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Results for “Model explainability”

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At least 541 records · Page 30Linked to original sources

PubMed2003

Structure/function analyses of human sex hormone-binding globulin: effects of zinc on steroid-binding specificity.

In humans, sex hormone-binding globulin (SHBG) binds and transports the biologically most important androgens and estrogens in the blood, and regulates the access of these steroids to their targets tissues. In addition to binding sex steroids, SHBG has specific binding sites for divalent cations including calcium and zinc. Zinc binding to a site at the entrance of the steroid-binding pocket in human SHBG has been shown to reduce its affinity for estrogens, while having no impact on the binding of C19 steroids. Crystallographic studies indicate that C18 and C19 steroids are bound in opposite orientations within the SHBG steroid-binding site, and we have obtained new information that supports a molecular model explaining the mechanism by which zinc alters the affinity of human SHBG for estrogens, by studying directly the estradiol-binding properties SHBG variants created by site-directed mutagenesis. In this model, the coordination of a zinc ion by the side chains of residues Asp65 and His136 eliminates a critical hydrogen bond between Asp65 and the hydroxyl at C3 of estrogens, such as estradiol and 2-methoxyestradiol, and causes disorder in a polypeptide loop segment that covers the steroid-binding site. The combination of these structural changes explains the specific decrease in the affinity of human SHBG for C18 steroids in the presence of a zinc ion.

Amino Acid Sequence↗

PubMed2000

A simple model for the interplay of predators, rodents and food.

This paper presents a simple mathematical model for multiannual population cycles, in particular the periods, for the triple of small rodents and their predators and food. The parameters used are average birth rates of rodents and predators. The period lengths fit observations of lemmings and voles rather well and the model explains why the observed periods cluster around 4 years and around 10 years.

Animals↗

PubMed1995

Effects of implicit memory on explicit recall: set size and word-frequency effects.

PIER was designed to explain findings related to tasks involving an encoding phase and a testing phase in which retrieval cues prime the recovery of what has been encoded. The model assumes that retrieval cues initiate the sampling of associated memories linked to the encoded information and that sampled memories are subjected to a recognition check to determine whether they meet criteria specified by the purpose of the retrieval. The model explains how the number of implicitly activated associates linked to a studied word affects its later recovery and predicts that words infrequently experienced will be recovered with greater likelihood than words frequently experienced. This prediction was tested and confirmed in two experiments in which the associative set size and the frequency of the studied words were manipulated. Implications for the study of implicit memory are considered.

Attention↗

PubMed1992

Charge density of total organic carbon in Finnish lakes.

Ion balance calculations were used to estimate the charge density of total organic carbon (TOC) and for calibrating the model of Oliver et al. (1983) for Finnish lakes. The database consisted of samples collected in the autumn of 1987 from 955 randomly selected lakes covering the whole country. Organic anion concentrations were estimated as the difference between Sigma cations and Sigma inorganic anions. The charge density of TOC was estimated by dividing the organic anion concentration by TOC. The median charge density was 7.5 microeq (mg TOC)(-1). In lakes with pH values lower than 4.5 the average charge density was 5.2 microeq (mg TOC)(-1) and it increased to 10.3 microeq (mg TOC)(-1) in lakes with pH higher than or equal to 7.0. The calibrated model explained 79% of the observed variability in organic anion concentration. The value for the total amount of acidic functional groups per mol organic carbon in the model of Oliver et al. (1983) was slightly higher compared to the Finnish calibration. At pH values higher than 4.4 the Finnish calibration gives lower organic anion concentrations compared to the model of Oliver et al. (1983).

Journal Article↗

PubMed1980

Three-receptor, clonal expansion model for selection of self-recognition in the thymus.

The postulate is made that dual recognition by T lymphocytes is due to two types of receptor, one encoded by antibody genes and one by a distinct multigene family with simple rules for expression. This postulate leads to a model explaining ontogenic and evolutionary selection for self recognition, T lymphocyte effector function, the apparent high frequency of alloreactive T cells and immune response gene activity. The model is contrasted with previous explanations of self-recognition phenomena.

Animals↗

PubMed2000

Nongenomically initiated steroid actions.

The classical theory of steroid hormone action comprises binding to an intracellular receptor followed by modulation of transcriptional and translational events. These cumbersome model explains the characteristic latency of these genomic steroid effects. Over the past two decades, increasing evidence for rapid nongenomic effects of steroids, incompatible with the traditional model, has accumulated. These alternative steroid effects have been described for all classes of steroids and a multitude of species and tissues with different mechanisms of action.

Aldosterone↗

PubMed2006

The extended health belief model applied to the experience of diabetes in young people.

OBJECTIVES: The experience of diabetes in young people was investigated within the theoretical framework of the extended health belief model (EHBM). It was anticipated that participant reports of high internal locus of control, high diabetes related empowerment, coping with hypoglycaemia, and diabetes related social support would be associated with good patient adherence to the self-care regime. METHOD: 118 participants aged 16-25 years with Type 1 diabetes mellitus were sent a set of questionnaires incorporating previously published scales measuring all variables of the EHBM. RESULTS: High levels of family support and low locus of control beliefs in powerful others to control their diabetes reduce the young person's perception of severity and vulnerability to diabetes related complications. High internal locus of control beliefs and high levels of self-efficacy predicted the benefits of adhering to the self-care regime as outweighing the costs of doing so. Adherence to self-care regime was predicted by high levels of family support. CONCLUSION: The final model explained 12% of the variance in the young person's adherence to diabetes self-care regime. It is proposed that the EHBM is an adequate model for understanding the socio-psychological factors present in the young person's appraisal of their diabetes, which, in turn, influences adherence to the diabetes self-care regime.

Adaptation, Psychological↗

PubMed1997

Another look at reasons for choosing and rejecting.

Students indicated which of two options they would choose or reject in a between-subjects task-frame manipulation. Alternatives had either high or low variability of feature values, corresponding to enriched and impoverished alternatives, respectively. Previous research has yielded mixed results of task framing, with Shafir (1993) demonstrating greater preference for the enriched alternative in choice than rejection but Ganzach (1995) demonstrating the opposite result. An accentuation model explained these differences by postulating that the greater demands for justification in the choice task lead to accentuation of differences between alternatives in choice. The accentuation model was tested against weight-change models in two experiments, one using various decision scenarios and the other using four-trait adjective descriptions of potential roommates. Results were consistent with accentuation theory and inconsistent with a systematic change in weighting of positive and negative attributes across choice and rejection tasks.

Decision Making↗

PubMed2003

Mathematical model of static platelet adhesion on a solid surface.

The scheme of platelet/surface interaction and a kinetic model of platelet adhesion on a solid surface are suggested. The elaborated approach takes into account the platelet activation by the surface and accumulation of free activated cells in the bulk of the liquid phase. This effect has an especially important role in static experimental conditions. The suggested model explains three types of adhesion kinetic curves, obtained in experiments in vitro: sigmoid curves with or without saturation and an exponential curve with saturation. According to the model, the curve shape is determined by material surface properties, platelet functionality, and experimental conditions of the platelet/surface interaction. The data of static platelet adhesion from platelet rich plasma on glass, siliconized glass, hexadecyltrichlorosilane monolayers, and low-density polyethylene are described mathematically with the proposed model. Numerical parameters are calculated from approximation of experimental data by the model. These parameters allow quantitative characterization of platelet interaction with the material surface.

Biocompatible Materials↗

PubMed1996

Prediction of the left ventricular mass from the electrocardiogram in systemic hypertension.

Although echocardiography provides a reliable method to determine left ventricular (LV) mass, it may not be available in all settings. Numerous electrocardiographic (ECG) criteria for the detection of LV hypertrophy have been developed, but few attempts have been made to predict the LV mass itself from the ECG. In a community-based survey program in the general population, 277 subjects were identified with untreated diastolic hypertension (diastolic blood pressure 95 to 115 mm Hg, 3 occasions) or isolated systolic hypertension (diastolic blood pressure <95 mm Hg and systolic blood pressure 160 to 220 mm Hg, 3 occasions). All subjects underwent ECG and echocardiography on the same day. A multiple linear regression analysis was performed using a random training sample of the data set (n = 185). The independent variables included both ECG and non-ECG variables. The resulting model was used to predict the LV mass in the remainder of the data set, the validation sample (n = 92). Using sex, age, body surface area, the S-voltage in V1 and V4, and the duration of the terminal P in V1 as independent variables, the model explained 45% of the variance (r = 0.67) in the training sample and 42% (r = 0.65) in the validation sample. This result exceeded that of 2 existing ECG models for LV mass (r = 0.40 and 0.41). The correlations between LV mass and combinations of ECG variables used for the detection of LV hypertrophy, such as the Sokolow-Lyon Voltage (r = 0.03) and the Cornell Voltage (r = 0.31), were comparatively low. In settings where echocardiography is not available or is too expensive and time-consuming, prediction of the LV mass from the ECG may offer a valuable alternative.

Adult↗

PubMed1978

Postreplication repair in mammalian cells after ultraviolet irradiation: a model.

A model is presented for bypass of ultraviolet-induced damage in DNA during replication. The overall process is initiated by the introduction of a single-strand break into parental DNA near the point of arrest of synthesis, followed by a transient crossing-over step similar to that envisaged in genetic recombination. The mechanism proposed provides an alternative explanation to existing models and is entirely consistent with available data on postreplication repair in mammalian cells. In addition the model explains the low level of recombination repair observed in mammalian cells.

DNA Repair↗

PubMed1997

Effect of image orientation contents on detection efficiency.

Contrast detection performance is known to be better for single component sinusoidal gratings than for sums of gratings at different orientations. A recent study Rovamo et al. (1994) (Investigative Ophthalmology and Visual Science, 35, 2611-2619) showed that spatial integration is less effective for multiple orientation component than for single component gratings. This suggests an explanation that the size of a spatial integration window depends on the orientation contents of the stimulus. To test this hypothesis we designed a computational detection model and tested it against new experimental data. The model generates a cross-correlation template, the extent of which is limited both in the spatial and spatial frequency domain. The template is a copy of the band-pass filtered signal weighted by a spatial window function. The spatial window function, which limits spatial integration, decreases with increasing orientation range of the stimulus. The experimental stimuli were composed of side-by-side located square shaped, one cycle, grating patches. The range of either grating orientations or phases within the patches as well as the number of patches in a stimulus were varied. We also measured detection efficiency for Bessel Jo images as a function of area. Human spatial integration became considerably weaker with increasing orientation range. The increasing phase range also reduced detection efficiency to some extent. Supporting the idea of the varying size of the spatial integration window, the computational model explained the orientation, phase, and Bessel Jo data well.

Contrast Sensitivity↗

PubMed1999

A vector-integration-to-endpoint model for performance of viapoint movements.

Viapoint (VP) movements are movements to a desired point that are constrained to pass through an intermediate point. Studies have shown that VP movements possess properties, such as smooth curvature around the VP, that are not explicable by treating VP movements as strict concatenations of simpler point-to-point (PTP) movements. Such properties have led some theorists to propose whole-trajectory optimization models, which imply that the entire trajectory is precomputed before movement initiation. This paper reports new experiments conducted to systematically compare VP with PTP trajectories. Analyses revealed a statistically significant early directional deviation in VP movements but no associated curvature change. An explanation of this effect is offered by extending the vector-integration-to-endpoint (VITE) model (Bullock, D., & Grossberg, S. (1988a). Neural dynamics of planned arm movements: Emergent invariants and speed-accuracy properties during trajectory formation. Psychological Review, 95, 49-90; Bullock, D., & Grossberg, S. (1988b). The VITE model: A neural command circuit for generating arm and articulator trajectories. In J.A.S. Kelso, A.J. Mandell & M.F. Schlesinger (Eds.), Dynamic patterns in complex systems (pp. 305-326). Singapore: World Scientific Publishers.), which postulates that voluntary movement trajectories emerge as internal gating signals control the integration of continuously computed vector commands based on the evolving, perceptible difference between desired and actual position variables. The model explains the observed trajectories of VP and PTP movements as emergent properties of a dynamical system that does not precompute entire trajectories before movement initiation. The new model includes a working memory and a stage sensitive to time-to-contact information. These cooperate to control serial performance. The structural and functional relationships proposed in the model are consistent with available data on forebrain physiology and anatomy.

Journal Article↗

PubMed1992

Normalization of cell responses in cat striate cortex.

Simple cells in the striate cortex have been depicted as half-wave-rectified linear operators. Complex cells have been depicted as energy mechanisms, constructed from the squared sum of the outputs of quadrature pairs of linear operators. However, the linear/energy model falls short of a complete explanation of striate cell responses. In this paper, a modified version of the linear/energy model is presented in which striate cells mutually inhibit one another, effectively normalizing their responses with respect to stimulus contrast. This paper reviews experimental measurements of striate cell responses, and shows that the new model explains a significantly larger body of physiological data.

Adaptation, Ocular↗

PubMed2002

Comparing prototype-based and exemplar-based accounts of category learning and attentional allocation.

Exemplar theory was motivated by research that often used D. L. Medin and M. M. Schaffer's (1978) 5/4 stimulus set. The exemplar model has seemed to fit categorization data from this stimulus set better than a prototype model can. Moreover, the exemplar model alone predicts a qualitative aspect of performance that participants sometimes show. In 2 experiments, the authors reexamined these findings. In both experiments, a prototype model fit participants' performance profiles better than an exemplar model did when comparable prototype and exemplar models were used. Moreover, even when participants showed the qualitative aspect of performance, the exemplar model explained it by making implausible assumptions about human attention and effort in categorization tasks. An independent assay of participants' attentional strategies suggested that the description the exemplar model offers in such cases is incorrect. A review of 30 uses of the 5/4 stimulus set in the literature reinforces this suggestion.

Attention↗

PubMed2003

When is early visual cortex activated during visual mental imagery?

Although many neuroimaging studies of visual mental imagery have revealed activation in early visual cortex (Areas 17 or 18), many others have not. The authors review this literature and compare how well 3 models explain the disparate results. Each study was coded 1 or 0, indicating whether activation in early visual cortex was observed, and sets of variables associated with each model were fit to the observed results using logistic regression analysis. Three variables predicted all of the systematic differences in the probability of activation across studies. Two of these variables were identified with a perceptual anticipation theory, and the other was identified with a methodological factors theory. Thus, the variability in the literature is not random.

Humans↗

PubMed1991

Protein aggregation in vitro and in vivo: a quantitative model of the kinetic competition between folding and aggregation.

Protein aggregation is frequently observed as a major side-reaction of protein folding. We present quantitative models explaining the formation of aggregates during protein folding in vitro and in vivo on the basis of a kinetic competition between correct folding and aggregation reactions. Both models are in good agreement with experimental data. The model implies that, in vitro, the yield of native protein obtained upon refolding is determined by the rates of the competing first order folding and second order aggregation reactions. Therefore, a high protein concentrations aggregation dominates over folding and leads to the formation of insoluble protein. For in vivo protein synthesis, the model shows that the yield of native protein is only dependent on the rate of folding, on the rate of aggregation and on the rate of protein synthesis. In the cell, several mechanisms, including "folding helpers" seem to have evolved, which influence these processes and thereby prevent unproductive side reactions.

Kinetics↗

PubMed1999

A closer look at General Dental Service orthodontics in England and Wales. I: Factors influencing effectiveness.

OBJECTIVE: To evaluate factors influencing effectiveness in General Dental Service (GDS) orthodontics. DESIGN: Retrospective analysis of systematic 2% sample of GDS (England and Wales) cases. METHOD: Records of cases were collected during 1991. Assessment involved occlusal indices and data from National Health Service forms for 1,411 cases. Multivariate analyses were used with Peer Assessment Rating Index (PAR) score at Finish as the outcome indicator. RESULTS: Dual arch fixed appliances: achieved lower Finish PAR scores than other appliances; only 1.5% of the variance was explained, by treatment time and Dental Health Component of the Index of Orthodontic Treatment Need (DHC). Finish PAR was unaffected by Starting PAR. All other appliances: the model explained 25% of the variance for Finish PAR, which varied with Starting PAR and DHC scores. Social class had effects of little clinical significance, but the data suggested availability of orthodontic treatment was poorer in 'manual class' areas. Orthodontic qualifications, number of arches treated and mixed dentition starts had no significant effects when submitted to multivariate analysis. CONCLUSIONS: The importance of appliance selection is reinforced: dual arch fixed appliances are generally more consistent. Lower social class areas may be poorly provided with orthodontic services.

Age Factors↗