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PubMed2001

Strange quark polarization of the nucleon: a parameter-independent prediction of the chiral potential model.

We perform a one-loop calculation of the strange quark polarization (Deltas) of the nucleon in an SU(3) chiral potential model. We find that if the intermediate quark excited states are summed over in a proper way, i.e., summed up to a given energy instead of given radial and orbital quantum numbers, Deltas turns out to be almost independent of all the model parameters: quark masses and potential strengths. The contribution from the quark-antiquark pair creation and annihilation " Z" diagrams is found to be significant. Our numerical results agree quite reasonably with experiments and lattice QCD calculations.

Journal Article↗

PubMed2006

Phase separation in a compressible 2D Ising model.

We perform a high precision Monte Carlo study of asymptotic domain growth in a compressible two-dimensional spin-exchange Ising model with continuous particle positions and zero total magnetization, and we investigate the effects of compressibility and lattice mismatch on the late-time domain growth law, R(t) = A + Bt(n). For mismatched systems, we measure significant deviations from the theoretically expected n = 1/3 late-time growth (n = 0.224 +/- 0.004), and for a compressible model with no mismatch, we measure only a slight deviation from n = 1/3. These results strongly suggest that the current understanding of the classes of domain growth is incomplete.

Journal Article↗

PubMed1993

Frequency discrimination in noise: comparison of cat performances with auditory-nerve models.

Pure-tone frequency discrimination (delta F) performances were measured in cats and compared to neural models of these delta F performances based on auditory-nerve data in cats. Animal psychophysical techniques were used to train cats to discriminate frequency changes for pulsed pure tones in background noise at both 1.0 and 3.0 kHz. A go-left, go-right procedure was employed, and delta F's were measured in noise as a function of signal level at a constant signal-to-noise ratio. In contrast to human listeners, cats showed increases in delta F at 1.0 kHz with increasing signal level. Model estimates of delta F's based on rate responses in the cat auditory nerve predict increasing delta F with increasing signal level, the trend observed in the cat psychophysical data. Model estimates of delta F's based on temporal (phase-locking) properties in cat auditory nerve, on the other hand, predict decreases in delta F that have been observed in previous data from human listeners [Dye and Hafter, J. Acoust. Soc. Am. 67, 1746-1753 (1980)]. These results suggest that for cats, average rate, rather than phase-locking, may be used by the central nervous system in performing frequency discrimination in background noise at 1.0 kHz. At 3.0 kHz cats showed little change in delta F as a function of signal level, a result similar to the trend for human listeners to show no change or slight increases in delta F with increases in signal level for tones in the 2- to 3-kHz range.

Animals↗

PubMed1997

Differential renal function measured by 99Tcm-DTPA and 99Tcm-DMSA in a complete unilateral renal obstruction rat model.

We performed a prospective study to determine whether 99Tcm-DTPA differential renal function (DRF) in the case of acute obstruction in a unipapillary kidney rat model shows a similar pattern of results as 99Tcm-DMSA in a multipapillary kidney model, and to exclude recirculation or a block of tubular transport of DMSA which might explain the discrepancy reported by Kelleher et al. In 15 male and 4 female Wistar rats (weight 300-350 g), 99Tcm-DMSA renal uptake was measured 24 h before complete obstruction of the right ureter, which served as reference values. Twenty-four hours after obstruction, 99Tcm-DTPA (18.5 MBq) renography and 99Tcm-DMSA (111 MBq) non-depth-corrected renal uptake measurements were performed in the 15 male rats; in the 4 female rats, 740 MBq of 99Tcm-DMSA were injected and absolute, non-depth-corrected renal uptake measured at 24, 48 and 72 h. 99Tcm-DMSA DRF of the right kidney ranged from 48 to 55% (mu: 51%; sigma: 2%) before obstruction and from 14 to 35% (mu: 24%; sigma: 6%) after obstruction, whereas 99Tcm-DTPA DRF ranged from 16 to 30% (mu: 25%; sigma: 4%). No significant differences were found between DRF measured by 99Tcm-DMSA and 99Tcm-DTPA (P = 0.18), or between DRF of the obstructed kidney as measured by 99Tcm-DMSA at 24, 48 and 72 h (P > 0.2). Hypothetically, the discrepancy between our findings and those of Kelleher et al. may be due to intratubular pressure. In conclusion, the present findings and those of Kelleher et al. suggest the differences in DRF following complete unilateral renal obstruction, as determined by DTPA and DMSA, are probably species-specific. Furthermore, recirculation and block of tubular transport are unlikely to contribute significantly to differences in DRF as measured by the two radiopharmaceuticals.

Animals↗

PubMed2002

A computationally efficient evolutionary algorithm for real-parameter optimization.

Due to increasing interest in solving real-world optimization problems using evolutionary algorithms (EAs), researchers have recently developed a number of real-parameter genetic algorithms (GAs). In these studies, the main research effort is spent on developing an efficient recombination operator. Such recombination operators use probability distributions around the parent solutions to create an offspring. Some operators emphasize solutions at the center of mass of parents and some around the parents. In this paper, we propose a generic parent-centric recombination operator (PCX) and a steady-state, elite-preserving, scalable, and computationally fast population-alteration model (we call the G3 model). The performance of the G3 model with the PCX operator is investigated on three commonly used test problems and is compared with a number of evolutionary and classical optimization algorithms including other real-parameter GAs with the unimodal normal distribution crossover (UNDX) and the simplex crossover (SPX) operators, the correlated self-adaptive evolution strategy, the covariance matrix adaptation evolution strategy (CMA-ES), the differential evolution technique, and the quasi-Newton method. The proposed approach is found to consistently and reliably perform better than all other methods used in the study. A scale-up study with problem sizes up to 500 variables shows a polynomial computational complexity of the proposed approach. This extensive study clearly demonstrates the power of the proposed technique in tackling real-parameter optimization problems.

Algorithms↗

PubMed2000

Performance of the Mortality Probability Models in assessing severity of illness during the first week in the intensive care unit.

OBJECTIVE: To extend the Mortality Probability Models (MPM) II severity system to time periods between 4 and 7 days after admission to the intensive care unit (ICU). DESIGN: Prospective inception cohort. SETTING: Fifteen adult medical and surgical ICUs in Spain. PATIENTS: A total of 1,441 patients aged > or =18 yrs consecutively admitted from April 1, 1995 through July 31, 1995. INTERVENTIONS: Prospective data collection during the stay of the patient in the ICU. Data collected included demographic information, length-of-stay and vital status at both ICU and hospital discharge, as well as all variables necessary for computing the MPM II system at admission and during the first 7 days of stay in the ICU MEASUREMENTS AND MAIN RESULTS: Calibration and discrimination of the four existing MPM II models (MPM0, MPM24, MPM48, and MPM72) were assessed in the study database. The MPM II system overestimated the mortality of patients with probabilities of death > or =0.4. The MPM24 model was customized. Models for time periods between 48 hrs and 7 days (MPM48 to MPMd7) were obtained using the same strategy that was used to develop the original MPM48 and the MPM72 models. The variable coefficients of the MPM24 model were kept fixed and the constant terms of the MPM48 to MPMd7 models were estimated by logistic regression. The constant term stabilized after the fourth day of admission and it was similar to the constant term of the MPM72 model. The customized MPM72 performed very well for days 4 to 7 after admission to the ICU. CONCLUSIONS: If the patient's condition stays the same day after day, the probability of dying in the hospital increases until 72 hrs, and then stabilizes. A severity measure that performs well at 72 hrs can be a useful tool for measuring severity at later time periods.

Critical Care↗

PubMed1984

Effect of a medical care evaluation program on physician knowledge and performance.

A model program designed to increase the educational value of medical care evaluation committee meetings was studied to determine its effect on the knowledge and clinical performance of participating physicians. The members of hospital committees in which the program was successfully implemented showed a statistically significant gain in knowledge of the topics discussed by their committees. In addition, several members made substantial changes in their patient care practices. These changes resulted not so much from the acquisition of new medical information as from a rethinking of patient management strategies, stimulated by peer discussion during committee meetings. A structure that encourages such discussions can be incorporated in other types of patient-care-oriented committee activities as well.

Clinical Competence↗

PubMed1981

Evaluation of information sequential aspects of schizophrenic performance. II. Research strategies and methodological issues.

This presentation deals with research issues surrounding the investigation of schizophrenic performance from the perspective of the experimental psychology of normal cognition. Topics of subject selection, influence of medication on subjects' performance, appropriateness of underlying models of performance developed with normal samples, and nonuniform measurement properties of dependent variables are dealt with. Relevant items of rationale for this branch of study are put forth and evaluated. Finally, the notion of critical deficit--whereby the importance of deterioration in a component of performance is judged according to the component's role in negotiating environmental demands (rather than the component's absolute magnitude of deterioration)--is detailed and exemplified.

Cognition↗

PubMed2002

The effects of piracetam on cognitive performance in a mouse model of Down's syndrome.

Piracetam is a nootropic agent that has been shown to improve cognitive performance in a number of animal model systems. Piracetam is reported to be used widely as a means of improving cognitive function in children with Down's syndrome (DS). In order to provide a preclinical assessment of the potential efficacy of piracetam, we examined the effects of a dose range of piracetam in the Ts65Dn mouse model of DS. Ts65Dn mice are trisomic for a region of mouse chromosome 16 with homology to human chromosome 21. Daily piracetam treatment at doses of 0, 75, 150, and 300 mg/kg ip was initiated in 6-week-old male Ts65Dn and euploid control mice. Following 4 weeks of treatment, mice were tested in the visible and hidden-platform components of the Morris water maze and were placed overnight in computerized activity chambers to assess effects on overall activity. Piracetam treatment was continued through the 4 weeks of testing. In control mice, 75 and 150 mg/kg/day piracetam improved performance in both the visible- and hidden-platform tasks. Although low doses of piracetam reduced search time in the visible-platform component in Ts65Dn mice, all piracetam doses prevented trial-related improvements in performance in Ts65Dn mice. The 300-mg/kg/day-piracetam dose was associated with a reversal of the nocturnal spontaneous hyperactivity in Ts65Dn. These data do not provide support for piracetam treatment for individuals with DS.

Animals↗

PubMedDate not supplied

Factors associated with pressure ulcers in adults in acute care hospitals.

OBJECTIVES: To identify and describe the relationship of factors associated with pressure ulcers in adults in acute care hospitals. DESIGN: Cross-sectional prevalence studies. SETTING: University teaching hospital. PATIENTS: Prevalence studies conducted from 1993 to 1995 with a total of 1992 subjects served as the derivation sample and a 1996 prevalence study with 581 subjects served as the validation sample. MAIN OUTCOME MEASURES: Pressure ulcers and the Braden risk assessment subscale scores. DATA ANALYSIS: Logistic regression analysis was used to derive a model that fit the data and performed well at identifying factors associated with pressure ulcers. Performance of the model, in terms of calibration, was statistically evaluated using the Hosmer-Lemeshow goodness-of-fit test. The effectiveness of the model, in terms of discrimination, was assessed by considering the cut-off values using 2 by 2 classification tables to measure the overall percentage of subjects correctly classified in the validation sample. MAIN RESULTS: Factors associated with pressure ulcers in adults in acute care hospitals were identified as age, male gender, sensory perception, moisture, mobility, nutrition, and friction/shear. Three interactions were also found to be associated with pressure ulcers; 2 interactions (age and sensory perception and moisture and sensory perception) were negatively associated and 1 interaction (nutrition and gender/male) was positively associated with pressure ulcers. The Hosmer-Lemeshow goodness-of-fit test for the derivation sample (0.76) and the validation sample (0.79) indicated that the model was well calibrated and a good fit. The overall percentage of subjects correctly classified using the validation sample was 88%, indicating that the model performed well. CONCLUSIONS: This study enhances the knowledge of the relationship of factors associated with pressure ulcers in adults in acute care populations and enhances the use and relative importance of particular Braden Scale subscales.

Acute Disease↗

PubMed1991

Teacher versus peer models revisited: effects on motor performance and self-efficacy.

This study reinvestigated and extended the findings of Landers and Landers (1973), which examined the influence of skilled and unskilled teacher and peer models on motor performance. Sixth grade females (N = 100) were randomly assigned to groups in a 2 x 2 (Model Type x Model Skill) factorial design or to a no-model group. In the treatment groups, subjects observed one of four unfamiliar models--(a) a skilled teacher, (b) an unskilled teacher, (c) a skilled peer, or (d) an unskilled peer--perform on the Bachman ladder task. Subjects performed 30 trials of the task and also completed self-efficacy questionnaires on three occasions. Results showed that subjects who watched a skilled model performed better than subjects who watched an unskilled model. Unlike the Landers and Landers study, no model type by model skill interaction was found. In addition, the skilled model group reported higher efficacy beliefs than the control group after performing and were more efficacious than the unskilled model subjects both after watching the model and after performing. These results suggest when students view unfamiliar models, the skill rather than the status of the model may be more salient.

Attention↗

PubMed2004

Factors associated with pressure ulcers in adults in acute care hospitals.

OBJECTIVES: To identify and describe the relationship of factors associated with pressure ulcers in adults in acute care hospitals. DESIGN: Cross-sectional prevalence studies. SETTING: University teaching hospital. PATIENTS: Prevalence studies conducted from 1993 to 1995 with a total of 1992 subjects served as the derivation sample and a 1996 prevalence study with 581 subjects served as the validation sample. MAIN OUTCOME MEASURES: Pressure ulcers and the Braden risk assessment subscale scores. DATA ANALYSIS: Logistic regression analysis was used to derive a model that fit the data and performed well at identifying factors associated with pressure ulcers. Performance of the model, in terms of calibration, was statistically evaluated using the Hosmer-Lemeshow goodness-of-fit test. The effectiveness of the model, in terms of discrimination, was assessed by considering the cut-off values using 2 by 2 classification tables to measure the overall percentage of subjects correctly classified in the validation sample. MAIN RESULTS: Factors associated with pressure ulcers in adults in acute care hospitals were identified as age, male gender, sensory perception, moisture, mobility, nutrition, and friction/shear. Three interactions were also found to be associated with pressure ulcers; 2 interactions (age and sensory perception and moisture and sensory perception) were negatively associated and 1 interaction (nutrition and gender/male) was positively associated with pressure ulcers. The Hosmer-Lemeshow goodness-of-fit test for the derivation sample (.76) and the validation sample (.79) indicated that the model was well calibrated and a good fit. The overall percentage of subjects correctly classified using the validation sample was 88%, indicating that the model performed well. CONCLUSIONS: : This study enhances the knowledge of the relationship of factors associated with pressure ulcers in adults in acute care populations and enhances the use and relative importance of particular Braden Scale sub-scales.

Acute Disease↗

PubMed2002

Comparison of performance of spherical and realistic head models in dipole localization from noisy EEG.

The performance of a three-shell spherical head model versus the performance of a realistic head model is investigated when solving the inverse problem with a single dipole, in the presence of noise. This is evaluated by calculating the average dipole location error for 1000 noisy scalp potential sets, originating from the same test dipole and having the same noise level. The average location errors are obtained utilizing a local linearization, which is validated with a Monte-Carlo simulation. When the difference between the average location error utilizing a spherical and a realistic head model, represented by deltaR, is large for a large number of test dipoles, then it is worth using the more computationally demanding realistic head model. However, if deltaR is small for a large number of test dipoles, then it does not matter which model is used. For 27 electrodes, an electroencephalogram (EEG) epoch of one time sample and spatially white Gaussian noise, we found that the importance of the realistic head model over the spherical head model reduces by increasing the noise level. We further found that increasing the number of scalp electrodes from 27 to 44 has limited impact on the importance of the realistic head model over the spherical head model in EEG dipole source analysis. By increasing the number of time samples to six, the performance of the realistic head model in the inverse calculation gains importance compared with the three-shell spherical head model. Finally, we used spatially and temporally correlated background EEG instead of Gaussian noise. The advantage of the realistic head model over the spherical head model is reduced when applying correlated noise compared to Gaussian noise.

Algorithms↗

PubMed2005

Intraarterial reteplase and intravenous abciximab for treatment of acute ischemic stroke. A preliminary feasibility and safety study in a non-human primate model.

We performed a preliminary feasibility and safety study using intravenous (IV) administration of a platelet glycoprotein IIb/IIIa inhibitor (abciximab) in conjunction with intraarterial (IA) administration of a thrombolytic agent (reteplase) in a primate model of intracranial thrombosis. We introduced thrombus through superselective catheterization of the intracranial segment of the internal carotid artery in 16 primates. The animals were randomly assigned to receive IA reteplase and IV abciximab ( n =4), IA reteplase and IV placebo ( n =4), IA placebo and IV abciximab ( n =4) or IA and IV placebo ( n =4). Recanalization was assessed by serial angiography during the 6-h period after initiation of treatment. Postmortem magnetic resonance (MR) imaging was performed to determine the presence of cerebral infarction or intracranial hemorrhage. Partial or complete recanalization at 6 h after initiation of treatment (decrease of two or more points in pre-treatment angiographic occlusion grade) was observed in two animals treated with IA reteplase and IV abciximab, three animals treated with IA reteplase alone and one animal treated with IV abciximab alone. No improvement in perfusion was observed in animals that received IV and IA placebo. Cerebral infarction was demonstrated on postmortem MR imaging in three animals that received IA and IV placebo and in one animal each from the groups that received IA reteplase and IV abciximab or IV abciximab alone. One animal that received IV abciximab alone had a small intracerebral hemorrhage on MR imaging. IA reteplase with or without abciximab appeared to be the most effective regimen for achieving recanalization in our model of intracranial thrombosis. Further studies are required in experimental models to determine the optimal dose, method of administration and efficacy of these medications in acute ischemic stroke.

Abciximab↗

PubMed1998

A semi-parametric Bayesian approach to generalized linear mixed models.

The linear mixed effects model with normal errors is a popular model for the analysis of repeated measures and longitudinal data. The generalized linear model is useful for data that have non-normal errors but where the errors are uncorrelated. A descendant of these two models generates a model for correlated data with non-normal errors, called the generalized linear mixed model (GLMM). Frequentist attempts to fit these models generally rely on approximate results and inference relies on asymptotic assumptions. Recent advances in computing technology have made Bayesian approaches to this class of models computationally feasible. Markov chain Monte Carlo methods can be used to obtain 'exact' inference for these models, as demonstrated by Zeger and Karim. In the linear or generalized linear mixed model, the random effects are typically taken to have a fully parametric distribution, such as the normal distribution. In this paper, we extend the GLMM by allowing the random effects to have a non-parametric prior distribution. We do this using a Dirichlet process prior for the general distribution of the random effects. The approach easily extends to more general population models. We perform computations for the models using the Gibbs sampler.

Bayes Theorem↗

PubMed1986

Cleft lip with or without cleft palate: reanalysis of a three-generation family study from England.

The study population consists of 424 three-generation families originally ascertained through nonsyndromic cleft lip with or without cleft palate (CL +/- P) surgical probands by Carter et al [J Med Genet 19:246-261, 1982] in London, England. Carter et al proposed that the multifactorial threshold model (MF/T) could explain the data. The goal of our study was to test that hypothesis, plus alternatives, rigorously. Two approaches were used: 1) Carter et al had proposed that these data were consistent with the predictions of the MF/T as presented by Carter [Br Med Bull 25:52-57, 1969]. However, we tested those predictions using standard chi 2 tests and found statistically significant departures from the predictions in these families. 2) Complex segregation analysis under the mixed model was performed. Again, the MF/T model could be rejected, as could a model of a major locus alone. The best-fitting model included both major locus and multifactorial components. When the data were analyzed in two parts based on the proband's phenotype (CL vs CL + P) there was some evidence of heterogeneity in that there was a significant proportion of sporadic cases in the families of CL probands but not in the families of CL + P probands. Our results provide no support for the MF/T model. The results from segregation analyses of CL +/- P in these families were most consistent with autosomal major gene inheritance plus multifactorial contributions.

Cleft Lip↗

PubMed1999

Culling of dairy cows. Part I. Effects of diseases on culling in Finnish Ayrshire cows.

The effects of 15 diseases on time until culling were studied in 39,727 Finnish Ayrshire cows that calved during 1993 and were followed until the next calving or culling. The diseases studied were: dystocia, milk fever, retained placenta, displacement of the abomasum, metritis, non-parturient paresis, ketosis, rumen disorders, acute mastitis, hypomagnesemia, lameness, traumatic reticuloperitonitis, anestrus, ovarian cysts, and teat injuries. Survival analysis, using the Cox proportional hazards model, was performed and diseases were modeled as time-dependent covariates. Different stages of lactation when culling can occur were also considered. Parity, calving season and herd were included as covariates in every model. Parity had a significant effect on culling, the risk of culling being four times higher for a cow in her sixth or higher parity than for a first parity cow. The effects of diseases varied according to when the diseases occurred and when culling occurred. Mastitis, teat injuries and lameness had a significant effect on culling throughout the whole lactation. Anestrus and ovarian cysts had a protective effect against culling at the time when they were diagnosed. In general, diseases affected culling decisions mostly at the time of their occurrence. The effect seemed to decrease with time from the diagnosis of the disease. However, milk fever, dystocia and metritis also had a significant effect on culling at the end of the lactation.

Animal Husbandry↗

PubMed1999

Identification of responders to a therapy: an example of validation of a predictive model.

The general objective of randomized clinical trials is to assess if the treatment effect on a given population is clinically meaningful. In this way, one obtains an average estimate of the treatment effect over the trial population. However, a growing need for medical practitioners is to be able to predict with sufficient precision the efficiency of a given treatment for a given patient. There is little information in the literature about this issue. We have previously proposed a treatment-startified Cox model including interaction between treatment and patient's covariates, to identify and predict the responders to a therapy. In this paper, we focus on the assessment of the predictive power of the model. The performance of the predictive model for a population and for an individual was statistically validated internally and externally from several aspects. The prediction correlates well with the observation. Thus, we suggest that this approach would be useful in identifying and predicting the responders to a therapy, subject to an appropriate and more extensive validation process in real setting.

Female↗