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Results for “Models, Theoretical”

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At least 1,099 records · Page 61Linked to original sources

PubMed1996

Motion discrimination of single targets: comparison of preliminary findings in normal subjects and patients with glaucoma.

BACKGROUND: Luminance, global motion and flicker sensitivities are affected in patients with primary open-angle glaucoma. Although no theoretical model has been put forward to explain the observed reduction in sensitivity in this patient group, these findings have often been attributed to diffuse and selective loss of large optic nerve fibres. METHODS: Movement processing was investigated using an optical projection system that generates smooth, continuous motion with control of speed, displacement and motion direction. Motion-displacement and direction-discrimination thresholds were measured in eight normal subjects and in three patients with diagnosed glaucoma. At each speed tested, targets were presented for a range of displacements and thresholds were extracted after probit analysis. The measurements were carried out both foveally and at 19 degrees in the periphery and provided the data necessary to develop and optimise a model of motion perception based on multiple time delays for the correlation of signals that map progressively more distant parts of the visual field. RESULTS: Our preliminary findings show that direction discrimination can be at chance level even for large displacements when motion is detected 80% of the time. Model simulations show that specific changes in the spatial sampling interval and the speed of transmission of the motion signals involved can cause the observed reduction in motion sensitivity and direction discrimination in patients with glaucoma. CONCLUSIONS: A model for motion detection and direction discrimination of single targets has been proposed to account for the measured functional relationship between motion displacement thresholds and target speed in normal subjects. Tested patients with glaucoma show reduced motion sensitivity and poor discrimination of motion direction. The type of degraded performance observed experimentally in glaucoma patients is also predicted by the model. Such predictions require specific changes in model parameters that may be indicative of changes in the retina caused by the disease.

Adult↗

PubMed1992

Structuring HIV prevention service delivery systems on the basis of social science theory.

In order to identify the optimal configuration of HIV prevention programs, it is necessary to examine different theoretical models of behavior change. Cognitive/decision-making theories of human behavior change are compared to social learning theories vis-a-vis their influence on the structure of service delivery systems. Cognitive/decision-making theories ascribe behavior change to the provision of new information and favor the development of homogeneous interventions providing clients with information about risk behaviors. These interventions are easily standardized across delivery sites and various target populations. Social learning theories view behavior change as a series of stages and recognize the influence of sociocultural variables. They favor multiple heterogeneous interventions in a variety of settings, with the provision of skills training as well as information. Ongoing HIV prevention research indicates that social learning theories provide a more accurate paradigm of human behavior change for the complex behaviors related to HIV risk. Public health agencies must therefore continue to strengthen organizational and referral relationships with community-based organizations that can provide the specialized prevention interventions called for by social learning theory. This will require ongoing collaboration and technical assistance.

Adolescent↗

PubMed1983

Intercalated discs as a cause for discontinuous propagation in cardiac muscle: a theoretical simulation.

A theoretical model of a cardiac muscle fiber (strand) based on core conductor principles and which includes a periodic intercalated disc structure has been developed. The model allows for examination of the mechanism of electrical propagation in cardiac muscle on a microscopic cell-to-cell level. The results of the model simulations demonstrate the discontinuous nature of electrical propagation in cardiac muscle and the inability of classical continuous cable theory to adequately describe propagation phenomena in cardiac muscle.

Action Potentials↗

PubMed1975

The absorption and scattering of light in bovine and human dental enamel.

The reflectance and transmission of thin slabs of dental enamel has been measured at all wavelenghts between 220 and 700 nm by means of an integrating sphere. From the results the true scattering and absorption coefficients have been computed. The theoretical model used is an extended two-flux model, which is presented and discussed. The absorption spectrum of the dissolved organic component of enemel was also determined. An absorption peak at 270 nm is common to all the spectra. This peak in the bovine enamel spectrum is about three times as high as in the spectrum of human enamel. The peak of the dissolved material is about as high as the peak of the corresponding enamel. Hence it is concluded that the organic component, presumably aromatic amino acids, is responsible for most or all of the observed optical absorption.

Absorption↗

PubMed1987

The effect of inactivation of calcium channels by intracellular Ca2+ ions in the bursting pancreatic beta-cells.

Based on recently determined ionic channel properties, a simple theoretical model for the burst activity of the pancreatic beta-cell is formulated in this paper. The model contains an inward voltage-activated Ca2+ current which is inactivated by intracellular calcium ions and an outward K+ current that is activated by the membrane potential. The probability of opening of the channel gates is represented by Boltzmann equations. Our model is applicable in a regime where an ATP-blockable K+ channel is inhibited. In this regime, glucose is treated as an activator for the rate of efflux of intracellular Ca2+ ions, and hence its effect is equated to kca, the efflux rate constant. In addition, intracellular H+ ion, which is a byproduct of the glycolytic metabolic process, is treated as a competitive inhibitor for Ca2+ ion. Since H+ is a competitive inhibitor (according to our assumption), its effect is equated to the strength of the Cai dissociation constant Kh. In the model, a Ca2+ binding site is assumed to exist in the inner membrane of the voltage-gated Ca2+ channel. The model predicts that a spike and burst electrical pattern can be generated by varying kca and that a given pattern may produce different levels of intracellular Ca2+ depending on Kh. In other words, it predicts that levels of [Ca2+]i can be separated from the electrical activity by controlling the concentration of glucose and pH appropriately. This may account for the experimental observation of Lebrun et al. (1985) that insulin secretion is not correlated to the burst of electrical activity.

Action Potentials↗

PubMed2004

Investigation on wind energy-compressed air power system.

Wind energy is a pollution free and renewable resource widely distributed over China. Aimed at protecting the environment and enlarging application of wind energy, a new approach to application of wind energy by using compressed air power to some extent instead of electricity put forward. This includes: explaining the working principles and characteristics of the wind energy-compressed air power system; discussing the compatibility of wind energy and compressor capacity; presenting the theoretical model and computational simulation of the system. The obtained compressor capacity vs wind power relationship in certain wind velocity range can be helpful in the designing of the wind power-compressed air system. Results of investigations on the application of high-pressure compressed air for pressure reduction led to conclusion that pressure reduction with expander is better than the throttle regulator in energy saving.

Air↗

PubMed1996

Income prospects and age-at-marriage.

"This paper provides an empirical investigation of a theoretical model of the marriage market. In the model, women are valued more for their ability to bear children and men are valued more for the ability to make money. Men cannot reveal their labor market ability to potential spouses until they enter the labor force. At the same time, the relevant information for evaluating females as spouses is revealed at a younger age. The model predicts that the income of males will be positively associated with age-at-first-marriage. We find empirical support for the model [based on U.S. data]. However, we also find the association between male earnings and age-at-first-marriage becomes negative for those who married after age 30, which was not predicted by the model. Consistent with the model, we do not find a strong relationship between earnings and age-at-first-marriage among females."

Americas↗

PubMed2006

Environmental security: a geographic information system analysis approach--the case of Kenya.

Studies into the relationships between environmental factors and violence or conflicts constitute a very debated research field called environmental security. Several authors think that environmental scarcity, which is scarcity of renewable resources, can contribute to generate violence or social unrest, particularly within states scarcely endowed with technical know-how and social structures, such as developing countries. In this work, we referred to the theoretical model developed by the Environmental Change and Acute Conflict Project. Our goal was to use easily available spatial databases to map the various sources of environmental scarcity through geographic information systems, in order to locate the areas apparently most at risk of suffering negative social effects and their consequences in terms of internal security. The analysis was carried out at a subnational level and applied to the case of Kenya. A first phase of the work included a careful selection of databases relative to renewable resources. Spatial operations among these data allowed us to obtain new information on the availability of renewable resources (cropland, forests, water), on the present and foreseen demographic pressure, as well as on the social and technical ingenuity. The results made it possible to identify areas suffering from scarcity of one or more renewable resources, indicating different levels of gravity. Accounts from Kenya seem to confirm our results, reporting clashes between tribal groups over the access to scarce resources in areas that our work showed to be at high risk.

Conflict, Psychological↗

PubMedDate not supplied

Potential air contamination during CO2 angiography using a hand-held syringe: theoretical considerations and gas chromatography.

PURPOSE: To assess air contamination in the hand-held syringes currently used for CO2 delivery and to determine whether there is an association between their position and the rate of air contamination. METHODS: Assessment of air contamination in the syringe (20 ml) included theoretical modeling, mathematical calculation, and gas chromatography (GC). The model was used with Fick's first law to calculate the diffusion of CO2 and the amount of air contamination. For GC studies, the syringes were placed in the upright, horizontal, and inverted positions and gas samples were obtained after 5, 10, 20, 30, and 60 min. All trials with each position for each sampling time were performed five times. RESULTS: The amounts of air contamination with time calculated mathematically were 5-10% less than those of GC. With the diffusivity of air-CO2 at 0.1599 cm2/sec (9.594 cm2/min), air contamination was calculated to be 60% at 60 min. With GC air contamination was 13% at 5 min, 31% at 20 min, 43% at 30 min, and 68% at 60 min. There was no difference in air contamination between the different syringe positions. CONCLUSION: Air contamination occurs in hand-held syringes filled with CO2 when they are open to the ambient air. The amounts of air contamination over time are similar among syringes placed in the upright, horizontal, and inverted positions.

Air Pollution, Indoor↗

PubMed2005

An asymptotic comparative analysis of the thermodynamics of non-covalent association.

There is an ambiguity in the theoretical models for computing association constants, the key observable in a laboratory, of non-covalent associations. We show that three different models give unique result asymptotically in the limit of strong associate. For weak associations, the disagreement reflects the nature of ill-defined "associated complex'' which can be defined, among various ways, either geometrically or thermodynamically depending on measurement techniques. Furthermore, even when the free energy of association is unique, the corresponding entropy and enthalpy can still be different from different types of measurements--a surprising source of entropy-enthalpy compensation. This work provides a mathematical basis for modeling non-covalent association processes in biology.

Algorithms↗

PubMed1995

Compatibility considerations for low-mass rigid-belt vehicles.

A number of staged impacts performed by our group with the aid of a test device representing a low-mass vehicle (LMV) indicates that a rigid-belt body (RBB) is a valid means for providing adequate occupant safety also for LMVs in the strict sense (curb mass less than 600 kg). The RBB concept raises the problem of compatibility, however. Ideally, the deformability of car front structures should increase with increasing vehicle weight in order to ascertain compatibility. Published data on frontal deformation characteristics substantiate in contrast that conventional cars today exhibit an opposite behaviour. To evaluate the compatibility properties of ultrastiff LMVs, two crash experiments were performed along with a theoretical model analysis. An LMV with a mass of 680 kg (including batteries, 50% mass of two dummies, instrumentation) designed according to the RBB concept and a conventional care of 1320 kg--(equivalent loading conditions as LMV)--were crashed at 56 km/h in a frontal direction against a deformable barrier (FMVSS 214). Furthermore, a mathematical model was based on estimated deformation characteristics of conventional vehicles to predict intrusion distances into the FMVSS barrier in hypothetical frontal crashes with 56 km/h. The results indicate that due to its low mass an LMV does not represent an excessive compatibility problem for other car occupants in spite of the stiff RBB characteristics.

Accidents, Traffic↗

PubMed1981

A statistical mechanical treatment of fatty acyl chain order in phospholipid bilayers and correlation with experimental data. A. Theory.

A theoretical model has been developed in order to describe the organization of acyl chains in phospholipid bilayers. Since the model is intended to reproduce highly quantitative experimental results such as the deuterium magnetic resonance (NMR) data and to supplement the experimental information, all the rotameric degrees of freedom, the excluded volume interactions and the van der Waals interactions have been considered. The model is a direct extension of a generalized van der Waals theory of nematic liquid crystals to flexible molecules. In this picture, the anisotropy of the short-range repulsive forces which are treated by a hard core potential is introduced as the dominant factor governing intrinsic order among the chains. The anisotropy of the attractive forces, which are approximated by a molecular field, plays a somewhat secondary role. The dependence of the energy of interaction on the relative chain conformations is approximated by two order parameters reflecting respectively the 'average shape' of the molecules and the 'average shape' in a 'mean orientation'. The influence of the interactions in the polar region on the lateral chain area is accounted for by an effective lateral pressure. In certain aspects the model has features in common with the Marcelja theory.

Acylation↗

PubMed1994

Polyhydroxy-mediated interactions between liposomes and bacterial biofilms.

A theoretical model has been developed for the interaction of the surface polymers of the bacterial glycocalyx with liposomes incorporating lipids with polyhydroxy headgroups such as phosphatidylinositol (PI). The theory is based on a lattice model and equations are derived for the potential energy of interaction between the surfaces of a bacterium and a liposome as a function of their separation. It is shown that a relatively small energy of interaction, less than that of a single hydrogen bond, between the polyhydroxyl headgroup of the liposomal lipid and bacterium surface polymer residues could give rise to a potential energy of interaction in excess of the classical double layer repulsive force and attractive dispersion force interactions. The most important prediction of the theory is that the potential energy of interaction goes through a minimum as a function of the polyhydroxy lipid (PI) concentration in the liposomal surface, thus predicting an optimal liposomal composition for adsorption of liposome to bacterium. This result is in concordance with the adsorption of dipalmitoylphosphatidylcholine-PI liposomes to a range of biofilms of oral and skin-associated bacteria on solid supports, where optimum levels of PI for adsorption have been found. The theory demonstrates that subtle changes in the composition of liposomal and bacterial surfaces involving relatively small interaction energies can markedly influence the nature of their interactions.

Bacteria↗

PubMed1980

Dual isotope separation technique for radioassay. The effect of cross counting between two simultaneously counted isotopes.

A theoretical model, describing the effects of cross counting on the dual-isotope-corrected bound count rate and on the precision with which it is estimated, is presented. Experimental support for the validity of the model is given. The bound count rate is shown to be independent of the amount of marker isotope incorrectly counted as label isotope. The precision with which the bound count rate is estimated appears to be dependent on the size of this cross counting correction, although other factors such as pipetting errors are of more importance. Implications for the choice of assay system are discussed.

Cobalt Radioisotopes↗

PubMed1995

Injection bias of DNA fragments in capillary electrophoresis with sieving.

The relative amounts of DNA fragments in a mixture injected into the capillary by electromigration or hydrodynamically by pressure were compared. Even if the electrophoretic mobilities of DNA fragments with different sizes are the same in a free solution in the sample vial, the size bias is brought about by the different mobilities in a sieving medium and by the electroosmosis. The experiments were performed in capillaries filled with a solution of liquified agarose, a replaceable sieving medium. The experimental results were compared with a theoretical model.

Bacteriophage phi X 174↗

PubMed1988

Oestrogen and progesterone interactions in the control of gonadotrophin and prolactin secretion.

Oestrogen and progesterone have marked effects on the secretion of the gonadotrophins and prolactin. During most of the oestrous or menstrual cycle the secretion of gonadotrophin is maintained at a relatively low level by the negative feedback of oestrogen and progesterone on the hypothalamic-pituitary system. The spontaneous ovulatory surge of gonadotrophin is produced by a positive feedback cascade. The cascade is initiated by an increase in the plasma concentration of oestradiol-17 beta which triggers a surge of luteinizing hormone releasing hormone (LHRH) and an increase in pituitary responsiveness to LHRH. The facilitatory action of oestrogen on pituitary responsiveness is reinforced by progesterone and the priming effect of LHRH. How oestrogen and progesterone exert their effects is not clear but the facilitatory effects of oestrogen take about 24 h, and the stimulation of LHRH release is produced by an indirect effect of oestradiol on neurons which are possibly opioid, dopaminergic or noradrenergic and which modulate the activity of LHRH neurons. In the rat, a spontaneous prolactin surge occurs at the same time as the spontaneous ovulatory gonadotrophin surge. The prolactin surge also appears to involve a positive feedback between the brain-pituitary system and the ovary. However, the mechanism of the prolactin surge is poorly understood mainly because the neural control of prolactin release appears to be mediated by prolactin inhibiting as well as releasing factors, and the precise role of these factors has not been established. The control of prolactin release is further complicated by the fact that oestradiol stimulates prolactin synthesis and release by a direct action on the prolactotrophes. Prolactin and gonadotrophin surges also occur simultaneously in several experimental steroid models. A theoretical model is proposed which could explain how oestrogen and progesterone trigger the simultaneous surge of LH and prolactin.

Animals↗

PubMed1989

Pulmonary diffusing capacity: implications of two-phase blood flow in capillaries.

The classical view of oxygen (O2) uptake in pulmonary capillaries assumes implicitly that capillary blood can be regarded as a continuous homogeneous hemoglobin solution. In this study a theoretical model was used to examine the role played by the particulate (two-phase) nature of blood on pulmonary oxygen exchange. Red cells were modelled as discrete hemoglobin (Hb) containing spheres flowing in single file suspension through a cylindrical capillary surrounded by a uniform annulus of alveolar tissue. The model accounted for the free diffusion of O2 from alveolar air space through tissue and plasma, free and Hb facilitated diffusion of O2 inside red cells, and the intracellular kinetics of O2-Hb binding. Oxygen uptake was driven by a specified O2 tension at the alveolar surface. The computed pulmonary diffusing capacity (DLO2) decreased with increasing spacing (Ls) between red cells. The reduction in DLO2 with increasing Ls was marshalled more by a reduction in membrane diffusing capacity (DMO2), than by the reduction in erythrocyte diffusing capacity (DeO2). The dependence of DMO2 on cell spacing stemmed from the manner in which O2 flowed across the alveolar surface into the discrete sinks (red cells) within the capillaries. The degree to which Ls influenced DMO2 was dependent on tissue and plasma layer thickness relative to red cell dimensions. The results indicate that the functional area of the alveolo-capillary membrane for O2 exchange depends on the red cell content of capillaries. Thus, DMO2 is not dictated solely by the morphology of the exchange apparatus (and physical parameters), but has functional determinants as well.

Capillaries↗

PubMed1988

Scheduling mammograms for asymptomatic women.

A decision theoretic model was used to investigate the relative importance of risk level, radiation hazard, mammographic accuracy, and cost in mammographic screening decision. The model uses woman-specific medical and family history facts and clinic-specific information regarding mammographic accuracy and practice to profile both woman and clinic, and to formulate periodic screening recommendations. Model parameters were varied extensively to investigate the sensitivity of screening schedules to input values. Multivariate risk was estimated within the program using published data from the Breast Cancer Detection Demonstration Project 5-year follow-up study. Radiation hazard estimates were developed from published radiation physics and radioepidemiologic risk data. Benchmark values for mammographic sensitivity and specificity under screening conditions were calculated from Breast Cancer Detection Demonstration Project data. Procedural costs used in the analysis were varied around values reflecting conditions at the Washington University Medical Center. Mortality advantages of early versus late breast cancer detection were accounted for using Health Insurance Plan of New York case survival rates. Results are compared with published screening policies to provide insight into implicit assumptions behind those policies. This analysis emphasizes the importance of accounting for variations in clinical accuracy under screening circumstances, in costs, in radiation exposure, and in woman-specific risk when recommending mammographic screening.

Aged↗