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PubMed1982

Explicating activity theory: a formal replication.

The principal intent of this paper is to replicate formally the work of Lemon, Bengtson, and Peterson (1972), the first researchers to carefully articulate a systematic statement of the activity theory of aging, but whose test of the theory resulted in disappointing ambiguous findings. Probability samples of three distinct types of retirement communities (N = 1209) provided far greater variation in background variables than existed in the original study. Behaviorally based activity scales were drawn from daily activity inventories rather than the single-item ordinal measures of activity items used in the original research. Multiple Classification Analysis was employed to sort out the effects of each activity type from the others and from the effects of age, gender, and health upon life satisfaction. Different from the results of Lemon, Bengtson, and Peterson, our findings, although mixed, lend strong support to the activity theory of aging. Informal activity contributed positively, strongly, and frequently to the life satisfaction of respondents. Solitary activities had no effect on life satisfaction. Formal activity had a negative effect. All activity effects were similar in the three communities. The implications of these findings for activity theory are discussed at length.

Activities of Daily Living↗
PubMed1985

[Mathematical approach to bacterial identification].

The identification of Enterobacteriaceae is examined in the context of the recent field of research assisted by multi-criteria decisions. In particular, the authors discuss the limitations of two approaches of assisted identification based on completely different mathematical concepts: the calculation of the probabilities (used in commercial centres) and the aggregation of pre-orders by a model of assisted decision such as Electra II. In order to reduce the errors of interpretation inherent in the use of any model, the authors propose the use of computer assisted systems of bacterial identification in bacteriology laboratories.

Bacteriological Techniques↗
PubMedDate not supplied

Using explicit decision rules to manage issues of justice, risk, and ethics in decision analysis: when is it not rational to maximize expected utility?

Concepts of justice, risk, and ethics can be merged with decision analysis by requiring the analyst to specify explicity a decision rule or sequence of rules. Decision rules are categorized by whether they consider: 1) aspects of outcome distributions beyond central tendencies; 2) probabilities as well as utilities of outcomes; and 3) means as well as ends. This formulation suggests that distribution-based decision rules could address both risk (for an individual) and justice (for the population). Rational choice under risk if choices are one-time only (vs. repeated events) or if one branch contains unlikely but disastrous outcomes might ignore probability information. Incorporating risk attitude into decision rules rather than utilities could facilitate use of multiattribute approaches to measuring outcomes. Certain ethical concerns could be addressed by prior specification of rules for allowing particular branches. Examples, including selection of polio vaccine strategies, are discussed, and theoretical and practical implications of a decision rule approach noted.

Beneficence↗
PubMed1989

Lateral diffusion of membrane proteins in protein-rich membranes. A simple hard particle model for concentration dependence of the two-dimensional diffusion coefficient.

A model for the effect of protein concentration on the rate of lateral diffusion of integral membrane proteins is presented, in which the proteins are represented by equivalent hard circular particles on a surface. As the density of particles increases, the probability of finding a vacancy immediately adjacent to a tracer particle into which it may diffuse decreases, resulting in a concomitant reduction of the tracer diffusion coefficient. Using scaled particle theory to calculate the concentration-dependent probabilities, a simple approximate result is obtained in closed form, that is compared with the results of previously published Monte Carlo lattice simulations and experimental observations.

Diffusion↗
PubMed2000

Fixation of clonal lineages under Muller's ratchet.

For clonal lineages of finite size that differ in their deleterious mutational effects, the probability of fixation is investigated by mathematical theory and Monte Carlo simulations. If these fitness effects are sufficiently small in one or both lineages, then the lineage with the less deleterious effects will become fixed with high probability. If, however, in both lineages the deleterious effects are larger than a threshold s(c), then the probability of fixation is independent of the fitness effects and depends only on the initial frequencies of the lineages. This threshold decreases with decreasing genomic mutation rate U and increases with population size N. (For N = 10(5), we have s(c) approximately = 0.1 if U = 1, and s(c) approximately = 0.015 if U = 0.1). Above the threshold, the competition is not driven by the ratio of mean fitnesses of the lineages, but by the relative sizes of the zero-mutation classes, which are independent of the fitness effects of the mutations. After the loss of the zero-mutation class of a lineage, the other lineage will spread to fixation with high probability and within a short time span. If the mutation rates of the lineages differ substantially, the lineage with the lower mutation rate is fixed with very high probability unless the lineage with the larger mutation rate has very slightly deleterious mutational effects. If the mutation rates differ by not more than a few percent, then the lineage with the higher mutation rate and the more deleterious effects can become fixed with appreciable probability for a certain range of parameters. The independence of the fixation probability on the fitness effects in a single population leads to dramatic effects in metapopulations: lineages with more deleterious effects have a much higher fixation probability. The critical value s(c), above which this phenomenon occurs, decreases as the migration rate between the subpopulations decreases.

Animals↗
PubMedDate not supplied

Determination of the hyperparameters of a prior probability model in survival analysis.

This paper outlines the mathematical theory required for eliciting the hyperparameters of a subjective conjugate distribution for the exponential survival model with censoring. The technique involves the quantification of expert knowledge based on determination by the expert of expected fractiles of a survival distribution in a particular clinical trial setting. Once the prior predictive distribution is determined and the fractiles elicited one can proceed, using iterative techniques, to solve for the hyperparameters. The restrictions and constraints of the hyperparameters as well as the fractiles are studied. The theory is then applied in a clinical trial setting.

Clinical Trials as Topic↗
PubMed1996

Superficial siderosis of the brain: roles for cerebrospinal fluid circulation, iron and the hydroxyl radical.

Superficial siderosis is associated with chronic blood loss into the cerebrospinal fluid. The pattern of hemosiderin deposition and clinical signs in superficial siderosis suggest that cerebrospinal fluid is recirculated into the ventricular system. Patterns of deposition of corpora amylacea and findings in normopressure communicating hydrocephalus also support the recirculation theory. 'Free' iron with excess production of hydroxyl radicals is the probable mechanism of tissue damage. The arachnoid villus-superior saggital sinus theory of cerebrospinal fluid circulation should be abandoned.

Brain Diseases↗
PubMed1984

Clinical inferences and decisions--III. Utility assessment and the Bayesian decision model.

It is accepted that errors of misclassifications, however small, can occur in clinical decisions but it cannot be assumed that the importance associated with false positive errors is the same as that for false negatives. The relative importance of these two types of error is frequently implied by a decision maker in the different weighting factors or utilities he assigns to the alternative consequences of his decisions. Formal procedures are available by which it is possible to make explicit in numerical form the value or worth of the outcome of a decision process. The two principal methods are described for generating utilities as associated with clinical decisions. The concept and application of utility is then expanded from a unidimensional to a multidimensional problem where, for example, one variable may be state of health and another monetary assets. When combined with the principles of subjective probability and test criterion selection outlined in Parts I and II of this series, the consequent use of utilities completes the framework upon which the general Bayesian model of clinical decision making is based. The five main stages in this general decision making model are described and applications of the model are illustrated with clinical examples from the field of ophthalmology. These include examples for unidimensional and multidimensional problems which are worked through in detail to illustrate both the principles and methodology involved in a rationalized normative model of clinical decision making behaviour.

Bayes Theorem↗
PubMed1996

The joint action of two mixed stimuli with a bivariate threshold.

The general mathematical statistical theory of a mixture of two stimuli in quantal bioassay or other binary response is investigated assuming the existence of thresholds. Univariate thresholds have long been studied. Different serious attempts on bivariate threshold theory have emerged since the late 1940s. Some have been based on biologically established fact mixed with analysis, while others have been more heuristic. In this paper we investigate the properties of correlations of thresholds. Joint actions are then interpreted according to those correlations and deviations from independence. Isoboles are lines, curves, planes, or surfaces of constant probability. They are also investigated, and their properties are used in proofs of important theorems. It is proved that additivity or additivism of two stimuli is present if and only if the thresholds of the two stimuli are perfectly correlated. Furthermore, additivity requires univariate dose-response functions to be everywhere concave on the nonnegative real numbers (doses). Finally, threshold theory, itself, dictates cumulative distributions of marginal thresholds that are everywhere nondecreasing and concave on the nonnegative reals. That is, they cannot have points of inflection. Such popular distributions as the normal and logistic have points of inflection at their respective medians. This does not invalidate their usefulness. They are simply modeling phenomena other than those which are the subject of this paper, namely, threshold theory.

Mathematical Computing↗
PubMed2004

Information-theory-based snake adapted to multiregion objects with different noise models.

We propose a segmentation technique adapted to objects composed of several regions with gray-level fluctuations described by different probability laws. This approach is based on information theory techniques and leads to a multiregion polygonal snake driven by the minimization of a criterion without any parameters to be tuned by the user. We demonstrate the improvements obtained with this approach as well as its low computational cost. This approach is compatible with applications such as object recognition and object tracking with nonrigid deformation in images perturbed by different types of optical noise.

Journal Article↗
PubMed2004

Economic context and pigeons' risk-taking: an integrative approach.

The present work tested pigeons' (Columba livia) risk sensitivity to changes in resource availability in the concurrent-chains choice procedure. Subjects were provided choices, generally between variable-ratio (VR) and fixed-ratio (FR) outcome schedules with equal or different mean values. Predictions from ecology's risk sensitivity, behavioral economic's rate maximization, and psychology's delay-reduction theory were contrasted under settings where budget was adversely affected by one of three manipulations in seven experiments: (1) session length was dramatically reduced; (2) choice-phase duration was substantially increased by increasing the response requirements in the choice phase, or (3) outcome duration was significantly increased by increasing the response requirements in the outcome phase while holding session length constant. Although preference measures were sensitive to every budget manipulation, the nature of the changes observed were dependent upon the manner in which resource availability was manipulated and the choice alternatives offered. For example, choice was affected in an opposite manner when budget was adversely altered by lengthening choice duration ("search time") as opposed to lengthening outcome duration ("handling time"). Findings revealed partial support for pigeons' sensitivity both to changes in budget and to changes in conditioned reinforcer value. The overall pattern of results thus favors an integrative approach to describing the effect of economic context on general risk sensitivity.

Animals↗
PubMed1999

Ionic effects beyond Poisson-Boltzmann theory.

Polyelectrolytes are electrolytes asymmetric both in charge and size. Their properties in solution are dominated by Coulombic forces, and without a detailed understanding of these interactions, no interpretation of experimental data is possible. This paper is a review of recent developments in the theory of highly asymmetric electrolytes of spherical shape resembling surfactant micelles. Three different models are discussed: (a) the cell model, which is focused on the small ion-macroion interaction; (b) the model that treats the solution as an effective one-component fluid of macroions; and (c) the isotropic model, where the solution is represented as a mixture of charged spheres. Traditionally, the electrostatic interactions are accounted for via the solution of the Poisson-Boltzmann equation. This theory, however, ignores the fluctuations around the most probable distribution and may yield poor results for systems with multivalent ions. This paper focuses on developments beyond the Poisson-Boltzmann theory; the results of computer simulations and integral equation theories represent the major part of the review.

Journal Article↗
PubMed1996

The relationship between portal venous and hepatic arterial blood flow. I. Experimental liver transplantation.

The relationship between the changes in portal venous and hepatic arterial blood flows, in the liver is a much disputed question, it has tremendous significance in the practice of transplantation, and an explanation has been available since 1981, when Lautt published the so-called "adenosine washout theory". According to our earlier observations the decrease of portal pressure or flow consistently led to an increase in hepatic artery flow. At the same time changes in hepatic artery flow or pressure seemed to produce only inconsistent effects on the portal circulation. In the present experiments liver transplantation (OLTX) was carried out on mongrel dogs by Starzl's method. Electromagnetic flow probes were placed on the hepatic artery and the portal vein before removal of recipient's liver, and after completion of all vascular anastomoses to the newly inserted liver, during the recirculatory phase of OLTX. The flow probes were connected to a Hellige electromagnetic flowmeter, portal venous and systemic arterial pressures were also recorded. The control HAF was 241 +/- 23 ml/min, the average PVF was 517 +/- 47 ml/min before removal of the recipient's liver. In the recirculatory phase of HAF increased, by 71 +/- 12% (p < 0.001). The PVF decreased in most animals after OLTX. The decrease was in average -40.2 +/- 3.5% (p < 0.001). The THBF calculated by adding the HAF and PVF showed a small, but not significant decrease recirculation. The systemic arterial pressure decreased slightly and portal vein pressure rose in most animals after OLTX. There was a substantial increase in portal inflow resistance and prehepatic arteriolar resistance and a decrease in hepatic artery resistance. The decrease of PVF after OLTX can be explained by progressive fluid accumulation in the liver parenchyma and increased sinusoidal and portal inflow resistance. The prolonged and continuous increase in hepatic artery flow during the recirculatory phase of OLTX may be due to the decrease of portal flow. The exact mechanism, by which a change in portal flow leads to arteriolar dilatation, can be most probably explained by the "adenosine washout theory" of Lautt.

Animals↗
PubMed1999

Scar and antiscar quantum effects in open chaotic systems.

We predict and numerically observe strong periodic orbit effects in the properties of weakly open quantum systems with a chaotic classical limit. Antiscars lead to a large number of exponentially narrow isolated resonances when the single-channel (or tunneling) opening is located on a short unstable orbit of the closed system; the probability to remain in the system at long times is thus exponentially enhanced over the random matrix theory prediction. The distribution of resonance widths and the probability to remain are quantitatively given in terms of only the stability matrix of the orbit on which the opening is placed. The long-time remaining probability density is nontrivially distributed over the available phase space; it can be enhanced or suppressed near orbits other than the one on which the lead is located, depending on the periods and classical actions of these other orbits. These effects of the short periodic orbits on quantum decay rates have no classical counterpart, and first appear on times scales much larger than the Heisenberg time of the system. All the predictions are quantitatively compared with numerical data.

Journal Article↗
PubMed2000

Testing a biosynthetic theory of the genetic code: fact or artifact?

It has long been conjectured that the canonical genetic code evolved from a simpler primordial form that encoded fewer amino acids [e.g., Crick, F. H. C. (1968) J. Mol. Biol. 38, 367-379]. The most influential form of this idea, "code coevolution" [Wong, J. T.-F. (1975) Proc. Natl. Acad. Sci. USA 72, 1909-1912], proposes that the genetic code coevolved with the invention of biosynthetic pathways for new amino acids. It further proposes that a comparison of modern codon assignments with the conserved metabolic pathways of amino acid biosynthesis can inform us about this history of code expansion. Here we re-examine the biochemical basis of this theory to test the validity of its statistical support. We show that the theory's definition of "precursor-product" amino acid pairs is unjustified biochemically because it requires the energetically unfavorable reversal of steps in extant metabolic pathways to achieve desired relationships. In addition, the theory neglects important biochemical constraints when calculating the probability that chance could assign precursor-product amino acids to contiguous codons. A conservative correction for these errors reveals a surprisingly high 23% probability that apparent patterns within the code are caused purely by chance. Finally, even this figure rests on post hoc assumptions about primordial codon assignments, without which the probability rises to 62% that chance alone could explain the precursor-product pairings found within the code. Thus we conclude that coevolution theory cannot adequately explain the structure of the genetic code.

Codon↗
PubMed2001

On Setting Response Criteria for Calibrated Subjective Probability Estimates.

Framing the issue of subjective probability calibration in signal-detection-theory terms, this paper first proves a theorem regarding the placement of well-calibrated response criteria and then develops an algorithm guaranteed to find such criteria, should they exist. Application of this algorithm to tasks varying in difficulty and number of response categories shows that perfect calibration is easiest to attain under median difficulty levels (d' approximately 1.4) and is practically or theoretically impossible to attain when the task is either very hard (d' approximately 0.5) or very easy (d' approximately 10). Implications for calibration research, including the hard-easy effect, are discussed. Copyright 2001 Academic Press.

Journal Article↗