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PubMed2000

Flicker adaptation can be explained by probability summation between ON- and OFF-mechanisms.

This study considered whether the dynamics of flicker adaptation can be explained in terms of probability summation over time between independent ON- and OFF-processors. Foveal thresholds were measured for flickering probes, and for static ON- and OFF-probes that have durations consistent with the flicker duty cycle. Thresholds were obtained using a probe-flash stimulus onset asynchrony paradigm. The outcome of the experiment suggests that flicker thresholds are lower than the component ON- and OFF-thresholds, and that the flicker response can be predicted by assuming probability summation between the ON- and OFF-mechanisms.

Adaptation, Ocular↗

PubMed2001

Pulmonary embolism: from clinical presentation to clinical probability assessment.

The rehabilitation of clinical assessment of venous thromboembolism (VTE) is certainly one of the major breakthroughs of the last decade in the field of pulmonary embolism (PE) diagnosis. Although the value of isolated clinical findings in suspected PE is limited due to the poor sensitivity and specificity of its symptoms and signs, their combination, whether empiric or explicit through prediction rules, allows a fairly accurate assessment of the likelihood of the disease in a given patient. This allows the identification of a large subset of patients with a low prevalence of the disease in whom a combination of noninvasive tests rules out PE effectively despite these tests' limited individual sensitivity. The best validated example is the combination of a low probability lung scan and a normal venous compression ultrasound in a patient with low clinical probability of PE, which is associated to a low 1 to 2% 3-month risk of VTE in such patients left untreated by anticoagulants. Finally, clinical assessment contributes significantly to the cost savings associated with modern noninvasive strategies.

Cost-Benefit Analysis↗

PubMed1985

[The position of the human fetus during pregnancy and the probability of spontaneous rotation to the vertex position in primi and multiparae].

Now that ultrasonography is a routine examination in pregnancy, breech presentations are frequently diagnosed. On the basis of 4066 individual examinations a graph was drawn indicating the relative frequency of breech, vertex and transverse presentations in the course of pregnancy. There are clear, and in some cases statistically significant differences between primiparae and multiparae with regard to the individual presentations. The analysis of several ultrasonographic examinations of one and the same patient makes a prediction possible of the likelihood of the foetus moving from breech to vertex presentation in a particular week of the pregnancy. Here there are clear differences between primiparae and multiparae: thus, the chance that a foetus in breech presentation in the 29th week will move spontaneously into vertex presentation in a primipara is 31.1%, while the degree of probability in multipara is 70.2%. In the 33rd week the probability is 15.5% in primiparae, as opposed to 57.5% in multiparae. From the 37th week on, spontaneous movement into vertex presentation is no longer likely to occur, in either primiparae or multiparae. In the 29th week, on the other hand, the likelihood of a foetus in vertex presentation moving into breech presentation in a primiparae is 0.6%, while in a multiparae it is considerably higher, at 2.3%. From the 33rd week on there is no likelihood of spontaneous movement into breech presentation in either group. Two tables, for primiparae and multiparae, respectively, show the likelihood of movement from one presentation to the other between the 13th and 41st weeks.(ABSTRACT TRUNCATED AT 250 WORDS)

Breech Presentation↗

PubMed2005

Transition probabilities of a string oscillator subject to impulsive collisions with a heavy mass point.

Impulsive linear collisions between a string oscillator (a one-dimensional particle in a box) and a mass point are studied quantum mechanically. In the limit of a very heavy mass point (which corresponds classically to many collisions during a single encounter) the transition probabilities are determined exactly. The result permits a discussion of the mixed quantum-classical regime where the collider becomes almost classical while the oscillator remains quantum mechanical. While the average transition probabilities P(m-->n) are well reproduced by the Ehrenfest mean-field approximation, the prediction for the superimposed high-frequency resonance structure is qualitatively wrong for a genuine quantum oscillator. Only if the oscillator is also almost classical and if (m-n)2 square root(mu) << m, where mu is the mass ratio collider/oscillator, this structure is correctly predicted by the Ehrenfest approximation.

Chemistry, Physical↗

PubMed2005

beta-Arrestin2, interacting with phosphodiesterase 4, regulates synaptic release probability and presynaptic inhibition by opioids.

Most mu-opioid receptor agonists recruit beta-arrestin2, with some exceptions such as morphine. Surprisingly, however, the acute analgesic effect of morphine is enhanced in the absence of beta-arrestin2. To resolve this paradox, we examined the effects of morphine and fentanyl in acute brain slices of the locus coeruleus and the periaqueductal gray from beta-arrestin2 knockout mice. We report that, in these mice, presynaptic inhibition of evoked inhibitory postsynaptic currents was enhanced, whereas postsynaptic G protein-coupled K(+) (Kir3/GIRK) currents were unaffected. The frequency, but not amplitude, of miniature inhibitory postsynaptic currents was increased in beta-arrestin2 knockout mice, indicating a higher release probability compared to WT mice. The increased release probability resulted from increased cAMP levels because of impaired phosphodiesterase 4 function and conferred an enhanced efficacy of morphine to inhibit GABA release. Thus, beta-arrestin2 attenuates presynaptic inhibition by opioids independent of mu-opioid receptor-driven recruitment, which may make beta-arrestin2 a promising target for regulating analgesia.

3',5'-Cyclic-AMP Phosphodiesterases↗

PubMed2000

Silent synapses in the developing hippocampus: lack of functional AMPA receptors or low probability of glutamate release?

At early developmental stages, silent synapses have been commonly found in different brain areas. These synapses are called silent because they do not respond at rest but are functional at positive membrane potentials. A widely accepted interpretation is that N-methyl-d-aspartate (NMDA) but not alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors are functionally expressed on the subsynaptic membrane. Here we show that, in both CA3 and CA1 hippocampal regions, AMPA-mediated synaptic responses can be detected already at early stages of postnatal development. However, some synapses appear silent because of a very low probability of glutamate release. They can be converted into functional ones by factors that enhance release probability such as paired-pulse stimulation, increasing the temperature or cyclothiazide (CTZ), a drug that blocks AMPA receptor desensitization and increases transmitter release. Conversely, conducting synapses can be switched off by increasing the frequency of stimulation. Although we cannot exclude that "latent AMPA receptors" can become functional after activity-dependent processes, our results clearly indicate that, in the neonatal hippocampus, a proportion of glutamatergic synaptic connections are presynaptically rather than postsynaptically silent.

6-Cyano-7-nitroquinoxaline-2,3-dione↗

PubMed1987

Alternative to the steady-state method: derivation of reaction rates from first-passage times and pathway probabilities.

An alternative method for deriving rate equations in enzyme kinetics is presented. An enzyme is followed as it moves along the various pathways allowed by the reaction scheme. The times spent in various sections of the scheme and the pathway probabilities are computed, using simple rules. The rate equation obtains as a function of times and probabilities. The results are equivalent to those provided by the steady-state formalism. While the latter applies uniformly to all schemes, the formalism presented here requires adaptation to each additional class of schemes. However, it has the merit of allowing one to leave unspecified many details of the scheme, including topological ones. Furthermore, it allows one to decompose a scheme into subschemes, analyze the parts separately, and use the intermediate results to derive the rate equation of the complete scheme. The method is applied here to derive general equations for one- and two-entry site enzymes.

Binding Sites↗

PubMed1977

Probability, personality, age, and risk taking.

Male and female college (n=22) and high school (n=18) students were presented information concerning probability and were also measured on the personality variables of independence of judgment, response set, Machiavellianism, and open-mindedness. An investigation of the risky shift phenomenon revealed that an understanding of probability did not vitiate the shift toward greater risk. Age was shown to be functionally related to risk taking, the teen years showing greater proclivity for risk than later years. Independence of judgment emerged as a powerful predictor of initial risk; Machiavelianism bore no relationship to risk taking. Both of these findings concerning personality variables were at odds with earlier work. An interpretation of contextual interaction was advanced as a possible explanation.

Adolescent↗

PubMed1986

Birth forecasting based on birth order probabilities, with application to U.S. data.

"A model for birth forecasting based on prediction of the so-called 'birth order probabilities' is constructed. The relation between this model and recent models of fertility prediction is derived. Birth forecasts with approximate probability limits for the U.S. for the period 1983-1997 are generated. The performance of the proposed model in predicting future fertility is tested by fitting time series models to part of the available series (1917-1982) and ultimately generating birth forecasts for the remainder of the period, then comparing these forecasts with the actual data." The accuracy of the fertility forecasts made are compared with those made by other methods.

Americas↗

PubMed2003

CS-dependent response probability in an auditory masked-detection task: considerations based on models of Pavlovian conditioning.

Experimental studies were performed using a Pavlovian-conditioned eyeblink response to measure detection of a variable-sound-level tone (T) in a fixed-sound-level masking noise (N) in rabbits. Results showed an increase in the asymptotic probability of conditioned responses (CRs) to the reinforced TN trials and a decrease in the asymptotic rate of eyeblink responses to the non-reinforced N presentations as a function of the sound level of the T. These observations are consistent with expected behaviour in an auditory masked detection task, but they are not consistent with predictions from a traditional application of the Rescorla-Wagner or Pearce models of associative learning. To implement these models, one typically considers only the actual stimuli and reinforcement on each trial. We found that by considering perceptual interactions and concepts from signal detection theory, these models could predict the CS dependence on the sound level of the T. In these alternative implementations, the animals response probabilities were used as a guide in making assumptions about the "effective stimuli".

Animals↗

PubMed2000

Comparative analysis of dose volume histogram reduction algorithms for normal tissue complication probability calculations.

A model for estimating radiotherapy treatment outcome through the probability of damage to normal tissue and the probability of tumour control is a useful tool for treatment plan optimization, dose escalation strategies and other currently used procedures in radiation oncology. Normal tissue complication estimation (NTCP) is here analysed from the point of view of the reliability and internal consistency of the most popular model. Five different dose volume histogram (DVH) reduction algorithms, applied to the Lyman model for NTCP calculation. were analysed and compared. The study was carried out for sets of parameters corresponding to quite different expected dose-response relationships. In particular, we discussed the dependence of the models on the parameters and on the dose bin size in the DVH. The sensitivity of the different reduction schemes to dose inhomogeneities was analysed, using a set of simple DVHs representing typical situations of radiation therapy routine. Significant differences were substantiated between the various reduction methods regarding the sensitivity to the degree of irradiation homogeneity, to the model parameters and to the dose bin size. Structural aspects of the reduction formalism allowed an explanation for these differences. This work shows that DVH reduction for NTCP calculation has still to be considered as a very delicate field and used with extreme care, especially for clinical applications, at least until the actual formulations are tuned against strong clinical data.

Algorithms↗

PubMed2006

Analysis of normal tissue complication probability of the lung using a reliability model.

The volume effect of normal tissues and organs is an important factor for predicting normal tissue complication probability (NTCP) following partial, heterogeneous irradiation of organs at risk, and reducing the late sequela by conformal radiation therapy. We have previously developed a reliability model for calculation of NTCP, assuming a parallel architecture of functional subunits (FSU), where a critical number (k) out of the total number of FSUs (N) must be intact for the organ to maintain its function. Published data on radiation-induced lethal pneumonitis and altered breathing rate following partial volume irradiation of the mouse lung were analysed, and critical fraction and corresponding spatial density distribution of FSUs were estimated using this model. The critical fraction (k/N) seemed to be similar for the two endpoints, and a value of 0.7 was found to provide good fit to the experimental data. The critical fraction did not vary throughout the lung, and variation in volume effect cannot therefore be attributed to heterogeneous tissue architecture. On the other hand, our analysis revealed that the observed variation in volume effect of mouse lung may be attributed to heterogeneous spatial distribution in FSU density or also the spatial variation in inactivation probability of the FSUs.

Animals↗

PubMed2000

Technical report. The application of probability-generating functions to linear-quadratic radiation survival curves.

PURPOSE: To illustrate how probability-generating functions (PGFs) can be employed to derive a simple probabilistic model for clonogenic survival after exposure to ionizing irradiation. METHODS: Both repairable and irreparable radiation damage to DNA were assumed to occur by independent (Poisson) processes, at intensities proportional to the irradiation dose. Also, repairable damage was assumed to be either repaired or further (lethally) injured according to a third (Bernoulli) process, with the probability of lethal conversion being directly proportional to dose. Using the algebra of PGFs, these three processes were combined to yield a composite PGF that described the distribution of lethal DNA lesions in irradiated cells. RESULTS: The composite PGF characterized a Poisson distribution with mean, chiD+betaD2, where D was dose and alpha and beta were radiobiological constants. This distribution yielded the conventional linear-quadratic survival equation. To test the composite model, the derived distribution was used to predict the frequencies of multiple chromosomal aberrations in irradiated human lymphocytes. The predictions agreed well with observation. This probabilistic model was consistent with single-hit mechanisms, but it was not consistent with binary misrepair mechanisms. CONCLUSIONS: A stochastic model for radiation survival has been constructed from elementary PGFs that exactly yields the linear-quadratic relationship. This approach can be used to investigate other simple probabilistic survival models.

Cell Survival↗

PubMed1998

Field performance measurements of half-facepiece respirators: developing probability estimates to evaluate the adequacy of an APF of 10.

To evaluate the protection provided by negative-pressure, half-facepiece respirators, ambient and in-facepiece samples were collected on workers in foundry, aircraft-painting, and steel-manufacturing operations. Protection was assessed by workplace protection factors (WPF). The appropriateness of the assigned protection factor (APF) for half-facepiece respirators was evaluated with a new approach using the WPF data from these and other studies previously published. The new approach utilizes binomial statistics based on the number of successes (no overexposure) and failures (overexposure) and is illustrated with a graphical representation of WPF data. With this consideration of the data, the probability of overexposure occurring during a wearing period for workers wearing the half-facepiece respirators represented by the studies referenced here was 0.5%, with a 95% confidence interval of 0.01 to 2.7%. If 50% in-facepiece sampling errors are considered, the probability of overexposure was 2.9%, with a 95% confidence interval of 1.1 to 6.3%. The authors believe the current APF of 10 for half-facepiece respirators is appropriate.

Air Pollutants, Occupational↗

PubMed1996

To forget or not to forget: the effect of probability of test on directed forgetting.

Two experiments investigated how individuals use explicit memory cues that designate different probabilities of test. As in typical directed forgetting studies, subjects received words explicitly cued as having either a 0% or a 100% chance of being on a subsequent memory test (i.e. forget and remember cues, respectively). In addition, some words were explicitly cued as having the potential to be either forgotten or remembered (i.e. a 50% cue). Recall of 50% words was between that of 0% and 100% words. In addition, the presence of 50% words lowered recall of the 100% words compared to that of a control group that did not receive 50% words, but received the same number of 100% words. A think-aloud task indicated that these results were due to 50% words being treated like either 100% of 0% words at encoding. The results are discussed in terms of the effect of different probabilities of test on the strategic processing and representation of information.

Adult↗

PubMed1998

On the probabilities of identity states in permutable populations.

Génin and Clerget-Darpoux recently discussed the derivation of the probabilities of identity states for populations in which there was some degree of kinship, primarily to allow the extension of the classical affected-sib-pair method to such populations. It is argued here that their derivation makes certain assumptions that are valid only for some very restricted population models and that are not needed for an appropriate treatment. Here the probabilities of the identity states of two individuals with a given genealogical relationship are specified in terms of the kinship parameters of the underlying population, from which the founders of the individuals' genealogy have been randomly selected. It is argued that an appropriate representation for a permutable population, one in which gene identity does not depend on the pattern of genes across individuals, requires three parameters. This representation is related to that of Génin and Clerget-Darpoux and to that of Weir.

Genetics, Population↗

PubMed2006

Test to determine sterile integrity of wrapped medical products at a probability of recontamination of 1 : 1,000,000.

OBJECTIVE: We developed a microbiological test to detect the penetration of airborne microorganisms through the packaging of medical products after sterilization, to meet the requirements of European standard EN 556. We applied this test method to transparent pouches. DESIGN: The microbial-barrier properties of the transparent pouches were determined using the microbial challenge test, in which the package was placed inside an exposure chamber and exposed to a defined aerosol of Saccharomyces cerevisiae. The atmospheric pressure in the chamber was periodically reduced by 0-75 millibars, to simulate weather-dependent pressure changes. Thermoresistant petri dishes filled with nutrient agar were integrated into the transparent pouches before sterilization. The packages were incubated after exposure. They were then opened and examined for colony growth. RESULTS: The number of recontaminated packages per test group (n=50) depended on the microbial bioload (defined as the number of colony-forming units per plate) to which the packages were exposed and on the size and number of decreases in atmospheric pressure. Results of multiple regression analysis showed a significant increase in the number of recontaminated packages in correlation with the product of the values for microbial bioload and the size and number of decreases in atmospheric pressure. When we analyzed the probability of recontamination of wrapped medical devices after 2 reductions in atmospheric pressure (30 millibars each) and with a surface microbial load of 10 colony-forming units per 64 cm(2), we estimated that the frequency of recontamination was 1 : 100,000. CONCLUSION: Multiple regression analysis showed that the proposed microbial challenge test is suitable to determine the probability of package recontamination at the 1 : 1,000,000 level.

Air Microbiology↗

PubMed1996

Some characteristics of tumour control probability for heterogeneous tumours.

Some mathematical characteristics of tumour control probability (TCP) have been examined in the light of potential intra-tumour variations in clonogenic cell characteristics. In particular, an explicit expression for the relationship between the dose distribution and the probability of local clonogen eradication is obtained which maximizes the TCP using both general and specific TCP models, for the case of fixed energy deposition into the tumour volume. Characteristics of this expression are considered for the cases of varying clonogen cell density (uniform radiosensitivity) and uniform cell density (uniform radiosensitivity), yielding the 'uniform TCP' and 'uniform dose' results respectively for TCP maximization. These situations are examined graphically to highlight the possible consequences of neglecting intra-tumour heterogeneity in dose prescription.

Cell Survival↗