PubMed Health⌕ Search

SEARCH · PubMed Health

Results for “threshold model”

Explore indexed PubMed citations for clinical trials, systematic reviews and public health research. Read source abstracts and follow each citation to its original PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 91 records · Page 5Linked to original sources
PubMed2001

Bayesian analysis of liability of clinical mastitis in Norwegian cattle with a threshold model: effects of data sampling method and model specification.

First-lactation records of Norwegian Cattle were used to infer heritability of liability to clinical mastitis with a threshold sire model. Mastitis was defined as a binary response (presence or absence) in a defined period of first lactation (opportunity period). Length of opportunity period (from 30 d before calving up to 120 or 300 d of lactation) had less effect on heritability estimates than data sampling methods (include or exclude records of cows culled before the end of the opportunity period) whereas sire ranking was more affected by the former. Including all cows, whether culled before the end of the opportunity period or not, gave a sharper and more symmetric posterior distribution of heritability of liability to clinical mastitis. When we analyzed data for all cows, model specification had a small effect on heritability estimates, while sire ranking was affected markedly. Posterior means of heritability range from 0.058 to 0.074. A model regressing on the length of the opportunity period for culled cows without mastitis, was shown favorable for the two opportunity periods using Bayes factors and the deviance information criterion for model comparison. This model, in which liability of mastitis depends on time to culling, may allow utilizing information from all first lactations in genetic evaluation, irrespectively of duration and culling outcome.

Animals↗
PubMedDate not supplied

Caregiving appraisal and interventions based on the progressively lowered stress threshold model.

The purpose of this article is to describe the impact of a theoretically driven, psychoeducational intervention based on the Progressively Lowered Stress Threshold (PLST) model on caregiving appraisal among community-based caregivers of persons with Alzheimer's disease and related disorders. A total of 241 subjects completed the year-long study in four sites in Iowa, Minnesota, Indiana, and Arizona. Caregiving appraisal was measured using the four factors of the Philadelphia Geriatric Center Caregiving Appraisal Scale: mastery, burden, satisfaction, and impact. Analysis of trends over time showed that the intervention positively affected impact, burden, and satisfaction but had no effect on mastery when measured against the comparison group. The PLST model was influential in increasing positive appraisal and decreasing negative appraisal of the caregiving situation.

Adult↗
PubMed1990

Applicability of quadratic and threshold models to motion discrimination in the rabbit retina.

Computational and behavioral studies suggest that visual motion discrimination is based on quadratic nonlinearities. This raises the question of whether the behavior of motion sensitive neurons early in the visual system is actually quadratic. Theoretical studies show that mechanisms proposed for retinal directional selectivity do not behave quadratically at high stimulus contrast. However, for low contrast stimuli, models for these mechanisms may be grouped into three categories: purely quadratic, quadratic accompanied by a rectification, and models mediated by a high level threshold. We discriminated between these alternatives by analyzing the extracellular responses of ON-OFF directionally selective ganglion cells of the rabbit retina to drifting periodic gratings. The data show that purely-quadratic or high-threshold systems do not account for the behavior of these cells. However, their behavior is consistent with a rectified-quadratic model.

Animals↗
PubMed2006

Genetic analysis of clinical mastitis data from on-farm management software using threshold models.

Producer-recorded clinical mastitis data from 77,791 cows in 418 herds were used to determine the potential for genetic improvement of mastitis resistance using data from on-farm management software programs. The following threshold sire models were applied: 1) a single-trait lactation model, where mastitis was recorded as 0 or 1 in first lactation only; 2) a 3-trait lactation model, where mastitis was recorded as 0 or 1 in each of the first 3 lactations, and 3) a 12-trait, lactation-segment model, where mastitis was recorded as 0 or 1 in each of 4 segments (0 to 50, 51 to 155, 156 to 260, and 261 to 365 d postpartum) in each of the first 3 lactations. Lactation incidence rates were 0.16, 0.20, and 0.24 in first, second, and third lactation, respectively, and incidence rates within various segments of these lactations ranged from 0.036 in late first lactation to 0.093 in early third lactation. Estimated heritability of liability to clinical mastitis ranged from 0.07 to 0.15, depending on the model and stage of lactation. Heritability estimates were higher in first lactation than in subsequent lactations, but estimates were generally similar for different segments of the same lactation. Genetic correlations between lactations from the 3-trait model ranged from 0.42 to 0.49, while correlations between segments within lactation from the 12-trait model ranged from 0.26 to 0.64. Based on the results presented herein, it appears that at least 2 segments are needed per lactation, because mastitis in early lactation is lowly correlated with mastitis in mid or late lactation. Predicted transmitting abilities of sires ranged from 0.77 to 0.89 for probability of no mastitis during the first lactation and from 0.36 to 0.59 for probability of no mastitis during the first 3 lactations. Overall, this study shows that farmer-recorded clinical mastitis data can make a valuable contribution to genetic selection programs, but additional systems for gathering and storing this information must be developed, and more extensive data recording in progeny test herds should be encouraged.

Animals↗
PubMed1998

Threshold models in radiation carcinogenesis.

Cancer incidence and mortality data from the atomic bomb survivors cohort has been analyzed to allow for the possibility of a threshold dose response. The same dose-response models as used in the original papers were fit to the data. The estimated cancer incidence from the fitted models over-predicted the observed cancer incidence in the lowest exposure group. This is consistent with a threshold or non-linear dose-response at low-doses. Thresholds were added to the dose-response models and the range of possible thresholds is shown for both solid tumor cancers as well as the different leukemia types. This analysis suggests that the A-bomb cancer incidence data agree more with a threshold or non-linear dose-response model than a purely linear model although the linear model is statistically equivalent. This observation is not found with the mortality data. For both the incidence data and the mortality data the addition of a threshold term significantly improves the fit to the linear or linear-quadratic dose response for both total leukemias and also for the leukemia subtypes of ALL, AML, and CML.

Cohort Studies↗
PubMed2006

Genetic and permanent environmental variability of twinning in Thoroughbred horses estimated via three threshold models.

Twinning is a serious problem in Thoroughbred horses, as most of the multiple pregnancies are miscarried and live foals present lower performance values. The aim of this study was to estimate the heritability and repeatability of twinning in Thoroughbred horses. A total of 12648 pregnancies of 2033 Thoroughbred mares born between 1929 and 1994 in 11 studs were included. The analysis was performed by the use of AI-REML algorithm under three threshold animal models. The first model included effects of stud, mare's birth period and interaction between them as fixed. The second model included the interaction between the stud and mare's birth period as random, whereas in the third model stud effects were also treated as random. The models were compared based on the deviances. Phenotypic and genetic trends were evaluated as well. The twinning rate was 3.5%. The study involved 443 twins and two triplets. Heritability estimates varied from 0.24 (model III) to 0.29 (model I), whereas repeatability estimates ranged from 0.33 (model I) to 0.34 (model III). The fluctuations of average annual genetic effects may result from segregation of the major gene in the population.

Algorithms↗
PubMed1985

A threshold model for punctuated gradualism.

A model for continuous and discontinuous evolutionary change is proposed that accommodates both punctuated equilibrium and phyletic gradualism. Natural selection operating on a gaussian distribution of phenotypes subjected to severe stress will result in a skewing of the normal distribution in the direction of the favoured phenotype. If a reproductive isolation threshold is imposed upon this model, rapid crossing of the threshold by large numbers of individuals within the skewed distribution will result in the sudden achievement of a critical descendant population mass. Mild directional selection however, will result in gentle shifts of the gaussian mean thus leading us to suggest that stasis and incipient speciatior are extremes of the same spectrum, defined by the rates of mean-shift in response to varying severities of selection pressure. Both punctuated and gradualistic evolution as accommodated in our model may further be described within Thom's cusp catastrophe theory.

Biological Evolution↗
PubMed2004

Two-threshold model for scaling laws of noninteracting snow avalanches.

The sizes of snow slab failure that trigger snow avalanches are power-law distributed. Such a power-law probability distribution function has also been proposed to characterize different landslide types. In order to understand this scaling for gravity-driven systems, we introduce a two-threshold 2D cellular automaton, in which failure occurs irreversibly. Taking snow slab avalanches as a model system, we find that the sizes of the largest avalanches just preceding the lattice system breakdown are power-law distributed. By tuning the maximum value of the ratio of the two failure thresholds our model reproduces the range of power-law exponents observed for land, rock, or snow avalanches. We suggest this control parameter represents the material cohesion anisotropy.

Journal Article↗
PubMed1994

On the antigenic diversity threshold model for AIDS.

In this paper stochastic simulations of a model for AIDS pathogenesis developed by Nowak and coworkers are presented. The results reveal a strong dependency on initial conditions, and the model predicts a nonmonotonic hazard rate for the transition to AIDS. Doubts are raised concerning the significance of the "antigenic diversity threshold," since it only partially characterizes the model dynamics.

Acquired Immunodeficiency Syndrome↗
PubMed1999

What price information? Modelling threshold probabilities of fetal loss.

This paper is an extension of previous work in which an alternative method of measuring the benefits of antenatal screening was proposed. The method is based on the elicitation of threshold probabilities of fetal loss at which women would be indifferent between having and not having an amniocentesis for the prenatal diagnosis of fetal abnormalities. The aim of this paper is to extend the previous work by modelling the preferences of a larger sample of women and investigating the consistency and validity of their responses. The threshold probabilities are elicited using standard gambles and modelled using Tobit estimation. The results indicate that it is possible to model these probabilities in this way and that it is possible to obtain a high degree of consistency in response to standard gamble questions. While establishing the validity of the responses is more difficult there is some evidence that the technique can provide valid responses.

Adult↗
PubMed2001

Requirement for sustained MAPK signaling in both CD4 and CD8 lineage commitment: a threshold model.

Although there is general agreement that the RAS/MAPK signaling pathway is required for positive selection of CD4 T cells in the thymus, the role of this pathway in CD8 lineage commitment remains controversial. We show here that the differentiation of isolated cultured thymocytes to the CD8 as well as CD4 T cell lineage is sensitive to MEK inhibition and that both CD4 and CD8 thymocyte differentiation requires sustained MEK signaling. However, CD4 lineage commitment is promoted by a stronger stimulus for longer duration than required for CD8 lineage commitment. Interestingly, CD4 lineage commitment is not irreversibly set even after 10 h of signaling, well past early changes in gene expression. These findings are presented in the context of a model of lineage commitment in which a default pathway of CD8 lineage commitment is altered to CD4 commitment if the thymocyte achieves a threshold level of active MAPK within a certain time frame.

Animals↗
PubMed2002

A new laser pain threshold model detects a faster onset of action from a liquid formulation of 1 g paracetamol than an equivalent tablet formulation.

AIMS: To discover whether a new infra-red laser method could detect a change in pain threshold after as mild an analgesic as paracetamol and whether an effervescent liquid formulation produced a faster onset of action than tablets. METHODS: This double-blind, placebo controlled randomized study used a portable, infra-red laser to measure "first pain" thresholds on the nondominant forearm in 12 normal volunteers before and after 1 g of paracetamol or placebo. The mean of six recordings was determined three times before dosing, the first being used as a familiarization procedure, and 14 times after dosing. RESULTS: We detected a small (2%), statistically significant difference in pain threshold between a liquid formulation of paracetamol and placebo at 30 and 60 min (P = 0.004 and P = 0.001), but not between tablets and placebo. Liquid also increased the threshold significantly compared with tablets at 60 min (P = 0.01). CONCLUSIONS: To detect such a small increase in pain threshold requires a highly consistent measure and the coefficient of variation was 2% for the study overall, surprisingly low for a subjective phenomenon. The reasons for this include minimizing reflectance by blacking the skin, using a nonhairy site, averaging six data points at each sample time and controlling closely the ambient conditions and the subjects' preparation for studies.

Acetaminophen↗
PubMed1981

[Polygenic threshold model and the phenogenetic aspects of human fingerprints].

Based on the existence of the two genetic complexes determining finger prints (SU - spiral and SR - despiral), the two-compartment multithreshold polygenic model for systematization of finger prints has been proposed. It was found that the radial loop is genotypically not identical to the ulnar loop, as it was thought before, but differs very much from the latter by its print. The relative height of thresholds for each of 10 fingers has been measured. The two embryonal gradients have been established: one with a positive, and the other with a negative correlation between the threshold heights.

Dermatoglyphics↗
PubMed2004

Modeling threshold phenomena, metabolic pathways switches and signals in chemostat-cultivated cells: the Crabtree effect in Saccharomyces cerevisiae.

The long-term Crabtree effect in Saccharomyces cerevisiae cultivated in aerobic chemostat at steady state has been studied for three different substrate concentrations in the feed of the bioreactor (data: J. Gen. Microbiol., 129 (1983) 653). We have shown that a model using two ways of transport/metabolization (T/M) of hyperbolic form, with high and low affinity for the substrate, allowed to represent correctly the main characteristics of the phenomenon. The model is based on an explicit form of the T/M kinetics when the bioreactor is considered as a polyphasic dispersed system (PDS). Mass balances analysis also allows to quantify the critical dilution rate value (threshold), Dc, of the transition between respiratory and respirofermentative mode, for which ethanol is produced. A good approximation for the threshold is Dc = V(S)0 Y(Xc, S) where Y(Xc,S) is the average yield coefficient before transition and V(S)0, the maximum specific rate of high affinity T/M pathway. The theoretical value is 0.3 h(-1), and is equal to the experimental value. We thus show in a quantitative way that the transition depends both on culture conditions (global characteristic of the system) and on strain properties (intrinsic characteristic of the microorganism as well). Using two different methods to calculate the residual substrate has carried out the comparison between the simulations end the experimental data. This allowed showing that the latter is not well represented by Monod's model and has confirmed that the affinity for the substrate varies according to the biomass. We have then shown how to calculate the most important specific rates (or metabolic flux) related to biomass, ethanol, oxygen, hydrogen, respiratory and fermentative CO(2) and H(2)O within the cellular phase. It has appeared that the oxygen uptake rate directly depends on high-affinity T/M pathway. This let us think that the regulation of the Crabtree effect in S. cerevisiae depends on the saturation of some glucose metabolization and transport pathways rather than on saturation of the respiratory chains. The specific rates analysis has also allowed us to show, at least in this case, that the metabolization rate (biosynthesis+fueling) had its maximum value on the whole dilution rates interval; metabolites excretion (ethanol and fermentative CO(2)) only intervenes to drain a "surplus" glucose flux. As a consequence, the transport capacity must be higher than the one of metabolization. Maximization of the metabolization specific rate could then be used as an optimization criterion in the stoichiometric calculation of metabolic flux (and not the specific growth rate maximization because growth is limited in a chemostat (mu = D)). We have also shown that the mass balances based on the T/M processes are in agreement with molar and elementary balances of the general stoichiometric equation for glucose respiration and fermentation under aerobic conditions. Thanks to the specific rates calculating the stoichiometric coefficients has done this. The total mass balance difference does not exceed 4%, which is compatible with the experimental carbon balance. Finally, we have emphasized that the ratio of biosynthesis flux and metabolization flux is constant before and after transition. This observation could be applied as soon as the free substrate concentration in the cellular phase is low. The paper succinctly describes the former theoretical results on which the model is built and sufficiently explains the algorithm for straightforward implementation.

Algorithms↗
PubMed1992

Threshold model in the genetics of age-dependent disease in twins: I. General principles as applied to Alzheimer disease.

In the context of the etiology and pathogenesis of Alzheimer disease (AD), we discuss assumptions under which categorical data on the phenotypes of twin pairs may be used to estimate standardized characteristics (correlation and the critical threshold) for an age-dependent multinomial process. Important topics include Erlangian, gamma, and Poisson processes, tetrachoric and trichoric functions, and degrees of freedom and how they relate to estimation from both an abstract and a practical standpoint. Under plausible assumptions about the age-dependence of a heritable trait, it is possible to generate sufficient degrees of freedom to test the genetic model and to explore age-dependence and the impact of environmental factors that may influence it systematically. Though general in scope, the model is focused on data from twin pairs. The statistical strategy is briefly outlined, but its properties are not examined in detail.

Aging↗
PubMed1979

Empirical examination of the threshold model of neuron firing.

An elementary model of neuronal activity involves temporal and spatial summation of postsynaptic currents that are elicited by presynaptic spikes and that, in turn, elicit postsynaptic potentials at a trigger zone; when the potential at the trigger zone exceeds a "threshold" level, a postsynaptic spike is generated. This paper describes three methods of estimating the "summation function", that is, the function of time that converts the synaptic current into potential at the trigger zone: namely, maximum likelihood, cross-correlation analysis and cross-spectral analysis. All three methods, when applied to input-output data collected on various neurons of Aplysia californica, give comparable results. As estimated, the summation function involved in the explored cells has an early positive-going swing that is large and brief. In the cell L5, but not in R2, there was also a late negative-going swing of longer duration.

Animals↗