we propose to consider organisms as open entities. To what theory does this conception lead? What would be the consequences of such a theory with regard to medical aspects?

Allergy and Immunology↗
PubMed1993

A survey on quantitative microdialysis: theoretical models and practical implications.

The existing methods of quantitative microdialysis are reviewed. The methods are divided into 8 groups, depending on the mathematical models and theoretical principles used to describe convective diffusion in the extracellular space of the brain. Special emphasis is made to describe each method from a historical perspective, showing its main contribution to recent knowledge, as well as its limitations and drawbacks. It is concluded that those methods based on explicitly derived equations for in vivo recovery are still too approximative and not suitable for routine application. Therefore, empirical models based on varying perfusion flow rates or concentrations of substances in the perfusion solution, found several practical implications. Methods using a reference substance as a marker of in vivo recovery are also discussed. The paper stresses the increasing importance of methods allowing the quantitative evaluation of microdialysis data whenever measuring neurotransmitter release, drug concentrations or pharmacokinetic variables.

Animals↗
PubMedDate not supplied

[Altered states of consciousness: theoretical models and concepts].

The main contemporary theoretical models and concepts of altered states of consciousness are reviewed. Analysis of the literature indicates that nowadays, psychology recognizes the existence of different levels of consciousness. The achievements of the psychology of consciousness are helpful in research, but they have a very limited application in psychiatric practice.

Arousal↗
PubMed2002

Radio-frequency heating of the cornea: theoretical model and in vitro experiments.

We present a theoretical model for the study of cornea heating with radio-frequency currents. This technique is used to reshape the cornea to correct refractive disorders. Our numerical model has allowed the study of the temperature distributions in the cornea and to estimate the dimensions of the lesion. The model incorporates a fragment of cornea, aqueous humor, and the active electrode placed on the cornea surface. The finite element method has been used to calculate the temperature distribution in the cornea by solving a coupled electric-thermal problem. We analyzed by means of computer simulations the effect of: a) temperature influence on the tissue electrical conductivity; b) the dispersion of the biological characteristics; c) the anisotropy of the cornea thermal conductivity; d) the presence of the tear film; and e) the insertion depth of the active electrode in the cornea, and the results suggest that these effects have a significant influence on the temperature distributions and thereby on the lesion dimensions. However, the cooling of the aqueous humor in the endothelium or the realistic value of the cornea curvature did not have a significant effect on the temperature distributions. An experimental model based on the lesions created in rabbit eyes has been used in order to compare the theoretical and experimental results. There is a tendency toward the agreement between experimental and theoretical results, although we have observed that the theoretical model overestimates the lesion dimension.

Animals↗
PubMed1991

Identification of regulatory properties of metabolic networks by graph theoretical modeling.

An earlier graph theoretical model of metabolic and gene-expression networks has been modified and extended to include the effect of electrical potentials on binding constants, representation of uncatalyzed processes, and treatment of parallel reactions catalyzed by a single enzyme. Formal operations on the graph, which are facilitated by a set of standardized guidelines, identify the feedback signals in the network and rank them according to their influence. The technique was applied to a model of glycolysis in ascites tumor cells in the absence and presence of 12.5 mM exogenous glucose. Feedback regulation was widely distributed and mostly due to binding of adenine nucleotide cofactors to the enzymes of the network. The major changes in feedback regulation on adding glucose is the relief of inhibition of hexokinase and phosphofructokinase and the activation of pyruvate kinase. We conclude that regulation of tumor cell glycolysis is not restricted to hexokinase or to (Na+,K+)-ATPase as was previously suggested by others.

Animals↗
PubMed2004

Carbon nanotube mats and fibers with irradiation-improved mechanical characteristics: a theoretical model.

We employ a theoretical model to calculate mechanical characteristics of macroscopic mats and fibers of single-walled carbon nanotubes. We further investigate irradiation-induced covalent bonds between nanotubes and their effects on the tensile strength of nanotube mats and fibers. We show that the stiffness and strength of the mats can be increased at least by an order of magnitude, and thus small-dose irradiation with energetic particles is a promising tool for making macroscopic nanotube materials with excellent mechanical characteristics.

Journal Article↗
PubMed2006

Analysis of longitudinal data: the integration of theoretical model, temporal design, and statistical model.

This article argues that ideal longitudinal research is characterized by the seamless integration of three elements: (a) a well-articulated theoretical model of change observed using (b) a temporal design that affords a clear and detailed view of the process, with the resulting data analyzed by means of (c) a statistical model that is an operationalization of the theoretical model. Two general varieties of theoretical models are considered: models in which the time-related change of primary interest is continuous, and those in which it is characterized by movement between discrete states. In addition, two general types of temporal designs are considered: the longitudinal panel design and the intensive longitudinal design. For each general category of theoretical models, some of the analytic possibilities available for longitudinal panel designs and for intensive longitudinal designs are discussed. The article concludes with brief discussions of two issues particularly relevant to longitudinal research--missing data and measurement--and a few words about exploratory research.

Humans↗
PubMed2005

Internal solitons in laboratory experiments: comparison with theoretical models.

Nonlinear internal solitary waves observed in laboratory experiments are discussed from the standpoint of their relation to different soliton theories, from the classical integrable models such as the Korteweg-de Vries, Gardner, Benjamin-Ono, and Joseph-Kubota-Ko-Dobbs equations and their modifications, through the nonintegrable models describing higher-order nonlinear effects, viscosity, rotation, and cylindrical spreading, to the strongly nonlinear models. First, these theoretical models are briefly described and, then, laboratory data and their comparison with the theory are presented.

Algorithms↗
PubMed1977

[A theoretical model of structural chromosome rearrangement induction].

A theoretical model to explain the occurrence of chromosome rearrangements at the DNA molecular level is proposed. It involves: (10) breakage of one strand of each DNA molecule; (20) enzymatic digestion by an exonuclease of a short sequence of bases adjacent to the lesion, hence the presence of a short monocatenar segment on the complementary strand; (30) association of two monocatenar sequences if they are sufficiently complementary hence the formation of an X-shaped heteroduplex; (40) repair synthesis of DNA and, during this repair, an exhange of strands identical to that occuring in sister-chromatid exchanges, hence the rearrangement. This model has the advantage of necessitating one single breakpoint per DNA molecule, and explains the efficiency of radiations with low linear transfer of energy, such as X- and gamma-rays. It also explains how rearrangements such as translocations can be totally balanced since they would occur base to base within identical sequences.

Chromosome Aberrations↗