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At least 145 records · Page 8Linked to original sources

PubMed1999

Noisy templates explain area summation.

The noisy template model is a variant of an ideal detector for a signal known except for contrast. The ideal detector cross-correlates the stimulus with a normalised template which is matched to the known signal pattern. The noisy template model simply adds noise to the matched template every time it is cross-correlated with the signal. This paper outlines the predictions of the noisy template model for area summation. The noisy template model explains Piper's Law, as does the ideal-observer, but it also explains critical area phenomena and the lack of area summation for contrast discrimination.

Computer Simulation↗

PubMed2002

Attention-deficit/hyperactivity disorder (ADHD) behaviour explained by dysfunctioning reinforcement and extinction processes.

Inattentiveness, overactivity and impulsiveness are presently regarded as the main clinical symptoms of attention-deficit/hyperactivity disorder (ADHD). Inattention is, however, a characteristic of most psychiatric disorders. It is argued that the ADHD Inattentive subtype may have heterogeneous origins and be qualitatively different from the ADHD Hyperactive/Impulsive subtype. At the neurobiological level, ADHD symptoms may to a large extent be caused by a dysfunctioning dopamine system: A dysfunctioning meso-limbo-cortical dopamine branch will produce altered reinforcement and extinction processes, on a behavioural level giving rise to deficient sustained attention, hyperactivity, motor and cognitive impulsiveness. A dysfunctioning nigro-striatal dopamine branch will cause 'extrapyramidal' symptoms. Our model disentangles the behaviours usually explained by 'executive functions' into cognitive impulsiveness, motor impulsiveness and deficient motor control. The various dopaminergic branches may not be equally dysfunctional in all individuals with ADHD. Etiologically, dopamine dysfunctioning will probably mainly be genetically determined while sometimes be induced by environmental factors like drugs of abuse or pollutants, which may explain geographical differences in prevalence rates.

Animals↗

PubMed1998

An evaluation of a measure of the proportion of the treatment effect explained by a surrogate marker.

Time-dependent markers, such as CD4 and viral load, are potential surrogate markers in AIDS clinical trials. A critical issue with surrogate markers is whether changes in these markers explain the beneficial effect of treatment on the real end point of the clinical trial. A statistic to measure the proportion of the treatment effect explained by the surrogate is p(FGS) = 1 - gamma/alpha, where alpha is the treatment effect coefficient in a Cox model and gamma is the treatment effect coefficient from a time-dependent Cox model adjusted for the marker. In this article we evaluate the statistical properties of p(FGS). Using a Monte Carlo study we show that the statistic is not well calibrated, because it can fall outside the range zero to one, even in very large samples. In the simulation study we consider situations where the time-dependent marker is measured with error at a fixed number of times. We show that a method of fitting a time-dependent Cox model involving smoothing the marker reduces the bias in the estimate of p(FGS) compared with the standard method of using the current or last observed marker value. We also show that the estimate of p(FGS) has considerable variability and can have wide confidence intervals. We conclude that p(FGS) is only likely to be useful in large trials with a strong treatment effect. The methods are illustrated using CD4 counts from an AIDS clinical trial of zidovidine versus placebo.

Acquired Immunodeficiency Syndrome↗

PubMed1988

Racial differences in the incidence of hypertensive end-stage renal disease (ESRD) are not entirely explained by differences in the prevalence of hypertension.

Blacks experience a disproportionate risk of end-stage renal disease (ESRD) compared with whites. The increased prevalence of hypertension in blacks has been suggested as an explanation for this increased risk. We were able to examine this possibility using hypertensive ESRD incidence rates in a population with well-characterized prevalence of hypertension and rate of its control. After adjusting rates of hypertensive ESRD for age, sex, and differences in the prevalence of hypertension by race, we found black:white (B:W) relative risk still to be increased. Prevalence estimates for moderate-severe hypertension and differences in the control of hypertension between the two race groups are of insufficient magnitude to explain the increase in adjusted relative risk. This observation provides further support for the possibility that there are racial differences in the susceptibility to renal damage from elevated BP, which may explain increased risk for hypertensive ESRD in blacks, or that hypertension is being erroneously diagnosed as the cause of ESRD in blacks when another cause is present.

Adult↗

PubMed2000

Social class and self-rated health: can the gradient be explained by differences in life style or work environment?

The purpose of the present paper is to describe differences in work environment and life style factors between social classes in Denmark and to investigate to what extent these factors can explain social class differences with regard to changes in self-rated health (SRH) over a 5 year period. We used data from a prospective study of a random sample of 5001 Danish employees, 18-59 years of age, interviewed at baseline in 1990 and again in 1995. At baseline we found higher prevalence in the lower classes of repetitive work, low skill discretion, low influence at work, high job insecurity, and ergonomic, physical, chemical, and climatic exposures. High psychological demands and conflicts at work were more prevalent in the higher classes. With regard to life style factors, we found more obese people and more smokers among the lower classes. The proportion with poor SRH increased with decreasing social class at baseline. The follow-up analyses showed a clear association between social class and worsening of SRH: The lower the social class, the higher the proportion with deterioration of SRH. There was no social gradient with regard to improved SRH over time. Approximately two thirds of the social gradient with regard to worsening of SRH could be explained by the work environment and life style factors. The largest contribution came from the work environment factors.

Adolescent↗

PubMed1999

The roles of social class of origin, achieved social class and intergenerational social mobility in explaining social-class inequalities in alcoholism among young men.

The aim of this study was to investigate the role of intergenerational health-related mobility in explaining social-class inequalities in alcoholism among young men. Data on social class of origin and on risk factors in childhood and adolescence, e.g. risk use of alcohol, were collected for 49,323 men, born 1949-51, at enlistment for compulsory military training in 1969/70. Information on achieved socioeconomic class was obtained from Sweden's 1975 census. Data on alcoholism diagnoses were collected from the national in-patient care register 1976-83. Risk indicators for alcoholism established in adolescence were found to be more common among downwardly mobile individuals, and also among stable manual workers, than among those who ended up as non-manual employees. Downwardly mobile individuals, and also stable manual workers, were also found to have an increased risk of alcoholism diagnosis. The increased relative risk could, to a considerable extent, be attributed to factors from childhood/adolescence. In this longitudinal study, it is shown that intergenerational social mobility associated with health-related factors, albeit not with illness itself, made a major contribution to explaining differences in alcoholism between social classes. Factors established in adolescence were important with regard to differences in alcoholism between social classes among young adults. But such adverse conditions did not seem to be well reflected by social class of origin.

Adult↗

PubMed1998

A comparative study of iatrogenesis, medication abuse, and psychiatric morbidity in chronic pain patients with and without medically explained symptoms.

We report a case control study comparing patients attending a pain clinic whose symptoms were not considered medically explained (cases) with those whose symptoms were considered medically explained (controls). Principal comparisons were psychiatric morbidity, medication use, and iatrogenic factors assessed by interview, and questionnaire measures of anxiety, depression, functional impairment, coping strategies and pain beliefs. Medically unexplained symptoms were associated with the presence of psychiatric morbidity (odds ratio = 3.4, 95% C.I.: 1.4,8.2), and of possible iatrogenic factors. There were no significant differences with regards to medication abuse/dependence.

Adult↗

PubMed2000

Greater late lumen loss after successful coronary balloon angioplasty in the proximal left anterior descending coronary artery is not explained by extent of vessel wall damage or plaque burden.

OBJECTIVES: We investigated whether the greater late lumen loss after coronary balloon angioplasty in the proximal left anterior descending artery (P-LAD) compared with that in other segments might be related to differences in vascular dimensions or morphology as determined by angiography and intravascular ultrasound imaging. BACKGROUND: The greater late lumen loss after angioplasty in the P-LAD that has been observed in several studies has not been explained. METHODS: We studied 178 patients and 194 coronary artery lesions by quantitative angiography and 30 MHz intravascular ultrasound imaging after successful balloon angioplasty. Vessel wall morphology was compared among three proximal and three nonproximal segments. Follow-up quantitative angiography for late lumen loss calculation was performed in 168 lesions. Multivariate analysis was used to determine predictors of late lumen loss. RESULTS: Absolute and relative late loss were significantly greater at the P-LAD compared with the pooled group of other segments (0.42 +/- 0.60 mm vs. 0.10 +/- 0.48 mm, p = 0.0008 and 0.14 +/- 0.24 vs. 0.03 +/- 0.17, p < 0.001). Also, a greater percentage of calcific lesions (65% vs. 44%, p = 0.034), a lower incidence of rupture (51% vs. 74%, p = 0.009) and a larger reference segment plaque area (5.4 +/- 2.2 mm2 vs. 4.7 +/- 1.9 mm2, p = 0.05) were found in the P-LAD. In multivariate analysis however, these variables were not predictive of late loss. CONCLUSIONS: Greater late lumen loss after coronary balloon angioplasty of the P-LAD is not explained by differences in atherosclerotic plaque burden or in vessel wall damage.

Aortic Dissection↗

PubMed2000

Cooperative equilibrium transitions coupled with a slow annealing step explain the sharpness and hysteresis of collagen folding.

Heat and guanidinium-induced denaturation curves of collagen III and its fragments were fitted by theoretical models to explain the extreme sharpness and the hysteresis between unfolding and refolding. It was shown that a recently proposed kinetic model for collagen denaturation does not account for the observed steepness, with physically reasonable values of activation energy and frequency factors in the Arrhenius equation. The extreme slope, which amounts to 0.38 per centigrade for collagen III at the midpoint of its transition, can only be explained by a highly cooperative equilibrium model. The refolding curve is shifted to lower temperatures by 6 degrees C for collagen III and reversible unfolding matching the initial profile of the native protein is observed only after long-time annealing. A simple formalism is proposed by which experimental denaturation and refolding curves are quantitatively described. The transition proceeds via many cooperative steps with slightly different equilibrium constants for unfolding and refolding. Hysteresis and annealing are caused by very slow steps, which are probably connected with a rearrangement of misfolded regions. These slow steps disappear with decreasing size of collagen fragments and hysteresis is not found for collagen model peptides.

Animals↗

PubMed1996

Short strong hydrogen bonds: can they explain enzymic catalysis?

It has been proposed that some remarkable enzymic catalytic effects can be explained by the existence of unusually strong hydrogen bonds within the enzyme's active site. Although such hydrogen bonds may be short, and may have unusual properties, there is no evidence that unusually strong hydrogen bonds exist in solution or in enzyme active sites. Thus there is no basis for invoking strong hydrogen bonds to explain enzymic rate enhancements.

Catalysis↗

PubMed1998

Untreated growth hormone deficiency explains premature mortality in patients with hypopituitarism.

Untreated growth hormone deficiency (GHD) might explain the increased mortality reported in patients with hypopituitarism. These statistics appear to be largely attributable to cardiovascular disease. Differences in fibrinolytic activity regulators, together with dyslipidaemias, abdominal obesity and raised blood pressure in hypopituitarism might explain this increased risk. There is growing evidence that treatment with growth hormone (GH) can rectify most of the cardiovascular abnormalities associated with GHD. The reported improved psychosocial well-being in response to GH treatment may also be important in lowering risk in this group of patients.

Cardiovascular Diseases↗

PubMed1999

Can biased symptom perception explain false-alarm choking sensations?

BACKGROUND: Breathlessness in asthma often cannot be explained with objective variables indicating airways obstruction. The hypothesis that unrealistic breathlessness results from false interpretation of sensations was tested. METHODS: Sixty-four children and adolescents with asthma, aged 9-18 years, were randomly assigned to; (1) standardized physical exercise for induction of general symptoms; (2) equipment causing itching through skin irritation; or (3) physical exercise combined with equipment causing itching through skin irritation. Pre-test and post-test measures were: lung function; breathlessness; general symptoms; itching; state anxiety; and worry. RESULTS: Lung function decreased within normal parameters (3.8%, 1.1%, 2.6%, respectively) and did not differ significantly between conditions. Breathlessness increased significantly after exercise, particularly in condition 3. Breathlessness correlated with general symptoms and worrying, but not with changes in lung function, age, or asthma severity. CONCLUSION: Biased symptom perception can explain unrealistic breathlessness. Prerequisites are situational cues triggering selective perception and ambiguous sensations associated with the anticipated (feared) physical state. Excessive breathlessness may often warrant objective confirmation by means of lung-function testing.

Adolescent↗

PubMed2000

Schizophrenia among Surinamese in the Netherlands: high admission rates not explained by high emigration rates.

BACKGROUND: A high risk of schizophrenia has been found among Caribbean immigrants in the Netherlands and Great Britain. One hypothesis to explain these findings is that patients with a diagnosis of schizophrenia or patients with symptoms of psychosis not specifically diagnosed, emigrate more than the general population. Such selection might account for high rates of Surinamese patients with a diagnosis of schizophrenia in Netherlands psychiatric hospitals. We examined this hypothesis. METHODS: The files of patients with schizophrenia or patients suffering from unspecified psychosis were selected from the archives of the National Psychiatric Hospital in Surinam. These patients were traced to investigate patterns of emigration. Data from the Central Bureau for Statistics in the Netherlands and from the Central Population Bureau in Surinam provided the percentage of the general Surinamese population that emigrated to the Netherlands. The difference between the two percentages was evaluated using the chi-squared test. Using the same method the percentage of remigration from the Netherlands among Surinamese patients was compared with the percentage of remigration among the general Surinamese population. RESULTS: Surinamese patients with schizophrenia or unspecified psychosis did not emigrate more frequently to the Netherlands compared with the general Surinamese population. They did remigrate significantly more frequently. CONCLUSION: High migration rates do not explain the high hospital admission rates for schizophrenia among Surinamese in the Netherlands.

Adolescent↗

PubMed2001

A homeostatic model of oxidative damage explains paradoxes observed in earlier aging experiments: a fusion and extension of older theories of aging.

The Rate of Living and the Threshold Theories of Aging are two contradicting approaches used to explain experimental facts about aging in fruit flies. In this paper we suggest an approach that unifies these theories and removes the contradiction. The approach involves quantitative description of the oxidative stress theory of aging, which is presented in the form of a mathematical homeostatic model. The crucial variable in the model is called 'homeostatic capacity', which is analogous to the classical notion of vitality. We model the process of aging as the age-related accumulation of damage produced by oxidative stress, which reduces the homeostatic capacity of the organism. The model is tested with the experimental data obtained in the classical experiments by Maynard Smith in 1958-1963. Our homeostatic model explains the well-known results of these experiments more accurate than any one of the early theories of aging. We form an hypothesis about the mechanisms underlying the results observed in the experiments and analyze a possible interplay of these mechanisms. Our virtual replication of Maynard Smith's classical experiments demonstrates that mathematical modeling can be a powerful tool to reveal and investigate the inherent genetic and physiological processes underlying the data observed in complicated insect experiments.

Aging↗

PubMed1997

Explained and unexplained medical symptoms in generalized anxiety and panic disorder: relationship to the somatoform disorders.

We have examined the numbers and types of symptoms in a sample of 90 patients with generalized anxiety disorder (GAD) and 77 patients with panic disorder (PD) collected from six different sites during the conduct of a multicenter clinical trial. This information was obtained utilizing the Health Questionnaire, a 47-item self-report list of medical symptoms, patterned after the Somatization Disorder section of the Diagnostic Interview Schedule. Although the patients in this sample had a wide variety of medically explained and unexplained physical symptoms, none of them qualified for a diagnosis of somatization disorder by DSM-III-R criteria. GAD and PD patients reported remarkably similar numbers of explained and unexplained medical symptoms. The panoply of somatic symptoms presented by these patients presents a formidable diagnostic challenge for clinicians. These findings suggest that the pattern of overutilization of medical services that is well documented for PD patients may also be found for GAD patients.

Adult↗

PubMed1992

General slowing alone cannot explain age-related search effects: reply to Cerella (1991)

Cerella (1991) has argued that the performance of older adults in the Fisk and Rogers (1991) study is a linear function of the performance of younger adults that is independent of task-specific cognitive requirements. We demonstrate that this is not the case. First, we show that the scatter plot analyses used by Cerella can hide the very task-specific age-related slowing they were designed to reveal. Second, we demonstrate that the percentage of variance explained by such analyses can be misleading. Third, we show that there are reliable differences across tasks in the parameters relating younger and older adults' performance. Finally, we argue that the general, task-independent proportionate slowing that Cerella suggested explains so much of the variance in age-related performance is actually an average slowing that is a function of a relatively small task-independent and a relatively large task-dependent factor.

Adult↗

PubMed1996

Usefulness of the protection motivation theory in explaining hearing protection device use among male industrial workers.

The present study examined the usefulness of personal variables: noise annoyance, and components of the protection motivation theory (R. W. Rogers, 1983) along with social-organizational factors in explaining hearing protection device (HPD) use among Israeli manufacturing workers. Participants were 281 men exposed to harmful noise levels for which routine HPD use is required by regulation. In practice, 3 HPD user groups were identified: nonusers (n = 38), occasional users (n = 125), and regular users (n = 118). HPD use was objectively verified. HPD use was primarily related to the personal variables but not to management pressure, coworker pressure, or family support. The most powerful predictors of HPD use were perceived self-efficacy (for long-term HPD use), perceived susceptibility (to hearing loss), and noise annoyance, together explaining 48% of the outcome variance. These findings have implications for interventions aimed at motivating workers to use HPDs regularly.

Adolescent↗

PubMed2000

On measures of explained variance in nonrecursive structural equation models.

Whereas measures of explained variance in a regression and an equation of a recursive structural equation model can be simply summarized by a standard R2 measure, this is not possible in nonrecursive models in which there are reciprocal interdependencies among variables. This article provides a general approach to defining variance explained in latent dependent variables of nonrecursive linear structural equation models. A new method of its estimation, easily implemented in EQS or LISREL and available in EQS 6, is described and illustrated.

Analysis of Variance↗