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PubMed1984

Spectral analysis of the surface electromyogram as a tool for studying rate modulation: a comparison between theory, simulation, and experiment.

Theoretical work suggests that if the interpulse intervals ( IPIs ) of motor unit action potential trains ( MUAPTs ) are independently and normally distributed, then spectral analysis of the electromyogram could be a useful tool for studying rate modulation by virtue of the presence of a peak in the power spectrum at the average firing frequency of all active motor units. It is shown in this paper that IPIs need not be normally distributed, specifically that the results are very much the same if the IPIs are distributed according to a Gamma probability density function ( PDF ). Simulation of the electromyogram based on this theory proved the applicability of the method. Experimental results obtained for the masseter, biceps brachii and first dorsal interosseus (FDI) muscles, however, were in disagreement with both theory and simulation except for the biceps muscle at force levels up to 20% of the maximal force and for the masseter and FDI muscles in 1 out of 5 subjects. This indicates that the models for MUAPTs hitherto used might not be generally correct. Apart from this discrepancy, our results reveal differences between masseter and FDI muscles on the one hand and the biceps brachii on the other, which indicate that motor unit synchronisation is much more pronounced in the latter muscle.

Action Potentials↗
PubMed2001

Money, kisses, and electric shocks: on the affective psychology of risk.

Prospect theory's S-shaped weighting function is often said to reflect the psychophysics of chance. We propose an affective rather than psychophysical deconstruction of the weighting function resting on two assumptions. First, preferences depend on the affective reactions associated with potential outcomes of a risky choice. Second, even with monetary values controlled, some outcomes are relatively affect-rich and others relatively affect-poor. Although the psychophysical and affective approaches are complementary, the affective approach has one novel implication: Weighting functions will be more S-shaped for lotteries involving affect-rich than affect-poor outcomes. That is, people will be more sensitive to departures from impossibility and certainty but less sensitive to intermediate probability variations for affect-rich outcomes. We corroborated this prediction by observing probability-outcome interactions: An affect-poor prize was preferred over an affect-rich prize under certainty, but the direction of preference reversed under low probability. We suggest that the assumption of probability-outcome independence, adopted by both expected-utility and prospect theory, may hold across outcomes of different monetary values, but not different affective values.

Adult↗
PubMed1985

A computer simulation of medical decision strategy performance.

This study demonstrates a computer model which can be used to compare the effects of errors in probability and utility estimation on the performance of Bayesian and alternative medical decision strategies. The model task requires choosing one of three treatments for a patient with one of three diseases based on the patient's state with respect to five binary cues and estimates of the prior probabilities of disease, the conditional probabilities of the cues and the utilities of the treatments. A classic decision analytic strategy uses Bayes' formula to calculate posterior probabilities of disease and chooses treatments based on maximization of expected value. Alternative strategies use likelihood ratios to calculate disease scores for each patient state and choose the treatment with highest payoff for the disease with the highest score. Two strategies with different cutoffs for the ratios are compared with a random strategy and a classic decision analytic strategy. The simulation results show that the payoffs for all strategies except the random strategy decline with increasing estimation error. The decision analytic strategy has the highest mean payoff at all levels of error. The differences between this optical strategy and the alternatives, however, decrease as estimation error increases, and the frequency with which the strategies based on simple diagnostic scoring rules outperform the formal Bayesian strategy increases.

Bayes Theorem↗
PubMed1994

The role of probability of reinforcement in models of choice.

A general account of choice behavior in animals, the cumulative effects model, has been proposed by Davis, Staddon, Machado, and Palmer (1993). Its basic assumptions are that choice occurs in an all-or-none fashion for the response alternative with the highest probability of reinforcement and that the probability of reinforcement for each response alternative is calculated from the entire history of training (total number of reinforced responses/total number of reinforced and nonreinforced responses). The model's reliance on probability of reinforcement as the fundamental variable controlling choice behavior subjects the cumulative effects model to the same criticisms as have been directed toward other related models of choice, notably melioration theory. Several different data sets show that the relative value of a response alternative is not predicted by the obtained probability of reinforcement associated with that alternative. Alternative approaches to choice theory are considered.

Animals↗
PubMed1986

A perspective on the role of decision analysis in clinical practice.

The upsurge in interest in clinical decision support techniques is not accidental. Clinical decision situations are more and more characterized by complexity and uncertainty, due to increasing diagnostic and therapeutic possibilities, the fast growth of medical knowledge, the increasing quality demands by the public, and the pressure towards cost-conscious decision making. On the other hand, information science is increasingly able to provide the tools for building decision-support systems. There are several approaches to the development of decision aids. The present paper is mainly concerned with one potentially very powerful method: decision analysis. Basically, decision analysis is a method for explicitating, structuring and analysing clinical decision situations for individual patients. Suggestions for therapy choice, and diagnostic and prognostic statements are obtained by integrating the probability and value judgments made during the analysis. A clinical decision analysis can conveniently be divided into four stages: defining and structuring the clinical problem, assessing the relevant probabilities and utilities, calculation of the preferred course of action, including sensitivity analysis, and formulation of the results of the analysis in a clinically useful way. An optimistic scenario of the future role of decision analysis in clinical medicine would describe--among others--decision-consultation departments, decision-oriented clinical data-bases and medical knowledge bases, research groups for development, testing, and updating of clinical strategies, decision-analysis courses in all levels of medical education, and decision-oriented clinical textbooks. Moreover, decision analysis will help in setting priorities in clinical research, in quality control and peer review, and in development of protocols for use by the paramedical personal. Some illustrations from applied decision analysis are given.

Decision Making, Computer-Assisted↗
PubMed1994

Stochastic population forecasts for the United States: beyond high, medium, and low.

"This article presents and implements a new method for making stochastic population forecasts that provide consistent probability intervals. We blend mathematical demography and statistical time series methods to estimate stochastic models of fertility and mortality based on U.S. data back to 1900 and then use the theory of random-matrix products to forecast various demographic measures and their associated probability intervals to the year 2065. Our expected total population sizes agree quite closely with the Census medium projections, and our 95 percent probability intervals are close to the Census high and low scenarios. But Census intervals in 2065 for ages 65+ are nearly three times as broad as ours, and for 85+ are nearly twice as broad. In contrast, our intervals for the total dependency and youth dependency ratios are more than twice as broad as theirs, and our ratio for the elderly dependency ratio is 12 times as great as theirs. These items have major implications for policy, and these contrasting indications of uncertainty clearly show the limitations of the conventional scenario-based methods."

Age Factors↗
PubMed1998

Approximately optimal designs for phase II clinical studies.

There is no consensus on determination of sample size in phase II clinical trials. The use of Bayesian decision theory has been proposed by Stallard (1), among others. In this article, optimal three-stage designs are obtained using decision theory. These are compared with procedures proposed by Schoenfeld (2), Ensign et al. (3), and Chen et al. (4) and the sequential probability ratio test of Wald (5) and Barnard (6). The three-stage procedures are shown to be close to the true optimal test; the sequential probability ratio test is easier to obtain and only marginally inferior. Because optimality of the decision-theory approach depends on accurate specification of costs and profits, an assessment is made of the sensitivity of the proposed procedures to a range of gain function parameter values.

Bayes Theorem↗
PubMed1997

Missed targets are more frequent than false alarms: a model for error rates in visual search.

In many visual search tasks, reaction times (RTs) for target detection are measured as a function of display size. The corresponding error rates are usually low but increase with increasing display size. Missed-target errors are more common than false alarms. In recent models of visual search, the error rates were attributed to a premature search termination and error rates increasing with display size were interpreted as indicating a speed-accuracy trade-off and an underestimation of search times per item (obtained from RT slopes). A model is described in which errors occur as a result of imperfect rather than incomplete search (i.e., it is assumed that there are task-specific probabilities of categorizing a target or a distractor incorrectly). Signal-detection theory is used to show that the observed error rate properties can be attributed to an optimized decision strategy. "Corrections" of RT data are thus questionable.

Humans↗
PubMed2004

Cue interaction and judgments of causality: contributions of causal and associative processes.

In four experiments, the predictions made by causal model theory and the Rescorla-Wagner model were tested by using a cue interaction paradigm that measures the relative response to a given event based on the influence or salience of an alternative event. Experiments 1 and 2 uncorrelated two variables that have typically been confounded in the literature (causal order and the number of cues and outcomes) and demonstrated that overall contingency judgments are influenced by the causal structure of the events. Experiment 3 showed that trial-by-trial prediction responses, a second measure of causal assessment, were not influenced by the causal structure of the described events. Experiment 4 revealed that participants became less sensitive to the influence of the causal structure in both their ratings and their predictions as trials progressed. Thus, two experiments provided evidence for high-level (causal reasoning) processes, and two experiments provided evidence for low-level (associative) processes. We argue that both factors influence causal assessment, depending on what is being asked about the events and participants' experience with those events.

Association Learning↗
PubMed2000

Who's afraid of Thomas Bayes?

Sometimes direct evidence is so strong that a prescription for practice is decreed. Usually, things are not that simple-leaving aside the possibility that important trade offs may be involved, direct comparative data may be imprecise (especially in crucial sub-groups) or subject to possible bias, or there may be no direct comparative evidence; but still decisions have to be made. In these circumstances, indirect evidence-the plausibility of effects-enters the frame. But how should we describe the extent of plausibility and, having done so, how can this be integrated with any direct evidence that might exist. Also, how can allowance be made in a transparent (that is, explicit) way for perceptions of the size of bias in the direct evidence. Enter the Reverend Thomas Bayes; plausibility (however derived-laboratory experiment, qualitative study or just "experience") is captured numerically as degrees of belief ("prior" to the direct data) and updated (by the direct evidence) to yield "posterior" probabilities for use in decision making. The mathematical model used for this purpose must explicitly take account of assumptions about bias in the direct data. This paradigm bridges theory and practice, and provides the intellectual scaffold for those who recognise that (numerically definable) probabilities, and values (also numerically definable) underlie decisions, but who also realise that subjectivity is ineluctable in science.

Bayes Theorem↗
PubMed1972

Detection of increments in noise intensity by monkeys.

Monkeys were trained to detect 100-msec increments in the intensity of continuous white noise. A response on one of two bars was reinforced with some probability if it conformed to the presence or absence of the increment on that trial. Stimulus parameters of background intensity, increment size, and probability of increment presentation were varied, and response probabilities and latencies were recorded. The task was analogous to the "yes-no" task used in human psychophysics. Data analysis within the context of signal-detection theory revealed response biasing toward one bar or the other to be related to the probability of increment presentation, whereas sensitivity depended on the combination of increment size and background noise intensity. Weber's law was found to hold for a large range of background intensities in that the sensitivity to relative intensity increments varied little. Performance was compared to that of an ideal observer that uses samples of the envelope of the noise waveform on which to base its responses.

Acoustic Stimulation↗
PubMed1998

Possible mechanisms of anosognosia: a defect in self-awareness.

Anosognosia of hemiplegia is of interest for both pragmatic and theoretical reasons. We discuss several neuropsychological theories that have been proposed to explain this deficit. Although for psychological reasons people might deny deficits, the denial hypothesis cannot account for the hemispheric asymmetries associated with this disorder and cannot explain why some patients might deny one deficit and recognize another equally disabling deficit. There is some evidence that faulty feedback from sensory deficits, spatial neglect and asomatognosia might be responsible for anosognosia in some patients. However, these feedback hypotheses cannot account for anosognosia in all patients. Although the hemispheric disconnection hypothesis is appealing, disconnection is probably only a rare cause of this disorder. The feedforward intentional theory of anosognosia suggests that the discovery of weakness is dependent on attempted action and some patients might have anosognosia because they do not attempt to move. We present evidence that supports this theory. The presence of one mechanism of anosognosia, however, does not preclude the possibility that other mechanisms might also be working to produce this disorder. Although a large population study needs to be performed, we suspect that anosognosia might be caused by several of the mechanisms that we have discussed. On the basis of the studies of impaired corporeal self-awareness that we have reviewed, we can infer that normal self-awareness is dependent on several parallel processes. One must have sensory feedback and the ability to attend to both one's body and the space where parts of the body may be positioned or acting. One must develop a representation of the body, and this representation must be continuously modified by expectations (feedforward) and knowledge of results (feedback).

Awareness↗
PubMed2005

Evolution of the theory of circulation.

The first known descriptions regarding the basic aspects of circulation was probably in 500 B.C., by the Greek thinker Alcmaeon of Croton who observed arteries and veins to be dissimilar in animal dissection, and this was followed by the description of the human heart as a three chambered organ by Aristotle in 350 B.C. Herophilus of Chalcedon, a Greek anatomist, confirmed the findings of dissimilarity between arteries and veins in human cadaver dissections in 300 B.C., and determines that arteries were thicker than veins and contained blood. The advancement by far in the learning of human circulation was made possible first by significant contribution of Galen followed by observations of Ibn al-Nafis, Servetus, Colombo, Cesalpino, Vesalius and Fabricius. In 17th century William Harvey, an English physician, made important advancements into the understanding of this important area of medicine, advancements that continued with the observations of Malpighi.

Blood Circulation↗
PubMed2006

Fluorescence and metastability of N2O2+: theory and experiment.

State-selective mass spectrometry has revealed one conclusive and another probable metastable state of the N2O2+ dication, assigned respectively as 1 3Pi at 38.5 eV and 2 3Pi at 42.5 eV. Photon coincidence experiments confirm that dissociation of 1 3Pi is preceded by a fluorescent transition to X 3Sigma- and also indicate that an identical mechanism occurs for 2 3Pi. Highly correlated MRCI calculations are performed at a range of N2O2+ geometries, from which both N-N and N-O bond stretching curves are generated. Substantial barriers along both coordinates are observed for 1 3Pi and 2 3Pi, although the increasing density of states at higher energy may allow spin-orbit or vibronic predissociation for 2 3Pi. Fragment emissions derived from N2O+ and N2O2+ are analyzed with the aid of glass filters, from which NO (X 2Pi<--A 2Sigma+) and vibrationally excited N2+ (X 2Sigmag+<--B 2Sigmau+) transitions are deduced.

Journal Article↗
PubMed2004

B chromosomes and Robertsonian fusions of Dichroplus pratensis (Acrididae): intraspecific support for the centromeric drive theory.

We tested the centromeric drive theory of karyotypic evolution in the grasshopper Dichroplus pratensis, which is simultaneously polymorphic for eight Robertsonian fusions and two classes of B chromosomes. A logistic regression analysis performed on 53 natural populations from Argentina revealed that B chromosomes are more probably found in populations with a higher proportion of acrocentric chromosomes, as the theory predicts. Furthermore, frequencies of B-carrying individuals are significantly negatively correlated with the mean frequency of different Robertsonian fusions per individual. No significant correlations between presence/absence or frequency of Bs, and latitude or altitude of the sampled populations, were found. We thus provide the first intraspecific evidence supporting the centromeric drive theory in relation to the establishment of B chromosomes in natural populations.

Animals↗
PubMed2004

Cranklike conformational transitions in polyethylene.

Molecular dynamics simulations of a variety of polymeric systems provide the evidence for two different kinds of conformational transitions: independent single bond transitions and cranklike transitions (or correlated bond transitions). While single bond transitions can be rationalized according to standard theories of activated processes controlled by the saddle point crossing, a more complex description is required for the other type of transitions. In a recent work devoted to the analysis of the simplified chain model with three rotors [B. Nigro and G. J. Moro, J. Phys. Chem. B 106, 7365 (2002)], a theory has been proposed for cranklike transitions represented as a kinetic process between equilibrium states differing by two torsional angles (i.e., two bond transitions). Moreover their rate coefficients were estimated on the basis of a local expansion about the bifurcation of the separatrices departing from the potential function maximum. In the present work the same theory is applied to a model for long alkyl chains in solution, in order to rationalize the behavior of cranklike transitions in polyethylene and to recognize the molecular features controlling them. We obtain probabilities of occurrence of cranklike transitions in substantial agreement with simulation results. Furthermore, the theory is capable of explaining the dependence of the rate on the separation between the two reactive bonds, as well as the dependence on the conformational state of the starting configuration. In particular, selection rules for next-to-nearest neighbor transitions are recovered from the shape of the separatrices and the location of the corresponding bifurcations.

Journal Article↗
PubMed2006

[Influence of erbium ion concentration on Judd-Ofelt parameters of Er3+ -doped tellurite glass].

Er3+ -doped tellurite glasses with four different concentrations were fabricated, and the oscillator strength of Er3+ in the tellurite glasses were calculated through the absorption spectra of the glasses. The Judd-Ofelt intensity parameter omega i, spontaneous transition probability A, fluorescence branching ratio beta, and radiative lifetime tau rad of Er3+ were calculated on the basis of Judd-Ofelt theory, and the effect of the erbium ion concentration on the above optical parameters was also discussed. The fluorescence spectra of Er3+: (4)I(13/2)--> (4)I(15/2) transition and the lifetime of Er3+: (4)I(13/2) level of the samples were measured. The stimulated emission cross-section of (4)I(13/2)--> (4)I(15/2) transition of the samples was finally calculated by using McCumber theory. The results show that with the increase in the Er3+ concentration, the oscillator strength and spontaneous transition probability A of Er3+ increase, while the fluorescence branching ratio beta of Er3+ shows little difference. The stimulated emission cross-section of Er3+: (4)I(13/2)--> (4)I(15/2) transition of the samples changes slightly with the increase in the Er3+ concentration. All the fluorescence effective line widths for the four different Er3+ concentration samples are nearly 50 nm.

English Abstract↗