Search PubMed⌕ Search

SEARCH · Search PubMed

Results for “Diffusion models”

Search indexed PubMed citations on genomics, clinical trials, systematic reviews and public health. Explore titles, authors and supplied subject terms, then open the PubMed record.

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

At least 55 records · Page 3Linked to original sources
PubMed2002

A diffusion model account of response time and accuracy in a brightness discrimination task: fitting real data and failing to fit fake but plausible data.

A brightness discrimination experiment was performed to examine how subjects decide whether a patch of pixels is "bright" or "dark," and stimulus duration, brightness, and speed versus accuracy instructions were manipulated. The diffusion model (Ratcliff, 1978) was fit to the data, and it accounted for all the dependent variables: mean correct and error response times, the shapes of response time distributions for correct and error responses, and accuracy values. Speed-accuracy manipulations affected only boundary separation (response criteria settings) in the model. Drift rate (the rate of accumulation of evidence) in the diffusion model, which represents stimulus quality, increased as a function of stimulus duration and stimulus brightness but asymptoted as stimulus duration increased from 100 to 150 msec. To address the argument that the diffusion model can fit any pattern of data, simulated patterns of plausible data are presented that the model cannot fit.

Attention↗
PubMed2003

The application of the convective diffusion model and the film equilibrium model to surfactant-facilitated dissolution of gliclazide.

Gliclazide is practically insoluble in water, and has low dissolution rate. Therefore, it was of interest to improve its dissolution rate using anionic and cationic surfactants. The intrinsic dissolution rates of gliclazide in solutions of sodium dodecyl sulfate (SDS) and in solutions of tetradecyltrimethyl ammonium bromide (TDTMAB) were measured using the rotating disk method to study the convective diffusion transport of drug-loaded micelles. Two different approaches were applied to the experimental data; the convective diffusion model and the film equilibrium model. The two approaches are based on the same fundamental assumptions differing only in their interpretation of the diffusional boundary layer. The results obtained from the film equilibrium model were less satisfactory, and in case of TDTMAB the model was inapplicable (negative diffusion coefficient). While excellent results were obtained from the convective diffusion model. The free solute diffusion coefficient (D(s)) obtained experimentally was 2.47 x 10(-5) cm(2)/s, and the diffusion coefficient of the drug-loaded SDS micelle (D(sm)) estimated was 1.74 x 10(-6) cm(2)/s. The drug-loaded SDS micelle radius was 14 A. The thickness of the diffusional boundary layer was 54 and 22 microm for the free solute and the drug-loaded SDS micelle, respectively. TDTMAB showed lower effect in improving the dissolution rate of gliclazide than SDS. The drug-loaded TDTMAB micelle diffusion coefficient was 1.03 x 10(-6) cm(2)/s. The radius of the drug-loaded TDTMAB micelle and the boundary layer thickness were 24 A and 19 microm, respectively.

Diffusion↗
PubMed1997

Accuracy of noninvasive in vivo measurements of photosensitizer uptake based on a diffusion model of reflectance spectroscopy.

This study compares the photosensitizer concentration measured noninvasively in vivo by diffuse reflectance spectroscopy with the results of postmortem tissue solubilization and fluorometric assay. The reflectance spectrometer consists of a fiber optic surface probe, spectrometer and charge-coupled device (CCD) array detector. The surface probe has eight detection fibers separated from the light source fiber by distances ranging from 0.85 to 10 mm. The imaging spectrometer disperses the light from each detector fiber onto the two-dimensional CCD array, while maintaining spatial separation of each individual spectrum. A single exposure of the CCD therefore captures the reflectance spectrum ar eight distances and over a range of 300 nm. From the spectra, the tissue's optical scattering and absorption coefficients are determined using a diffusion model of light propagation. Changes in the tissue absorption are used to estimate the photosensitizer concentration. Normal New Zealand White rabbits were injected with aluminum phthalocyanine tetrasulfonate (AlPcS4) and probe measurements made 24 h after injection on the dorsal skin, on muscle after surgically turning the skin back and on liver. For skin, the noninvasive estimate to proportional to the true concentration but low by a factor of 3. Based on Monte Carlo modeling of multilayered systems, this underestimate is attributed to the layered structure of the skin and nonuniform AlPcS4 distribution. A comparison of the noninvasive concentration estimates to the postmortem assay results finds good agreement for liver tissue even though application of the diffusion model is not strictly justified.

Animals↗
PubMed1992

Time course of transmitter release calculated from simulations of a calcium diffusion model.

A three-dimensional presynaptic calcium diffusion model developed to account for characteristics of transmitter release was modified to provide for binding of calcium to a receptor and subsequent triggering of exocytosis. When low affinity (20 microM) and rapid kinetics were assumed for the calcium receptor triggering exocytosis, and stimulus parameters were selected to match those of experiments, the simulations predicted a virtual invariance of the time course of transmitter release to paired stimulation, stimulation with pulses of different amplitude, and stimulation in different calcium solutions. The large temperature sensitivity of experimental release time course was explained by a temperature sensitivity of the model's final rate limiting exocytotic process. Inclusion of calcium tail currents and a saturable buffer with finite binding kinetics resulted in high peak calcium transients near release sites, exceeding 100 microM. Models with a single class of calcium binding site to the secretory trigger molecule failed to produce sufficient synaptic facilitation under this condition. When at least one calcium ion binds to a different site having higher affinity and slow kinetics, facilitation again reaches levels similar to those seen experimentally. It is possible that the neurosecretory trigger molecule reacts with calcium at more than one class of binding site.

Animals↗
PubMed1980

Scale-invariance in reaction-diffusion models of spatial pattern formation.

We propose a reaction-diffusion model of spatial pattern formation whose solutions can exhibit scale-invariance over any desired range for suitable choices of parameters in the model. The model does not invoke preset polarity or any other ad hoc distinction between cells and provides a solution to the French flag problem without sources at the boundary. Furthermore, patterns other than the polar pattern that usually arises first in a growing one-dimensional system described by Turing's model can be obtained. Evidence is given that suggests that the model may apply in the slug stage of Dictyostelium discoideum.

Cell Differentiation↗
PubMed1995

New hepatocellular diffusion model for analysis of hepatobiliary transport processes of drugs.

A new hepatocellular diffusion model was developed to kinetically evaluate the hepatobiliary transport processes of drugs in the perfusion system, based on the physiological structure of the liver. Since the equations describing the hepatocellular diffusion phenomena were derived as image forms in the Laplace domain, the fast inverse Laplace transform (FILT) was adopted to manipulate the image equations. Cefixime and cefpiramide were selected as model drugs. The concentrations in the perfusate and the excreted amounts into the bile were simultaneously measured at appropriate intervals after the rapid administration of each drug into the portal vein. The hepatocellular diffusion model was fitted to the biliary excretion profiles from rat livers, by means of a nonlinear least squares program, MULTI(FILT). According to this model, the hepatobiliary transport process of drug is kinetically separated into three steps, that is, the diffusion into and through the hepatocytes, the transfer from the hepatocytes into the bile canaliculi, and the movement through the bile canaliculi to the outlet of bile duct. These steps are characterized by the diffusion rate constant through hepatocytes (kdif), the permeability rate constant into the bile canaliculi (kbmc) and the transit time through the bile canaliculi to the outlet of bile duct (tcan), respectively. It was demonstrated that kdif of cefixime (0.023 min-1) was significantly smaller than that of cefpiramide (0.044 min-1), while the differences in kbmc and tcan were not obvious between cefixime and cefpiramide. kbmc and tcan of both drugs were about 1.2 min-1 and about 1.0 min, respectively. These parameters were correlated to the excretion ratio into the bile (Fbile) and the mean transit time from the sinusoid through the hepatocytes to the outlet of bile duct (tbile).

Animals↗
PubMed1994

Synaptic transmission in a diffusion model for neural activity.

Equations for a diffusion neuronal model describing the production of nerve impulses have been derived for the case in which the noisy depolarizations of the membrane potential are restricted by the reversal potentials. Identifying neuron firing intervals with the first-passage-time distribution for the associated process allows us to compute the interspike interval statistics and relate them to the parameters characterizing the neuronal input. Methods for approximation of the first two moments of the interspike intervals are proposed. The analytical results are numerically illustrated and simultaneous computer simulations were performed for the same purpose. A comparison of the achieved results is made with those from other diffusion models or the models with discontinuous trajectories. The model is proposed mainly as an alternative to the Ornstein-Uhlenbeck neuronal model.

Action Potentials↗
PubMed1980

Diffusion model for genotype-dependent migration.

A diffusion model for the joint action of genotype-dependent migration and genotype-independent population regulation is derived and investigated. The diploid, monoecious population occupies a finite chain of equally spaced colonies; generations are discrete and nonoverlapping; mating is random in each colony. A boundary-value problem is deduced for the gene frequencies at a multiallelic locus in the absence of mutation, selection, and random drift. This problem is studied for two alleles and constant and uniform population density and drift and diffusion coefficients. All equilibria are shown to be monotone, and explicit conditions for a protected polymorphism are established. Two examples of asymptotically stable clines are presented. It is demonstrated that genotype dependence of the mean displacements is necessary to produce or maintain spatial differentiation of gene frequencies.

Journal Article↗
PubMed2004

Stable squares and other oscillatory turing patterns in a reaction-diffusion model.

We study the Brusselator reaction-diffusion model under conditions where the Hopf mode is supercritical and the Turing band is subcritical. Oscillating Turing patterns arise in the system when bulk oscillations lose their stability to spatial perturbations. Spatially uniform external periodic forcing can generate oscillating Turing patterns when both the Turing and Hopf modes are subcritical in the autonomous system. Most of the symmetric patterns show period doubling in both space and time. Patterns observed include squares, rhombi, stripes, and hexagons.

Journal Article↗
PubMed2005

Exact dynamics of a reaction-diffusion model with spatially alternating rates.

We present the exact solution for the full dynamics of a nonequilibrium spin chain and its dual reaction-diffusion model, for arbitrary initial conditions. The spin chain is driven out of equilibrium by coupling alternating spins to two thermal baths at different temperatures. In the reaction-diffusion model, this translates into spatially alternating rates for particle creation and annihilation, and even negative "temperatures" have a perfectly natural interpretation. Observables of interest include the magnetization, the particle density, and all correlation functions for both models. Two generic types of time dependence are found: if both temperatures are positive, the magnetization, density, and correlation functions decay exponentially to their steady-state values. In contrast, if one of the temperatures is negative, damped oscillations are observed in all quantities. They can be traced to a subtle competition of pair creation and annihilation on the two sublattices. We comment on the limitations of mean-field theory and propose an experimental realization of our model in certain conjugated polymers and linear chain compounds.

Journal Article↗
PubMed1997

Monte Carlo uncertainty analysis of a diffusion model for the assessment of halogen gas exposure during dosing of brominators.

Monte Carlo simulation was incorporated into a diffusion-based exposure assessment model for the estimation of worker exposure to halogen gases during dosing of 500-lb sacks of a bromine-based biocide (BCDMH) into brominators. Indoor and outdoor dosing scenarios were modeled for small and large brominators. The diffusion model used describes a concentration gradient of halogen as a function of distance and time from the source instead of ascribing worst-case single point value estimates to the variables used in the diffusion model. Monte Carlo simulation was used to describe a distribution of values for each appropriate model variable. Using a personal computer and Monte Carlo simulation software, 10,000 iterations of the diffusion model were performed for four different dosing scenarios using random and independent samples from the distributions entered. The corresponding output distributions of predicted exposures were then calculated and displayed graphically for each scenario. The results of the Monte Carlo simulation predict that outdoor dosing of either small or large brominators with BCDMH is highly unlikely to result in an exceedance of the working occupational exposure limit for total halogen. In most ambient wind speed conditions, diffusion prevents appreciable airborne exposure to workers in the immediate vicinity of the brominator. Although relatively uncommon, dosing of brominators indoors in the assumed absence of local exhaust ventilation may generate airborne concentrations of total halogen that exceed the working short-term occupational exposure limit. Although very limited and inconclusive, field trial monitoring of BCDMH transfer operations indoors resulted in halogen concentrations well within the distribution of concentrations predicted by the Monte Carlo simulation of the diffusion model.

Air Pollution↗
PubMed2001

Field theory for a reaction-diffusion model of quasispecies dynamics.

RNA viruses are known to replicate with extremely high mutation rates. These rates are actually close to the so-called error threshold. This threshold is in fact a critical point beyond which genetic information is lost through a second-order phase transition, which has been dubbed as the "error catastrophe." Here we explore this phenomenon using a field theory approximation to the spatially extended Swetina-Schuster quasispecies model [J. Swetina and P. Schuster, Biophys. Chem. 16, 329 (1982)], a single-sharp-peak landscape. In analogy with standard absorbing-state phase transitions, we develop a reaction-diffusion model whose discrete rules mimic the Swetina-Schuster model. The field theory representation of the reaction-diffusion system is constructed. The proposed field theory belongs to the same universality class as a conserved reaction-diffusion model previously proposed [F. van Wijland et al., Physica A 251, 179 (1998)]. From the field theory, we obtain the full set of exponents that characterize the critical behavior at the error threshold. Our results present the error catastrophe from a different point of view and suggest that spatial degrees of freedom can modify several mean-field predictions previously considered, leading to the definition of characteristic exponents that could be experimentally measurable.

Biophysical Phenomena↗
PubMed2004

Nurses' adoption of technology: application of Rogers' innovation-diffusion model.

This qualitative study applied Everett Rogers' innovation-diffusion model to analyze nurses' perceptions toward using a computerized care plan system. Twelve nurses from three respiratory intensive care units in Taiwan voluntarily participated in a one-on-one, in-depth interview. Data were analyzed by constant comparative analysis. The content that emerged was compared with the model's five innovation characteristics (relative advantage, compatibility, complexity, trialability, and observability), as perceived by new users. Results indicate that Rogers' model can accurately describe nurses' behavior during the process of adopting workplace innovations. Related issues that emerged deserve further attention to help nurses make the best use of technology.

Adaptation, Psychological↗
PubMed2006

A finite locus effect diffusion model for the evolution of a quantitative trait.

A diffusion model is constructed for the joint distribution of absolute locus effect sizes and allele frequencies for loci contributing to an additive quantitative trait under selection in a haploid, panmictic population. The model is designed to approximate a discrete model exactly in the limit as both population size and the number of loci affecting the trait tend to infinity. For the case when all loci have the same absolute effect size, formal multiple-timescale asymptotics are used to predict the long-time response of the population trait mean to selection. For the case where loci can take on either of two distinct effect sizes, not necessarily with equal probability, numerical solutions of the system indicate that response to selection of a quantitative trait is insensitive to the variability of the distribution of effect sizes when mutation is negligible.

Evolution, Molecular↗
PubMed2025

Generating synthetic genotypes using diffusion models.

SUMMARY: In this paper, we introduce the first diffusion model designed to generate complete synthetic human genotypes, which, by standard protocols, one can straightforwardly expand into full-length, DNA-level genomes. The synthetic genotypes mimic real human genotypes without just reproducing known genotypes, in terms of approved metrics. When training biomedically relevant classifiers with synthetic genotypes, accuracy is near-identical to the accuracy achieved when training classifiers with real data. We further demonstrate that augmenting small amounts of real with synthetically generated genotypes drastically improves performance rates. This addresses a significant challenge in translational human genetics: real human genotypes, although emerging in large volumes from genome wide association studies, are sensitive private data, which limits their public availability. Therefore, the integration of additional, insensitive data when striving for rapid sharing of biomedical knowledge of public interest appears imperative. AVAILABILITY AND IMPLEMENTATION: All non proprietary data and the code to replicate the experiments is available on Github.

Humans↗