Threshold Assumption Research Articles

Bridging Membrane and Reaction Systems – Further Results and Research Topics

This paper continues an investigation into bridging two research areas concerned with natural computing: membrane computing and reaction systems. More specifically, the paper considers a transfer of two assumptions/axioms of reaction systems, non-permanency and the threshold assumption, into the framework of membrane computing. It is proved that: 1 spiking neural P systems with non-permanency of spikes assumption characterize the semilinear sets of numbers, and 2 symport/antiport P systems with threshold assumption translated as ω multiplicity of objects can solve SAT in polynomial time. Also, several open research problems are stated.

The effect of non-targeted cellular mechanisms on lung cancer risk for chronic, low level radon exposures

Purpose: The goal of the present study was to investigate the effect of non-targeted mechanisms on the shape of the lung cancer risk function at chronic, low level radon exposures relative to direct cellular radiation effects. This includes detrimental and protective bystander effects, radio-adaptive bystander response, genomic instability and induction of apoptosis by surrounding cells.Methods: To quantify the dependence of these mechanisms on dose, analytical functions were derived from the experimental evidence presently available. Alpha particle intersections of bronchial target cells during a given exposure period were simulated by a Transformation Frequency-Tissue Response (TF-TR) model, formulated in terms of cellular hits within the cycle time of the cell and then integrated over the whole exposure period.Results: In general, non-targeted effects like genomic instability and bystander effects amplify the biological effectiveness of a given radiation dose, while induction of apoptosis and adaptive response will decrease the risk values. While these observations are related to the absolute number of lung cancer cases, normalization to the epidemiologically observed risk at 0.675 Gy suggests that the effect of such mechanisms on the shape of the dose-response relationship may be different. Indeed, genomic instability and adaptive response cause a substantial reduction of the risk at low doses, while induction of apoptosis and detrimental bystander effects slightly increase the risk.Conclusions: Predictions of lung cancer risk, including these mechanisms, exhibit a distinct sublinear dose-response relationship at low exposures, particularly for very low exposure rates. However, the relatively large error bars of the epidemiological data do not currently allow the prediction of a statistically significant deviation from the Linear – No Threshold (LNT) assumption.

Adaptation Longitudinal Driving Behavior, Mental Workload, and Psycho-Spacing Models in Fog

Adverse weather conditions have a substantial effect on traffic flow. However, the adaptation effects in longitudinal driving behavior that underlie this impact are unclear, as are the determinants. A driving simulator experiment was performed with a repeated-measures design and 25 participants. The adaptation effects in actual longitudinal driving behavior and the physiological indicators of mental workload (i.e., heart rate and heart rate variability) were measured under two conditions: normal visibility and fog. Significant adaptation effects in longitudinal driving behavior and a significant increase in mental workload were observed. A new estimation method was used to investigate the extent to which fog influenced the position of so-called action points in the (Δv, s) plane of a psycho-spacing model, where Δv was relative speed and s was spacing. In addition, multivariate regression analysis was applied to investigate the extent to which an influence could be observed on acceleration and on jumps in acceleration at the action points. Large differences in the positions of action points in the (Δv, s) plane, acceleration, and jumps in acceleration were observed between conditions; therefore, car-following patterns closely resemble those predicted by psycho-spacing theory. However, a large degree of inter- and intradriver heterogeneity was observed, possibly caused by differences in mental workload within and between drivers. This heterogeneity indicates that the assumption of deterministic perceptual thresholds is unrealistic and necessitates the development of a data-driven stochastic model based on the principles of psycho-spacing models.

Variations in recollection: The effects of complexity on source recognition.

Whether recollection is a threshold or signal detection process is highly controversial, and the controversy has centered in part on the shape of receiver operating characteristics (ROCs) and z-transformed ROCs (zROCs). U-shaped zROCs observed in tests thought to rely heavily on recollection, such as source memory tests, have provided evidence in favor of the threshold assumption, but zROCs are not always as U-shaped as threshold theory predicts. Source zROCs have been shown to become more linear when the contribution of familiarity to source discriminations is increased, and this may account for the existing results. However, another way in which source zROCs may become more linear is if the recollection threshold begins to break down and recollection becomes more graded and Gaussian. We tested the "graded recollection" account in the current study. We found that increasing stimulus complexity (i.e., changing from single words to sentences) or increasing source complexity (i.e., changing the sources from audio to videos of speakers) resulted in flatter source zROCs. In addition, conditions expected to reduce recollection (i.e., divided attention and amnesia) had comparable effects on source memory in simple and complex conditions, suggesting that differences between simple and complex conditions were due to differences in the nature of recollection, rather than differences in the utility of familiarity. The results suggest that under conditions of high complexity, recollection can appear more graded, and it can produce curved ROCs. The results have implications for measurement models and for current theories of recognition memory.

A critical experimental study of the classical tactile threshold theory

BackgroundThe tactile sense is being used in a variety of applications involving tactile human-machine interfaces. In a significant number of publications the classical threshold concept plays a central role in modelling and explaining psychophysical experimental results such as in stochastic resonance (SR) phenomena. In SR, noise enhances detection of sub-threshold stimuli and the phenomenon is explained stating that the required amplitude to exceed the sensory threshold barrier can be reached by adding noise to a sub-threshold stimulus. We designed an experiment to test the validity of the classical vibrotactile threshold. Using a second choice experiment, we show that individuals can order sensorial events below the level known as the classical threshold. If the observer's sensorial system is not activated by stimuli below the threshold, then a second choice could not be above the chance level. Nevertheless, our experimental results are above that chance level contradicting the definition of the classical tactile threshold.ResultsWe performed a three alternative forced choice detection experiment on 6 subjects asking them first and second choices. In each trial, only one of the intervals contained a stimulus and the others contained only noise. According to the classical threshold assumptions, a correct second choice response corresponds to a guess attempt with a statistical frequency of 50%. Results show an average of 67.35% (STD = 1.41%) for the second choice response that is not explained by the classical threshold definition. Additionally, for low stimulus amplitudes, second choice correct detection is above chance level for any detectability level.ConclusionsUsing a second choice experiment, we show that individuals can order sensorial events below the level known as a classical threshold. If the observer's sensorial system is not activated by stimuli below the threshold, then a second choice could not be above the chance level. Nevertheless, our experimental results are above that chance level. Therefore, if detection exists below the classical threshold level, then the model to explain the SR phenomenon or any other tactile perception phenomena based on the psychophysical classical threshold is not valid. We conclude that a more suitable model of the tactile sensory system is needed.

Robust Stabilization of Multi Input Plants With Saturating Actuators

This technical note proposes the use of a time-varying sliding surface for stabilizing linear, possibly unstable, Multi-Input plants subject to saturating actuators, in the presence of bounded matched uncertainties. The present work generalizes our previous contributions in several different directions. First, the constructive procedure is generalized, and a result about asymptotical stabilization is given, under the usual assumption of the saturation threshold being larger than the bound on uncertainties. Another goal of the technical note is to widen the set of plants considered including linear Multi-Input systems. Simulation results show both the effectiveness of the control technique and the low computational burden required.

Return Migration and Optimal Tax Rules—a Hysteresis Case

The paper provides some insights into the return behavior of migrant workers. In particular, we investigate the effects of migration on optimal tax rules when individuals have entrenched habits. We model the migration process starting from the choice being made by an individual. Individuals are seen as having entrenched habits which signify various sources of resistance to a change. The presence of habit threshold assumption creates a situation, where the utility change necessary to attract a given number of people back has to be greater than the change that previously caused them to emigrate. Habit formation making people to resist changes lowers the elasticity of return migration. So, the habit-hysteresis factor seems to be a good reason to reduce responsiveness of migration to changes in tax rates. The main consequence of this conclusion is that a habit-hysteresis factor makes a tax policy less effective as a migration policy. To perform a migration policy, governments have to look at some alternative policy tools. Int Adv Econ Res (2009) 15:487 DOI 10.1007/s11294-009-9225-8

Prediction of Fatigue Thresholds of Steels with Surface Defects

The resistance-curve (R-curve) method was used to assess the effect of the diameter of drilled holes on the fatigue threshold of a carbon steel (JIS S45C) and a spring steel (SUP190). The specimens of the carbon steel with various hole diameters were subjected to tension-compression fatigue tests, and those of the spring steel to rotating bending and torsional fatigue tests. The fatigue crack made near the fatigue thresholds of drill-holed specimens of the carbon steel was modeled by the ring crack around the equator of the semi-cubic hole or by the quarter-circle at the corner of the cylindrical hole. When the diameter of drill holes was d = 0.05 and 0.1mm, the experimental data of the fatigue threshold agreed well with the prediction obtained under the assumption of the propagation threshold of the ring crack. The assumption of the quarter-circle crack gave a better prediction for the diameters of d = 0.3 and 0.5mm. For the case of the spring steel, on the other hand, the threshold value predicted from the crack propagation was far below the experimental data. The critical value of the stress intensity factor for crack nucleation was determined in order to obtain a better agreement with the experimental data. The critical value thus determined was nearly double the threshold stress intensity range for crack propagation. The nucleation condition of fatigue cracks controlled the fatigue threshold of the spring steel.

Security and potential level preferences with thresholds

The security level models of Gilboa [1988. A combination of expected utility and maxmin decision criteria. Journal of Mathematical Psychology, 32, 405–420] and of Jaffray [1988. Choice under risk and the security factor: An axiomatic model. Theory and Decision, 24, 169–200] as well as the security and potential level model of Cohen [1992. Security level, potential level, expected utility: A three-criteria decision model under risk. Theory and Decision, 33, 101–104] and Essid [1997. Choice under risk with certainty and potential effects: A general axiomatic model. Mathematical Social Sciences, 34, 223–247] successfully accommodate classical Allais paradoxes while they offer an interesting explanation for their occurrence. However, experimental data suggest a systematic violation of these models when lotteries with low probabilities of bad or good outcomes are involved. In our opinion, one promising candidate for the explanation of these violations is the assumption of thresholds in the perception of security and potential levels. The present paper develops an axiomatic model that allows for such thresholds, so that the derived representation of preferences can accommodate the observed violations of the original security and potential level models.

Relationships between Marine Stratus Cloud Optical Depth and Temperature: Inferences from AVHRR Observations

AbstractStudies using International Satellite Cloud Climatology Project (ISCCP) data have reported decreases in cloud optical depth with increasing temperature, thereby suggesting a positive feedback in cloud optical depth as climate warms. The negative cloud optical depth and temperature relationships are questioned because ISCCP employs threshold assumptions to identify cloudy pixels that have included partly cloudy pixels. This study applies the spatial coherence technique to one month of Advanced Very High Resolution Radiometer (AVHRR) data over the Pacific Ocean to differentiate overcast pixels from the partly cloudy pixels and to reexamine the cloud optical depth–temperature relationships. For low-level marine stratus clouds studied here, retrievals from partly cloudy pixels showed 30%–50% smaller optical depths, 1°–4°C higher cloud temperatures, and slightly larger droplet effective radii, when they were compared to retrievals from the overcast pixels. Despite these biases, retrievals for the overcast and partly cloudy pixels show similar negative cloud optical depth–temperature relationships and their magnitudes agree with the ISCCP results for the midlatitude and subtropical regions. There were slightly negative droplet effective radius–temperature relationships, and considerable positive cloud liquid water content–temperature relationships indicated by aircraft measurements. However, cloud thickness decreases appear to be the main reason why cloud optical depth decreases with increasing temperature. Overall, cloud thickness thinning may explain why similar negative cloud optical depth–temperature relationships are found in both overcast and partly cloudy pixels. In addition, comparing the cloud-top temperature to the air temperature at 740 hPa indicates that cloud-top height generally rises with warming. This suggests that the cloud thinning is mainly due to the ascending of cloud base. The results presented in this study are confined to the midlatitude and subtropical Pacific and may not be applicable to the Tropics or other regions.

LATTICE BOLTZMANN MODELING OF THROMBOSIS IN GIANT ANEURYSMS

We propose a numerical model of blood flow and blood clotting whose purpose is to describe thrombus formation in cerebral aneurysms. We identify possible mechanisms that can cause occurence of spontaneous thrombosis in unruptured giant intracranial aneurysms. Our main claim is that, under normal conditions, there is a low shear rate threshold below which thrombosis starts and growths. This assumption is supported by several evidences from literature. The proposed mechanisms are incorporated into a Lattice Boltzmann (LB) model for blood flow and platelets adhesion and aggregation. Numerical simulations show that the low shear rate threshold assumption together with aneurysm geometry account well for the observations.

Ordered Discrete-Choice Selection Models and Local Average Treatment Effect Assumptions: Equivalence, Nonequivalence, and Representation Results

Abstract This note shows that the local average treatment effect (LATE) assumptions of Angrist and Imbens are weaker than imposing an ordered, discrete-choice selection model if one imposes the standard assumption of constant thresholds in the latter. However, the note extends results of Vytlacil to show that the LATE assumptions are equivalent to an ordered, discrete-choice selection model if one allows for random thresholds in the latter. A nonparametric representation result for ordered, discrete-choice models is produced as a by-product of these results.

No-threshold dose–response curves for nongenotoxic chemicals: Findings and applications for risk assessment

We tested the hypothesis that no threshold exists when estradiol acts through the same mechanism as an active endogenous estrogen. A Michaelis–Menten (MM) equation accounting for response saturation, background effects, and endogenous estrogen level fit a turtle sex-reversal data set with no threshold and estimated the endogenous dose. Additionally, 31 diverse literature dose–response data sets were analyzed by adding a term for nonhormonal background; good fits were obtained but endogenous dose estimations were not significant due to low resolving power. No thresholds were observed. Data sets were plotted using a normalized MM equation; all 178 data points were accommodated on a single graph. Response rates from ∼1% to >95% were well fit. The findings contradict the threshold assumption and low-dose safety. Calculating risk and assuming additivity of effects from multiple chemicals acting through the same mechanism rather than assuming a safe dose for nonthresholded curves is appropriate.

Differentiating Future Commitments on the Basis of Countries’ Relative Historical Responsibility for Climate Change: Uncertainties in the ‘Brazilian Proposal’ in the Context of a Policy Implementation

During the negotiations on the Kyoto Protocol, Brazil proposed allocating the greenhouse gas emission reductions of Annex I Parties according to the relative effect of a country’s historical emissions on global temperature increase. This paper analyses the impact of scientific uncertainties and of different options in policy implementation (policy choices) on the contribution of countries’ historical emissions to indicators of historical responsibility for climate change. The influence of policy choices was found to be at least as large as the impact of the scientific uncertainties analysed here. Building on this, the paper then proceeds to explore the implications of applying the Brazilian Proposal as a climate regime for differentiation of future commitments on the global scale combined with an income threshold for participation of the non-Annex I regions. Under stringent climate targets, such a regime leads to high emission reductions for Annex I regions by 2050, in particular for Europe and Japan. The income threshold assumptions strongly affect the Annex I reductions, even more than the impact of another burden-sharing key. A variant of the Brazilian Proposal, allocating emission reductions on the basis of cumulative emissions since 1990, would lead to a more balanced distribution of emission reductions.

Study of thermal behaviors in <inline-formula><math display="inline" overflow="scroll"><msub><mi>CO</mi><mrow><mn>2</mn></mrow></msub></math></inline-formula> laser irradiated glass

To understand mechanisms underlying laser-induced damage of BK7 and fused silica, we calculate the temperature field of the substrates with CO2 laser irradiating at a given laser power and beam radius. We find that the two glasses show different thermal behaviors. A model is developed for estimating the time t to heat the surface of the substrates up to a particular temperature T with cw CO2 laser irradiation. We calculate theoretically the duration t that the samples are irradiated, from the beginning to visual catastrophic damage, with the assumption of damage threshold determined by the critical temperature. The duration t that the samples are irradiated, from the beginning to visual catastrophic damage, is investigated experimentally as well. Here we take the melting point or softening point as the critical temperature, given the thermomechanical coupling properties, which is enough to cause damage for BK7. Damage features are characterized by the sound of visual cracks. Finally, we calculate stresses induced by laser heating. The analysis of stress indicates that the damage of BK7 is due to the stresses induced by laser heating.

Large effects from small exposures. I. Mechanisms for endocrine-disrupting chemicals with estrogenic activity.

Information concerning the fundamental mechanisms of action of both natural and environmental hormones, combined with information concerning endogenous hormone concentrations, reveals how endocrine-disrupting chemicals with estrogenic activity (EEDCs) can be active at concentrations far below those currently being tested in toxicological studies. Using only very high doses in toxicological studies of EEDCs thus can dramatically underestimate bioactivity. Specifically: a) The hormonal action mechanisms and the physiology of delivery of EEDCs predict with accuracy the low-dose ranges of biological activity, which have been missed by traditional toxicological testing. b) Toxicology assumes that it is valid to extrapolate linearly from high doses over a very wide dose range to predict responses at doses within the physiological range of receptor occupancy for an EEDC; however, because receptor-mediated responses saturate, this assumption is invalid. c) Furthermore, receptor-mediated responses can first increase and then decrease as dose increases, contradicting the assumption that dose-response relationships are monotonic. d) Exogenous estrogens modulate a system that is physiologically active and thus is already above threshold, contradicting the traditional toxicological assumption of thresholds for endocrine responses to EEDCs. These four fundamental issues are problematic for risk assessment methods used by regulatory agencies, because they challenge the traditional use of extrapolation from high-dose testing to predict responses at the much lower environmentally relevant doses. These doses are within the range of current exposures to numerous chemicals in wildlife and humans. These problems are exacerbated by the fact that the type of positive and negative controls appropriate to the study of endocrine responses are not part of traditional toxicological testing and are frequently omitted, or when present, have been misinterpreted.

Exposure misclassification and threshold concentrations in time series analyses of air pollution health effects.

Linear, no-threshold relationships are typically reported for time series studies of air pollution and mortality. Since regulatory standards and economic valuations typically assume some threshold level, we evaluated the fundamental question of the impact of exposure misclassification on the persistence of underlying personal-level thresholds when personal data are aggregated to the population level in the assessment of exposure-response relationships. As an example, we measured personal exposures to two particle metrics, PM2.5 and sulfate (SO4(2-)), for a sample of lung disease patients and compared these with exposures estimated from ambient measurements Previous work has shown that ambient:personal correlations for PM2.5 are much lower than for SO4(2-), suggesting that ambient PM2.5 measurements misclassify exposures to PM2.5. We then developed a method by which the measured:estimated exposure relationships for these patients were used to simulate personal exposures for a larger population and then to estimate individual-level mortality risks under different threshold assumptions. These individual risks were combined to obtain the population risk of death, thereby exhibiting the prominence (and the value) of the threshold in the relationship between risk and estimated exposure. Our results indicated that for poorly classified exposures (PM2.5 in this example) population-level thresholds were apparent at lower ambient concentrations than specified common personal thresholds, while for well-classified exposures (e.g., SO4(2-)), the apparent thresholds were similar to these underlying personal thresholds. These results demonstrate that surrogate metrics that are not highly correlated with personal exposures obscure the presence of thresholds in epidemiological studies of larger populations, while exposure indicators that are highly correlated with personal exposures can accurately reflect underlying personal thresholds.

Adaptation of the timing of bud burst of Norway spruce to temperate and boreal climates

The adaptation of the annual cycle of development of boreal and temperate trees to climatic conditions has been seen as a result of stabilizing selection caused by two opposite driving forces of natural selection, i.e. the tolerance of unfavorable conditions during the frost exposed season (survival adaptation) and the effective use of growth resources during the growing season (capacity adaptation). In this study, two theories of the effects of climate on the adaptation of the timing of bud burst of trees were evaluated. This was done with computer simulations by applying a temperature sum model for predicting the timing of bud burst of different Norway spruce genotypes on the basis of air temperature data from various climatic conditions. High geographical variation in the temperature response of bud burst, typical for Norway spruce, was included in the theoretical analyses. The average timing of bud burst and the corresponding risk of occurrence of damaging frost during the susceptible period after bud burst were calculated for each genotype in each climate. Two contrasting theories of the stabilizing selection were evaluated, i.e. the overall adaptedness of each genotype was evaluated either 1) by assuming a fixed threshold for the risk of frost damage, or 2) by assuming a tradeoff between the risk of frost damage and the length of the growing season. The tradeoff assumption produced predictions of between provenance variation in bud burst which correspond more closely with empirical observations available in literature, compared to the fixed threshold assumption.

Designing and Implementing Performance Management of Social Care Block Contracts in Fareham/Gosport Social Services Locality

The article provides an account of an approach to managing contract performance in respect of block‐purchased social care services. The approach has confirmed quality threshold assumptions, and confirmed that the toolkit is effective at differentiating performance levels, informing improvement notices and measuring progress towards desired performance thresholds.

Endocrine disruptors: present issues, future directions.

A variety of natural products and synthetic chemicals, known collectively as endocrine-disrupting compounds (EDCs), mimic or interfere with the mechanisms that govern vertebrate reproductive development and function. At present, research has focused on (i) the morphological and functional consequences of EDCs; (ii) identifying and determining the relative potencies of synthetic and steroidal compounds that have endocrine-disrupting effects; (iii) the mechanism of action of EDCs at the molecular level; and (iv) the recognition that in "real life," contamination usually reflects mixtures of EDCs. Future research must examine (i) the interactive nature of EDCs, particularly whether the threshold concept as developed in traditional toxicological research applies to these chemicals; (ii) when and how EDCs act at the physiological level, particularly how they may organize the neural substrates of reproductive physiology and behavior; (iii) the various effects these compounds have on different species, individuals, and even tissues; and (iv) how adaptations may evolve in natural populations with continued exposure to EDCs. Several predictions are offered that reflect these new perspectives. Specifically, (i) the threshold assumption will be found not to apply to EDCs because they mimic the actions of endogenous molecules (e.g., estrogen) critical to development; hence, the threshold is automatically exceeded with exposure. (ii) Behavior can compound and magnify the effects of EDCs over successive generations; that is, bioaccumulated EDCs inherited from the mother not only influence the morphological and physiological development of the offspring but also the offsprings' reproductive behavior as adults. This adult behavior, in turn, can have further consequences on the sexual development of their own young. (iii) The sensitivity of a species or an individual to a compound is related to species (individual)-typical concentrations of circulating gonadal steroid hormones. Related to this is the recent finding that alternate forms of the putative receptors are differentially distributed, thereby contributing to the different effects that have been observed. (iv) Except in extraordinary situations, populations often continue to exist in contaminated sites. One possible explanation for this observation that needs to be considered is that animals can rapidly adapt to the nature and level of contamination in their environment. It is unlikely that successive generations coincidentally become insensitive to gonadal steroid hormones fundamentally important as biological regulators of development and reproduction. Rather, adaptive alterations in the genes that encode steroid receptors may occur with chronic exposure to EDCs, allowing the sex hormone receptor to discriminate natural steroids from EDCs.