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Results for “Model explainability”

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At least 595 records · Page 33Linked to original sources

PubMed2005

Tunneling rates in electron transport through double-barrier molecular junctions in a scanning tunneling microscope.

The scanning tunneling microscope enables atomic-scale measurements of electron transport through individual molecules. Copper phthalocyanine and magnesium porphine molecules adsorbed on a thin oxide film grown on the NiAl(110) surface were probed. The single-molecule junctions contained two tunneling barriers, vacuum gap, and oxide film. Differential conductance spectroscopy shows that electron transport occurs via vibronic states of the molecules. The intensity of spectral peaks corresponding to the individual vibronic states depends on the relative electron tunneling rates through the two barriers of the junction, as found by varying the vacuum gap tunneling rate by changing the height of the scanning tunneling microscope tip above the molecule. A simple, sequential tunneling model explains the observed trends.

Aluminum↗

PubMed2006

On the origin of the extracellular action potential waveform: A modeling study.

Although extracellular unit recording is typically used for the detection of spike occurrences, it also has the theoretical ability to report about what are typically considered intracellular features of the action potential. We address this theoretical ability by developing a model system that captures features of experimentally recorded simultaneous intracellular and extracellular recordings of CA1 pyramidal neurons. We use the line source approximation method of Holt and Koch to model the extracellular action potential (EAP) voltage resulting from the spiking activity of individual neurons. We compare the simultaneous intracellular and extracellular recordings of CA1 pyramidal neurons recorded in vivo with model predictions for the same cells reconstructed and simulated with compartmental models. The model accurately reproduces both the waveform and the amplitude of the EAPs, although it was difficult to achieve simultaneous good matches on both the intracellular and extracellular waveforms. This suggests that accounting for the EAP waveform provides a considerable constraint on the overall model. The developed model explains how and why the waveform varies with electrode position relative to the recorded cell. Interestingly, each cell's dendritic morphology had very little impact on the EAP waveform. The model also demonstrates that the varied composition of ionic currents in different cells is reflected in the features of the EAP.

Action Potentials↗

PubMed2005

MSP dynamics drives nematode sperm locomotion.

Most eukaryotic cells can crawl over surfaces. In general, this motility requires three sequential actions: polymerization at the leading edge, adhesion to the substrate, and retraction at the rear. Recent in vitro experiments with extracts from spermatozoa from the nematode Ascaris suum suggest that retraction forces are generated by depolymerization of the major sperm protein cytoskeleton. Combining polymer entropy with a simple kinetic model for disassembly we propose a model for disassembly-induced retraction that fits the in vitro experimental data. This model explains the mechanism by which disassembly of the cytoskeleton generates the force necessary to pull the cell body forward and suggests further experiments that can test the validity of the models.

Actins↗

PubMed2001

A new and accurate prediction model for growth response to growth hormone treatment in children with growth hormone deficiency.

OBJECTIVE: To identify parameters which predict individual growth response to recombinant human GH (rhGH) therapy and to combine these parameters in a prediction model. DESIGN: Fifty-eight prepubertal patients with GH deficiency (17 females) participated in this prospective multicenter trial with 1 year of follow-up. METHODS: Auxological measurements, parameters of GH status and markers of bone metabolism were measured at baseline and at 1, 3 and 6 months after the start of rhGH treatment. Correlations with height velocity during the first 12 months of treatment (HV+12) were calculated. Prediction models were derived by multiple regression analysis. RESULTS: The model which best predicted HV+12 combined the following parameters: pretreatment bone age retardation as a fraction of chronological age, pretreatment serum levels of IGF-I, urinary levels of deoxypyridinoline (a marker of bone resorption) after 1 month of treatment and height velocity after 3 months of treatment. This model explained 89% of the variation in HV+12 with a standard deviation of the residuals of 0.93 cm/year. Defining successful rhGH therapy as a doubling of pretreatment height velocity, the model had a specificity of 90% and a sensitivity of 100% in predicting therapeutic success. CONCLUSIONS: This model is an accurate and practicable tool to predict growth response in GH-deficient children. It may help to optimize rhGH therapy by individual dose adjustment and contribute to improved overall outcomes.

Adolescent↗

PubMed2004

[Dynamic modeling of potato phenological development].

By means of systematic demonstration, this study found the mathematical relationship among several temperature driven nonlinear models for crop growth, and amended the limitation of the past nonlinear model in biological usage and the improved model with more general definition. Integrated with four years' field experimental data and Gauss function based temperature effectiveness model submitted in this study, the phenological development model for potato crop was established under different ecological conditions. The phenological development model explained the digital contribution of soil temperature increment on potato growth, and verified by the experimental data from different years and different conditions. There were strict co-relationships between modeling data and experimental data.

Computer Simulation↗

PubMed1988

A Markov process based model of the neurotransmitter-receptor binding process in the brain.

A non-deterministic model, based on the assumption that the binding of a neurotransmitter to its receptor is a Markov process, is developed. This model is capable of describing the kinetic processes that occur when a neurotransmitter binds to its receptor under normal and abnormal (pathological) conditions. The model assumes the presence of four states and develops transition probability functions for these states, the expected number of visits to a given state, and the probability of ever reaching a given state. The model explains the probable causes of related disease situations such as Parkinson's disease and schizophrenia, and develops transition probability function and other related statistical properties appropriate to these disease situations. Model parameters of the normal condition were generated by a computer simulation program and compared to those of the pathological conditions. The results of these sample calculations support the assumptions made in the model for normal and disease situations.

Brain↗

PubMed2004

GRIND/ALMOND investigations on CysLT1 receptor antagonists of the quinolinyl(bridged)aryl type.

One of the current routes in developing antiasthmatics is CysLT(1) receptor antagonism. For a training set of 54 CysLT(1) receptor antagonists of the quinolinyl(bridged)aryl type we developed chemometric QSAR models applying GRID independent descriptors (=GRIND). PLS analysis resulted in a two-component model explaining 67% of the variance for CysLT(1) receptor binding (r2=0.67, SDEC = 0.47, q2=0.54). GRIND variables 11-50 and 22-55 are responsible for high-affinity binding; variable 11-62 is detrimental. The predictivity of the above chemometric model is tested with a set of 69 CysLT(1) receptor antagonists, exhibiting varying chemical similarity to the training set. Nearly 50% of the test set are quite well predicted. The quality of prediction coincides in part with chemical subclassification: phenylene bridged compounds are quite well predicted; for structures with bridging heterocycles predictions are rather poor. For explaining the outlier behavior, a PLS discriminant analysis including the training set and the strongest outliers of the test set was performed. The scores plot of discriminant PLS shows an almost complete separation between the two subsets. A PLS coefficients plot explains which GRIND variables are important for the discrimination between the training set and the outliers of the test set.

Asthma↗

PubMed2001

Computer modeling and molecular dynamics simulations of ligand bound complexes of bovine angiogenin: dinucleotide topology at the active site of RNase a family proteins.

We have undertaken the modeling of substrate-bound structures of angiogenin. In our recent study, we modeled the dinucleotide ligand binding to human angiogenin. In the present study, the substrates CpG, UpG, and CpA were docked onto bovine angiogenin. This was achieved by overcoming the problem of an obstruction to the B1 site by the C-terminus and identifying residues that bind to the second base. The modeled complexes retain biochemically important interactions. The docked models were subjected to 1 ns of molecular dynamics, and structures from the simulation were refined by using simulated annealing. Our models explained the enzyme's specificity for both B1 and B2 bases as observed experimentally. The nature of binding of the dinucleotide substrate was compared with that of the mononucleotide product. The models of these complexes were also compared with those obtained earlier with human angiogenin. On the basis of the simulations and annealed structures, we came up with a consensus topology of dinucleotide ligands that binds to human and bovine angiogenins. This dinucleotide conformation can serve as a starting model for ligand-bound complex structures for RNase A family of proteins. We demonstrated this capability by generating the complex structure of CpA bound to eosinophil-derived neurotoxin (EDN) by fitting the consensus topology of CpA to the crystal structure of native EDN.

Animals↗

PubMed2003

Determinants of health policy impact: a theoretical framework for policy analysis.

This paper addresses the role of policy and evidence in health promotion. The concept of von Wright's "logic of events" is introduced and applied to health policy impact analysis. According to von Wright (1976), human action can be explained by a restricted number of determinants: wants, abilities, duties, and opportunities. The dynamics of action result from changes in opportunities (logic of events). Applied to the policymaking process, the present model explains personal wants as subordinated to political goals. Abilities of individual policy makers are part of organisational resources. Also, personal duties are subordinated to institutional obligations. Opportunities are mainly related to political context and public support. The present analysis suggests that policy determinants such as concrete goals, sufficient resources and public support may be crucial for achieving an intended behaviour change on the population level, while other policy determinants, e.g., personal commitment and organisational capacities, may especially relate to the policy implementation process. The paper concludes by indicating ways in which future research using this theoretical framework might contribute to health promotion practice for improved health outcomes across populations.

Evidence-Based Medicine↗

PubMed2005

QSPR modeling of pseudoternary microemulsions formulated employing lecithin surfactants: application of data mining, molecular and statistical modeling.

Data mining, computer aided molecular modeling, descriptor calculation, genetic algorithm and multiple linear regression analysis techniques were combined together to generate predictive quantitative structure property relationship (QSPR) models explaining the formation of lecithin-based W/O microemulsions. Ninety-four microemulsion phase diagrams were collected from five different references published over the past few years. Computer-based molecular modeling techniques were then applied on the components of the collected microemulsion systems to generate corresponding plausible three-dimensional (3D) structures. The resulting 3D models were utilized to calculate a group of molecular physicochemical descriptors. Thereafter, genetic algorithm and backward stepwise regression analysis were separately assessed as means for selecting optimal descriptor sets for statistical modeling. The selected descriptors were correlated with microemulsion existence areas employing multiple linear regression analysis. The resulting W/O models were statistically validated and found to be of significant predictive power. The models allowed better understanding of the process of microemulsion formation. Unfortunately, all QSPR modeling efforts directed towards O/W microemulsions failed completely.

Emulsions↗

PubMed2000

A human physiologically-based model for glycyrrhzic acid, a compound subject to presystemic metabolism and enterohepatic cycling.

PURPOSE: To analyze the role of the kinetics of glycyrrhizic acid (GD) in its toxicity. A physiologically-based pharmacokinetic (PBPK) model that has been developed for humans. METHODS: The kinetics of GD, which is absorbed as glycyrrhetic acid (GA), were described by a human PBPK model, which is based on a rat model. After rat to human extrapolation, the model was validated on plasma concentration data after ingestion of GA and GD solutions or licorice confectionery, and an additional data derived from the literature. Observed interindividual variability in kinetics was quantified by deriving an optimal set of parameters for each individual. RESULTS: The a-priori defined model successfully forecasted GA kinetics in humans, which is characterized by a second absorption peak in the terminal elimination phase. This peak is subscribed to enterohepatic cycling of GA metabolites. The optimized model explained most of the interindividual variance, observed in the clinical study, and adequately described data from the literature. CONCLUSIONS: Preclinical information on GD kinetics could be incorporated in the human PBPK model. Model simulations demonstrate that especially in subjects with prolonged gastrointestinal residence times, GA may accumulate after repeated licorice consumption, thus increasing the health risk of this specific subgroup of individuals.

Adult↗

PubMed2004

The indoor environmental index and its relationship with symptoms of office building occupants.

An index for indoor environmental quality, the Indoor Environmental Index (IEI), was developed. This study aggregates the Indoor Air Pollution Index, an index found in the literature, and a new index: the Indoor Discomfort Index. The average of these two indices is the IEI, which is calculated using concentrations of eight pollutants and two comfort variables measured in 100 office buildings in the United States. The database used was developed for the U.S. Environmental Protection Agency Building Assessment Survey Evaluation study. A symptom index also is developed to denote persistent occupant symptoms. The IEI and the symptom index are used to investigate the relationship between indoor environmental quality and symptoms. Two simple linear regression models were formulated; these models explain 67 and 79% of the variation in the average symptom index, with the variation of the average IEI depending on the method of averaging used in the construction of the models. In addition, a conceptual explanation is provided for the empirical or regression models formulated. The IEI and the associated models relating indoor environmental quality with the office occupant symptom index may be used as management tools, as illustrated with an example.

Air Pollution, Indoor↗

PubMed2006

Arterial wave reflections and mortality in haemodialysis patients--only relevant in elderly, cardiovascularly compromised?

BACKGROUND: Chronic kidney disease (CKD) patients have a 3-30-fold increased risk of death compared with the general population. This mortality difference is even more pronounced in younger subjects. Two markers of aortic stiffness--aortic pulse wave velocity (PWV) and augmentation index (AIx)--have been prospectively related to all-cause and cardiovascular (CV) mortality in end-stage renal disease (ESRD) populations. The aims of our study were first, to confirm the important deleterious effect of arterial stiffness in uraemia and second, to assess the impact on survival of increased AIx in a relatively young non-diabetic dialysis population, with minimal CV disease. METHODS: Ninety-two patients (mean age 42.6 +/- 11.2 years) were included in the study and followed for a period of 61 +/- 25 months. None of the patients had diabetes mellitus, and only 3.3% had prior history of CV disease. AIx was determined by applantation tonometry using a SphygmoCor device (AtCor, PWV Inc., Westmead, Sydney, Australia). RESULTS: Mean AIx in our study population was 19.9 +/- 13.7%; other significant haemodynamic parameters were: systolic blood pressure (SBP) 129 +/- 24 mmHg, pulse pressure 35.3 +/- 17.5 mmHg with 27.2% of the study population receiving angiotensin-converting enzyme inhibitors (ACE-I). On univariate analysis, in our group AIx correlated with: body weight (P < 0.001), radial SBP (P < 0.001) and haemoglobin levels (P < 0.05). There was no correlation between AIx and any of the echocardiographic parameters. In the stepwise multiple regression analysis, the only independent predictors for AIx were weight (P < 0.001), SBP (P < 0.001) and haemoglobin (P < 0.05) with the model explaining 33% of the AIx variability (adjusted R(2) = 0.33). During the follow-up period, 15 deaths were recorded. In the Cox analysis (P = 0.014; chi square 20.7 for the model) the only independent predictors for all-cause mortality were age (P = 0.001), left ventricular mass index (P = 0.032) and ACE-I therapy (P = 0.039) while AIx did not reach statistical significance. There was no difference in patients' survival when divided by AIx tertiles, assessed by the log rank test (P = 0.78). CONCLUSION: Our results fail to support the notion that an increased effect of wave reflections on central arteries is a strong and independent predictor of mortality in all ESRD patients on haemodialysis. The effect of arterial wave reflections might be in fact dependent on patient age and concurrent comorbidity status.

Adult↗

PubMedDate not supplied

Widowhood and other demographic associations of pain in independent older people.

OBJECTIVES: To determine if psychosocial factors, as suggested by the demographic variables of widowhood and living alone, are associated with pain, particularly severe pain, in a representative sample of independent older people. DESIGN: One thousand older people (65+) randomly selected from independent residents living in a major city were surveyed about their health status (Health Status of Older People Study). Demographic characteristics, including age, gender, education, income, living alone, widowhood, and childlessness, were analyzed by logistic regression for their association with pain report of differing severity. Path analysis was used to confirm the association with pain severity and further define the role of mood disturbance in mediating this relationship. RESULTS: The prevalence of any pain report for the preceding 12 months was 56.3%. This was reduced when using more restrictive criteria, such that moderate-to-severe pain "at worst" and "at present" was found in 48.7% and 4.1% of the sample, respectively. After adjusting for type 1 error rate, the status of living alone was primarily associated with moderate-to-severe pain at worst, and being a widow(er) was associated with moderate-to-severe pain at present. The latter association had an estimated odds ratio greater than 3 and was characterized by more recent bereavement. Using path analysis, the model that severe pain was secondary to mood disturbance of widowhood, particularly recent bereavement, was tested and confirmed. The model explained 17% of the variance of pain severity in widow(er)s. CONCLUSION: The mood disturbance related to spousal bereavement aggravates pain in older people. This lends support to the biopsychosocial model of pain.

Aged↗

PubMed2006

Nurse intention to remain employed: understanding and strengthening determinants.

AIM: This paper reports a study testing a hypothesized model of the determinants of nurse intention to remain employed in current hospitals of employment. BACKGROUND: Previous research has shown that stronger nurse intention to remain employed is associated with higher job satisfaction, higher organizational commitment, higher perceived manager support, lower burnout, higher work group cohesion, being older, having more years of nursing experience and having lower levels of education. METHODS: A descriptive survey design was adopted. Over 13,000 Ontario, Canada nurses were invited to complete a mailed survey between February and May 2003. The Ontario Nurse Survey includes instruments and items measuring job satisfaction, burnout, professional nursing practice environment, demographic characteristics of nurse respondents and items about intention to remain employed. Two multiple regression models, one including all variables and the other using a stepwise method, were used to test the proposed model. RESULTS: Regression models explained 34% of variance in nurse intention to remain employed. The strongest predictors were nurse age, overall nurse job satisfaction and years of employment in the current hospital. Although the proposed model hypothesized six categories of predictors of intention to remain employed, only four of these were statistically significant determinants of nurse intention to remain: job satisfaction, personal characteristics of nurses, work group cohesion and collaboration, and organizational commitment of nurses. The other two categories of predictors, nurse burnout and nurse manager ability and support, may be predictors of job satisfaction and have indirect effects on intention to remain employed that are mediated through job satisfaction. CONCLUSION: Possible strategies to strengthen predictors of intention to remain employed include employment practices that reflect moral integrity, incorporate clear communication systems, maximize employee involvement in decision-making, promote praise and recognition, and establish a shared vision and goals.

Adult↗

PubMed2001

The structure of self-rated health among community-dwelling older adults with stroke.

OBJECTIVE: To examine whether depressive symptomatology is a third fundamental component of the structure of self-rated health, in addition to two other components (physical disease and functional disability) among community-dwelling older adults with stroke. DATA SOURCES AND STUDY SETTING: A total of 591 community-dwelling older adults with stroke were identified from the 1993 Asset and Health Dynamics among the Oldest-Old (AHEAD) national survey of community-dwelling older adults. STUDY DESIGN: A cross-sectional study. Structural equation modeling was applied to compare a widely used two-factor model of self-rated health with a model adding depression as a third possible factor. PRINCIPLE FINDINGS: The hypothesized three-factor model explained additional 21% more variance of self-rated health of older adults with stroke (R2 = 79%, NNFI = 0.95, CFI = 0.96, RMSEA = 0.04) as compared with the two-factor biomedical model (R2 = 58%, NNFI = 0.95, CFI = 0.98, RMSEA = 0.05). The three-factor model was statistically different from the two-factor model. CONCLUSIONS: Greater attention should be given to the theoretical structure of self-rated health of older adults with stroke, particularly, the significant impact of depression on their self-rated health.

Activities of Daily Living↗

PubMed1989

Examination of specific nutrition/health behaviors using a social cognitive model.

Nutrition intervention programs are not always successful. In some cases, an insufficient understanding of the interrelationships among factors influencing health behaviors may be responsible for the failures. This study used social cognitive theory, a framework for studying behaviors, to structure the relationships between measurable factors important to the frequency of health-oriented food consumption. We developed a model that incorporated factors for social environment, reinforcement, commitment, behavior modeling, knowledge, and attitude relative to the frequency of consumption of four beverages (whole milk, low-fat/skin milk, regular soda, and diet soda). Four-hundred fifty-seven middle-aged adults (mean age, 47 years; 58% female) and 709 college students (mean age, 21 years; 50% female) responded to a written questionnaire designed as a self-report on frequency of consumption and measures for 10 social cognitive variables. For all four beverages, the model explained 35% or more of the variance in frequency of consumption, thus confirming its predictive power. We used the statistical approach known as path analysis to examine the relationships within the model. The analysis demonstrated that factors influencing the consumption varied between the two age groups (e.g., nutrition knowledge was related to attitude in adult soda-drinking models but not in student soda-drinking models) and between forms of the beverages (e.g., for student models, nutrition knowledge was related to taste enjoyment for low-fat/skim milk but not for whole milk).(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗

PubMed2001

Mechanisms of rapid sympatric speciation by sex reversal and sexual selection in cichlid fish.

Mechanisms of speciation in cichlid fish were investigated by analyzing population genetic models of sexual selection on sex-determining genes associated with color polymorphisms. The models are based on a combination of laboratory experiments and field observations on the ecology, male and female mating behavior, and inheritance of sex-determination and color polymorphisms. The models explain why sex-reversal genes that change males into females tend to be X-linked and associated with novel colors, using the hypothesis of restricted recombination on the sex chromosomes, as suggested by previous theory on the evolution of recombination. The models reveal multiple pathways for rapid sympatric speciation through the origin of novel color morphs with strong assortative mating that incorporate both sex-reversal and suppressor genes. Despite the lack of geographic isolation or ecological differentiation, the new species coexists with the ancestral species either temporarily or indefinitely. These results may help to explain different patterns and rates of speciation among groups of cichlids, in particular the explosive diversification of rock-dwelling haplochromine cichlids.

Animals↗