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PubMed2006

Rotational transitions and diffraction in D2 scattering from the LiF(001) surface: theory and experiment.

High probabilities of energy transfer from translation to molecular rotations are observed in the scattering of n-D(2) from LiF(001) at an incident beam energy of 85.3 meV. For the 100 incidence direction, close-coupling calculations yield ratios of the rotationally inelastic (j=0-->2) and (j=1-->3) peaks to the rotationally elastic specular peaks (G=0) that are in reasonable agreement with experiment, as are the ratios of the rotationally elastic diffraction peak intensities to the specular peak intensities. The agreement between theory and experiment is also quite good for the rotationally inelastic diffractive (-1-1) transitions for (j=1-->3), but rather poor for (j=0-->2). The calculations show that the interaction between the electrostatic field of the surface ions and the quadrupole moment of the D(2) molecule efficiently promotes the (j=0-->2) and (j=1-->3) transitions. If this electrostatic interaction is excluded from the potential model, the ratios of the (j=0-->2) and (j=1-->3) rotationally inelastic peaks to the corresponding specular peaks show a large discrepancy with experiment, underlining the importance of this interaction. The close-coupling calculations show a somewhat worse agreement with experiment for the 110 incidence direction. In particular, the sharp peaks observed experimentally in the ratios of the peak intensities of the rotationally inelastic G=0 (j=0-->2) and (j=1-->3) to the rotationally elastic G=0 transitions as a function of incident angle are not reproduced by the calculations. The theoretical ratios of the peak intensities of the rotationally elastic diffraction to G=0 transitions are shifted to lower incidence angles with respect to experiment. The rotationally inelastic diffractive (-10) transitions present an interesting resonance phenomenon for the (j=0-->2) rotational transition. This resonance is predicted by both theory and experiment, although at rather different incident angles.

Journal Article↗
PubMed2000

Probable values of the cosmological constant in a holographic theory

We point out that for a large class of universes, holography implies that the most probable value for the cosmological constant is zero. In four space-time dimensions, the probability distribution takes the Baum-Hawking form, dP approximately exp(cM(2)(p)/Lambda)dLambda.

Journal Article↗
PubMed1990

Mechanism(s) for the metabolism of mitoxantrone: electron spin resonance and electrochemical studies.

Mitoxantrone has been reported to lack certain properties that characterize quinone containing antitumor agents that undergo enzymatic reduction. These properties are the stimulation of NADPH oxidation, the stimulation of oxygen consumption by microsomes and reductases and, the absence of oxygen free radicals during these reactions. Having these properties implies the presence of a futile redox cycle that requires the generation and the oxidation of a semiquinone free radical. It would follow that if mitoxantrone does not redox cycle in the presence of reductases, then the semiquinone free radical is not produced or, if it is formed, it reacts quickly to form diamagnetic products. However, using liver microsomes, there are reports of the formation of the mitoxantrone free radial anion. In this paper we investigated the mitoxantrone free radical anion generated electrochemically and found that in the presence of oxygen it behaved like other semiquinones. That is, it is oxidized to the parent compound (presumably generating oxygen free radicals), indicating the ability to redox cycle. The reduction potential to generate such free radical in aqueous medium is very high (-0.79 V) when compared to diaziquone (-0.36 V) and Adriamycin (-0.6 V). This suggests that mitoxantrone may not be a substrate for reductases. Under reductive conditions with purified NADPH cytochrome P-450 reductase which very easily reduces diaziquone and Adriamycin, mitoxantrone was not reduced. However, under the same conditions, mitoxantrone was oxidized by the prototype oxidase horseradish peroxidase with the production of a mitoxantrone free radical. This oxidation was accompanied by a drastic change in color and the formation of a dark precipitate. Because microsomes contain a variety of enzymes, we suggest that the previously observed free radical in microsomes is probably due to the oxidation of mitoxantrone. In this theory, this product is probably a polymer which would not require oxygen to be formed. Thus, under oxidative conditions, the mitoxantrone free radical cation will also display impaired redox activity.

Biotransformation↗
PubMed2005

An investigation of the discrete and continuum models of water behavior in hydrogel contact lenses.

PURPOSE: To investigate the water behavior in hydrogels of differing equilibrium water content (EWC) and to use these results to investigate the opposing discrete model (thermodynamically different classes of water exist in hydrogels) and continuum model (water behaves as a consequence of nonequilibrium conditions) of nonfreezing water in swollen hydrogels. METHODS: Differential scanning calorimetry (DSC) was used to obtain melting thermograms for five lenses of each of 12 hydrogel lens types. The gravimetric content of the nonfreezing water was subsequently determined from an integrated endotherm for water. The effect of anneal time on the amount of nonfreezing water obtained in samples was investigated before undertaking these measurements. The glass transition temperature (Tg) of each of the lens types was obtained with DSC to investigate how the Tg was related to the amount of nonfreezing water found in the hydrogels. RESULTS: Melting enthalpy increased with increasing anneal time and leveled off at 6 hours. Low-EWC lenses showed endotherms with a single melting peak at 0 degrees C. Materials with a higher EWC showed more complicated melting endotherms, with a broad shoulder occurring at temperatures below 0 degrees C. There was a statistically significant positive correlation between EWC and freezing water (R=0.95, P<0.0001), but the amount of nonfreezing water was similar for all lens materials. There was a statistically significant positive correlation between the Tg of the hydrogels and the theoretically derived weight fraction of nonfreezing water in the hydrogels required to bring the Tg of the gels down to 0 degrees C (R=0.99, P<0.0001) but not with the experimentally derived values (R=0.29, P=0.07). CONCLUSIONS: Low-EWC hydrogels have the lowest free-to-bound water ratios. That the experimentally derived values of nonfreezing water are different from the theoretically derived values creates doubt with the continuum model theory. The best model probably employs a combination of the discrete and continuum theories.

Calorimetry↗
PubMed1995

Trophozoite-to-cyst ratio increases during recovery from Pneumocystis carinii pneumonia in rats.

Pneumocystis carinii has two developmental stages: the trophozoite and the cyst. The differential specificity of monoclonal antibodies SMoAb (cysts only) and CMoAb (cysts and trophozoites) was used to analyse the trophozoite-to-cyst (T: C) ratio during recovery from P. carinii pneumonia (PCP) in a dexamethasone model in Wistar rats. In tissue homogenates the cysts decreased from week 0 to week 4 (4.10 +/- 1.57 10(6)/g vs 0.069 +/- 0.021 10(6)/g, p < 0.001), whereas the trophozoites decreased much less, leading to a change in T: C ratio from week 0 and 1 (72 +/- 33 and 49 +/- 14, respectively) to week 2 and 4 (1752 +/- 652 and 1163 +/- 482, p < 0.001). A similar tendency was found in immunofluorescence-labelled histological samples quantitated with the VIDAS image analyser. These results demonstrate a hitherto unrecognized and significant change in T: C ratio during recovery from PCP. This change could not be monitored using bronchoalveolar fluid, probably due to adherence of the trophozoites to the alveolar wall. Changes in T: C ratio indicate that in particular the cyst form of P. carinii is sensitive to the host response mounted during recovery, and support the theory that trophozoites probably also multiply in an extracystic asexual fission.

Animals↗
PubMed2005

Theory of mind--evolution, ontogeny, brain mechanisms and psychopathology.

The ability to infer other persons' mental states and emotions has been termed 'theory of mind'. It represents an evolved psychological capacity most highly developed in humans. The evolutionary origins of theory of mind can be traced back in extant non-human primates; theory of mind probably emerged as an adaptive response to increasingly complex primate social interaction. This sophisticated 'metacognitive' ability comes, however, at an evolutionary cost, reflected in a broad spectrum of psychopathological conditions. Extensive research into autistic spectrum disorders has revealed that theory of mind may be selectively impaired, leaving other cognitive faculties intact. Recent studies have shown that observed deficits in theory of mind task performance are part of a broad range of symptoms in schizophrenia, bipolar affective disorder, some forms of dementia, 'psychopathy' and in other psychiatric disorders. This article reviews the evolutionary psychology of theory of mind including its ontogeny and representation in the central nervous system, and studies of theory of mind in psychopathological conditions.

Animals↗
PubMed1983

Steady-state performance on fixed-, mixed-, and random-ratio schedules.

Three groups of rats pressed a lever for milk reinforcers on various simple reinforcement schedules (one schedule per condition). In Group M, each pair of conditions included a mixed-ratio schedule and a fixed-ratio schedule with equal average response:reinforcer ratios. On mixed-ratio schedules, reinforcement occurred with equal probability after a small or a large response requirement was met. In Group R, fixed-ratio and random-ratio schedules were compared in each pair of conditions. For all subjects in these two groups, the frequency distributions of interresponse times of less than one second were very similar on all ratio schedules, exhibiting a peak at about .2 seconds. For comparison, subjects in Group V responded on variable-interval schedules, and few interresponse times as short as .2 seconds were recorded. The results suggest that the rate of continuous responding is the same on all ratio schedules, and what varies among ratio schedules is the frequency, location, and duration of pauses. Preratio pauses were longer on fixed-ratio schedules than on mixed-ratio or random-ratio schedules, but there was more within-ratio pausing on mixed-ratio and random-ratio schedules. Across a single trial, the probability of an interruption in responding decreased on fixed-ratio schedules, was roughly constant on random-ratio schedules, and often increased and then decreased on mixed-ratio schedules. These response patterns provided partial support for Mazur's (1982) theory that the probability of instrumental responding is directly related to the probability of reinforcement and the proximity of reinforcement.

Journal Article↗
PubMed2006

Statistical mechanical theory for steady state systems. IV. Transition probability and simulation algorithm demonstrated for heat flow.

Two microscopic transition theorems are given for the probability of nonequilibrium work performed on a subsystem of a thermal reservoir along the trajectory in phase space of the subsystem. The resultant transition probability is applied to the case of heat flow down an applied temperature gradient. A combined molecular dynamics and Monte Carlo algorithm is given for such a nonequilibrium steady state. Results obtained for the thermal conductivity are in good agreement with previous Green-Kubo and nonequilibrium molecular dynamics results.

Journal Article↗
PubMed2001

Reparametrization-covariant theory for on-line learning of probability distributions.

We discuss the on-line learning of probability distributions in a reparametrization covariant formulation. Reparametrization covariance plays an essential role not only to respect an intrinsic property of "information" but also for pattern recognition problems. We can obtain an optimal on-line learning algorithm with reparametrization invariance, where the conformal gauge connects a covariant formulation with a noncovariant one in a natural way.

Journal Article↗
PubMed1989

Expected utility theory and risky choices with health outcomes.

Studies of people's attitude towards risk in the health sector often involve a comparison of the desirability of alternative medical treatments. Since the outcome of a medical treatment cannot be known with certainty, patients and physicians must make a choice that involves risk. Each medical treatment may be characterized as a gamble (or risky option) with a set of outcomes and associated probabilities. Expected utility theory (EUT) is the standard method to predict people's choices under uncertainty. The author presents the results of a survey that suggests people are very risk averse towards gambles involving health-related outcomes. The survey also indicates that there is significant variability in the risk attitudes across individuals for any given gamble and that there is significant variability in the risk attitudes of a given individual across gambles. The variability of risk attitudes of a given individual suggests that risk attitudes are not absolute but are functions of the parameters in the gamble.

Adult↗
PubMedDate not supplied

Evolutionary theories of aging. 1. The need to understand the process of natural selection.

BACKGROUND: In a Forum article Le Bourg (1998) criticized recent tests of evolutionary theories of aging and suggested alternative explanations for the long lifespan of ant queens and the positive relationship between body size and lifespan in mammals. Moreover, he attempts to criticize evolutionary theories of aging by showing that explanations other than evolutionary theories of aging probably account for the variation in human lifespan across countries. OBJECTIVE: Here we show that the arguments of Le Bourg suffer several problems. First, many of the arguments reveal a misunderstanding of the process of natural selection. Second, some of the arguments reflect a lack of knowledge of evolutionary theories of aging (e.g. pre-reproductive mortality is not predicted to influence lifespan of organisms contrary to what is claimed). Finally, his final example on lifespan in humans simply is a straw-man because serious evolutionary biologists are well aware of the importance of confounding variables and would certainly not make the type of conclusion suggested by Le Bourg. CONCLUSION: Although a critical discussion of evolutionary theories of aging is welcome, we believe that the alternative explanations proposed by Le Bourg are implausible and reflect a misunderstanding of the process of natural selection.

Aging↗
PubMed2003

The puzzle of acupuncture.

Acupuncture is based on the theory of channels, which serve as pathways for energy (Qi). On the course of the channels, acupuncture points are described, and by stimulation of these points, therapeutic effects may be achieved. This system is very complex and both channels and acupuncture points are anatomically invisible. Unlike in Western medicine, scientists fail to trace both the origin and the progress of acupuncture theory. Having developed in its full form not later than the 2nd century BC, it never underwent fundamental change. On the other hand, it has become a part of modern Western medicine as an effective therapy and the existence of acupuncture points, specified thousands of years ago, has been demonstrated by modern science. It is hardly probable that acupuncture theory, although dating back to ancient times, could have originated in primitive civilization. The origin of the energy channel theory does not fit into the traditional developmental scheme. The existence of the theory cannot be explained other than by its being a product of a highly developed civilization.

Acupuncture↗
PubMed2005

Score statistics of global sequence alignment from the energy distribution of a modified directed polymer and directed percolation problem.

Sequence alignment is one of the most important bioinformatics tools for modern molecular biology. The statistical characterization of gapped alignment scores has been a long-standing problem in sequence alignment research. Using a variant of the directed path in random media model, we investigate the score statistics of global sequence alignment taking into account, in particular, the compositional bias of the sequences compared. Such statistics are used to distinguish accidental similarity due to compositional similarity from biologically significant similarity. To accommodate the compositional bias, we introduce an extra parameter p indicating the probability for positive matching scores to occur. When p is small, a high scoring alignment obviously cannot come from compositional similarity. When p is large, the highest scoring point within a global alignment tends to be close to the end of both sequences, in which case we say the system percolates. By applying finite-size scaling theory on percolating probability functions of various sizes (sequence lengths), the critical p at infinite size is obtained. For alignment of length t, the fact that the score fluctuation grows as chi(t)1/3 is confirmed upon investigating the scaling form of the alignment score. Using the Kolmogorov-Smirnov statistics test, we show that the random variable , if properly scaled, follows the Tracy-Widom distributions: Gaussian orthogonal ensemble for p slightly larger than pc and Gaussian unitary ensemble for larger p. Although these results deepen our understanding of the distribution of alignment scores, the use of these results in practical applications remains somewhat heuristic and needs to be further developed. Nevertheless, the possibility of characterizing score statistics for modest system size (sequence lengths), via proper reparametrization of alignment scores, is illustrated.

Algorithms↗
PubMedDate not supplied

[Popper and the problem of induction in epidemiology].

In this article we are discussing a few of the contributions by the Austro-British philosopher Karl R. Popper, one of our most influential contemporary thinkers, whose epistemological and socio-political theories have also penetrated the sphere of epidemiology. We are focusing mainly on the so-called problem of induction. We sustain, in line with Popper, that the scientific method does not use inductive reasoning, but rather hypothetical-deductive reasoning. Although the movement from the data evaluating a hypothesis to a conclusion on the latter goes from the specific to the general, that is, in an inductive direction, the induction does not exist as a reasoning process or inference. That is, there is no method that enables us to infer or to verify hypotheses or theories (we cannot explore all of the possible situations to see whether the theory stands up), or even to render them very probable. Besides, scientists look for highly informative theories, not highly probable ones. What we actually do is to propose a hypothesis as a tentative solution to a problem, to confront the prediction deduced from the hypothesis with actual experience, and evaluate whether the hypothesis is rejected or not by the facts. As theories cannot be verified, we can only accept them if they withstand an attempt to reject them. Consequently, the test of a theory consists of criticism or a serious attempt at falsification, that is, the elimination of error within a theory, in order to reject it if it is false. The objective is, thus, the search for true theories. For this purpose, the scientific method uses a systematic set of methodological (not logical) rules, that is, decisions. These methodological rules or principles can be summed up in two: [symbol: see text]be inventive and critical!, that is, propose bold hypotheses and subject them to severe tests of experience. Logic plays its role mainly by allowing us to deduce from a hypothesis the predictions to be confronted with the facts or evidence. This is applicable both to statistical inference as well as to causal inference. We argue that the criteria of causality used in epidemiology are none other than rules of the method designed for the same purpose: they are concerned with eliminating or reducing errors (chance, bias...) on testing a causal hypothesis. Consequently, the so-called ausal inference, the step from evidence to causal theory, is not a logical inductive or probabilistic process but rather a decision based on the evaluation of a causal hypothesis thanks to methodological rules such as the criteria of causality. We believe that the interest of the debate between the Popperian and the inductivist epidemiologists is not merely a matter of words, as, if we are aware that we do not operate inductively, that we cannot establish firmly hypotheses, not even affirm them probabilistically, we will presumably adopt a humbler attitude and look more for the errors in our theories than for their facile examples of confirmation.

Causality↗
PubMed1994

Root probabilities for intraspecific gene trees under neutral coalescent theory.

The rooting of intraspecific gene or haplotype trees has proven to be difficult using the traditional techniques of rooting species trees such as outgroups. As an alternative, we apply neutral coalescent theory to the problems of determining the root of an intraspecific gene tree and the relative ages of the haplotypes. By using a recursive equation, exact root probabilities can be calculated for small cladograms and sample sizes. These exact probabilities indicate that root probabilities are not very sensitive to the parameter theta, which is four times the mutation rate times the inbreeding effective size. The exact probabilities also indicate that root probabilities are not very sensitive to the absolute numbers of each haplotype (allele) class, only their relative proportions. The exact method, unfortunately, is not feasible for implementation with larger data sets with present algorithms and computers. However, the exact results suggested a simple heuristic for determining outgroup weights; that is, finding the haplotype that is the oldest in the sample and that can serve as an outgroup for the remainder of the haplotype tree. Computer simulations revealed that these outgroup weights are strongly correlated with actual age and are much better indicators of haplotype age than is the haplotype frequency, another commonly used indicator of relative age.

Gene Frequency↗