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PubMed2002

Packet theory of conditioning and timing.

Packet theory is based on the assumption that the momentary probability of producing a bout or packet of responding is controlled by the conditional expected time function. Bouts of head entry responses of rats into a food cup appear to have the same characteristics across a range of conditions. The conditional expected time function is the mean expected time remaining until the next food delivery as a function of time since an event such as food or stimulus onset. The conditional expected time function encodes mean interval duration as well as the distribution form so that both the mean response rate and form of responding in time can be predicted. Simulations of Packet theory produced accurate quantitative predictions of: (1) the effect of reinforcement density (mean food-food interval) and distribution form on responding; (2) scalar variance in fixed interval responding; (3) CS-US and intertrial interval effects on the strength of conditioning; and (4) the effect of the ratio of cycle:trial time on the strength of conditioning.

Journal Article↗
PubMed2005

Cultural theories of nursing responsive to human needs and values.

PURPOSE: To present an alternative to the recent proposal that the extant theories of nursing be replaced with culture-specific theories of nursing in order to have theories of nursing that are culturally responsive. METHOD: A philosophical analysis of the implications for nursing practice of adopting the proposal serves as the basis for recommending a different philosophically based theoretical solution. Anticipated probable objections to that recommendation are considered. CONCLUSION: Culture-specific theories of nursing might allow nurses to be culturally sensitive. However, because nurses must be humanly responsive to the needs of people as human beings and not just as cultural beings, cultural theories of nursing should be developed within the precepts of a transcultural theory of nursing grounded in a conception of objective human needs and values.

Culture↗
PubMed2006

A semi-stochastic model for Salmonella infection in a multi-group herd.

A multi-group semi-stochastic model is formulated to identify possible causes of why different strains of Salmonella develop so much variation in their infection dynamics in UK dairy herds. The model includes demography (managed populations) and various types of transmission: direct, pseudovertical and indirect (via free-living infectious units in the environment). The effects of herd size and epidemiological parameters on mean prevalence of infection and mean time until fade out are investigated. Numerical simulation shows that higher pathogen-induced mortality, shorter infectious period, more persistent immune response and more rapid removal of faeces result in a lower mean prevalence of infection, a shorter mean time until fade out, and a greater probability of fade out of infection within 600 days. Combining these results and those for the deterministic counterpart could explain differences in observed epidemiological patterns and help to identify the factors inducing the decline in reported cases of epidemic strains such as DT104 in cattle. We further investigate the effect of group structure on the probability of a major outbreak by using the stochastic threshold theory in homogeneous populations and that in heterogeneous populations. Numerical studies suggest that group structure makes major outbreaks less likely than would be the case in a homogeneous population with the same basic reproduction number. Moreover, some control strategies are suggested by investigating the effect of epidemiological parameters on the probability of an epidemic.

Animals↗
PubMed2006

The comparator theory fails to account for the selective role of within-compound associations in cue-selection effects.

In a human causal learning experiment, we investigated cue selection effects to test the comparator theory (Denniston, Savastano, & Miller, 2001; Miller & Matzel, 1988). The theory predicts that the occurrence of cue selection is independent of whether the relevant learning trials are presented in a standard forward manner or in a backward manner and that within-compound associations are of equal importance in both cases. We found that the strength of the cue-selection effect was positively correlated with knowledge of within-compound associations in the backward condition but not in the forward condition. Furthermore, cue-selection effects were less pronounced in the former than in the latter condition. These results are at variance with the comparator hypothesis but are in agreement with a modified associative theory and with the suggestion that retrospective revaluation might be due to rehearsal processes.

Adult↗
PubMed2005

Prospect theory on the brain? Toward a cognitive neuroscience of decision under risk.

Most decisions must be made without advance knowledge of their consequences. Economists and psychologists have devoted much attention to modeling decisions made under conditions of risk in which options can be characterized by a known probability distribution over possible outcomes. The descriptive shortcomings of classical economic models motivated the development of prospect theory (D. Kahneman, A. Tversky, Prospect theory: An analysis of decision under risk. Econometrica, 4 (1979) 263-291; A. Tversky, D. Kahneman, Advances in prospect theory: Cumulative representation of uncertainty. Journal of Risk and Uncertainty, 5 (4) (1992) 297-323) the most successful behavioral model of decision under risk. In the prospect theory, subjective value is modeled by a value function that is concave for gains, convex for losses, and steeper for losses than for gains; the impact of probabilities are characterized by a weighting function that overweights low probabilities and underweights moderate to high probabilities. We outline the possible neural bases of the components of prospect theory, surveying evidence from human imaging, lesion, and neuropharmacology studies as well as animal neurophysiology studies. These results provide preliminary suggestions concerning the neural bases of prospect theory that include a broad set of brain regions and neuromodulatory systems. These data suggest that focused studies of decision making in the context of quantitative models may provide substantial leverage towards a fuller understanding of the cognitive neuroscience of decision making.

Animals↗
PubMed2002

Double photoionization and transfer ionization of he: shakeoff theory revisited.

The shakeoff theory of Aberg [Phys. Rev. A 2, 1726 (1970)] is revisited. With the sudden approximation, we calculate the shakeoff probability when one of the electrons in He is ejected with a finite velocity. This theory is used to examine ratios of cross sections for double to single photoionization and transfer ionization to single electron capture. It is also shown that the momentum distribution of the shakeoff electron provides a means to measure the correlation of the ground state wave function directly.

Journal Article↗
PubMed2005

A theory of attending and reinforcement in conditional discriminations.

A model of conditional discrimination performance (Davison & Nevin, 1999) is combined with the notion that unmeasured attending to the sample and comparison stimuli, in the steady state and during disruption, depends on reinforcement in the same way as predicted for overt free-operant responding by behavioral momentum theory (Nevin & Grace, 2000). The rate of observing behavior, a measurable accompaniment of attending, is well described by an equation for steady-state responding derived from momentum theory, and the resistance to change of observing conforms to predictions of momentum theory, supporting a key assumption of the model. When probabilities of attending are less than 1.0, the model accounts for some aspects of conditional-discrimination performance that posed problems for the Davison-Nevin model: (a) the effects of differential reinforcement on the allocation of responses to the comparison stimuli and on accuracy in several matching-to-sample and signal-detection tasks where the differences between the stimuli or responses were varied across conditions, (b) the effects of overall reinforcer rate on the asymptotic level and resistance to change of both response rate and accuracy of matching to sample in multiple schedules, and (c) the effects of fixed-ratio reinforcement on accuracy. Some tests and extensions of the model are suggested, and the role of unmeasured events in behavior theory is considered.

Attention↗
PubMed1999

Variation Thresholds for Extinction and Their Implications for Conservation Strategies.

We examine the degree to which fitting simple dynamic models to time series of population counts can predict extinction probabilities. This is both an active branch of ecological theory and an important practical topic for resource managers. We introduce an approach that is complementary to recently developed techniques for estimating extinction risks (e.g., diffusion approximations) and, like them, requires only count data rather than the detailed ecological information available for traditional population viability analyses. Assuming process error, we use four different models of population growth to generate snapshots of population dynamics via time series of the lengths commonly available to ecologists. We then ask to what extent we can identify which of several broad classes of population dynamics is evident in the time series snapshot. Along the way, we introduce the idea of "variation thresholds," which are the maximum amount of process error that a population may withstand and still have a specified probability of surviving for a given length of time. We then show how these thresholds may be useful to both ecologists and resource managers, particularly when dealing with large numbers of poorly understood species, a common problem faced by those designing biodiversity reserves.

extinction risk↗
PubMed1995

Exact tests for association between alleles at arbitrary numbers of loci.

Associations between allelic frequencies, within and between loci, can be tested for with an exact test. The probability of the set of multi-locus genotypes in a sample, conditional on the allelic counts, is calculated from multinomial theory under the hypothesis of no association. Alleles are then permuted and the conditional probability calculated for the permuted genotypic array. The proportion of arrays no more probable than the original sample provides the significance level for the test. An algorithm is provided for counting genotypes efficiently in the arrays, and the powers of the test presented for various kinds of association. The powers for the case when associations are generated by admixture of several populations suggest that exact tests are capable of detecting levels of association that would affect forensic calculations to a significant extent.

Algorithms↗
PubMed2005

On the stability of IrCl6(3-) and other triply charged anions: solvent stabilization versus ionic fragmentation and electron detachment for the IrCl6(3-).(H2O)n n = 0-10 microsolvated clusters.

The intrinsic gas-phase stability of the IrCl(6)(3-) trianion and its microsolvated clusters, IrCl(6)(3-).(H(2)O)(n) n = 1-10, have been investigated using density functional theory (DFT) calculations. Although IrCl(6)(3-) is known to exist as a stable complex ion in bulk solutions, our calculations indicate that the bare trianion is metastable with respect to decay via both electron detachment and ionic fragmentation. To estimate the lifetime of IrCl(6)(3-), we have computed the electron tunneling probability using an adaption of the Wentzel-Kramer-Brillouin theory and predict that the trianion will decay spontaneously via electron tunneling on a time scale of 2.4 x 10(-13) s. The global minimum structure for IrCl(6)(3-).H(2)O was found to contain a bifurcated hydrogen bond, whereas for IrCl(6)(3-).(H(2)O)(2), two low energy minima were identified; one involving two bifurcated water-ion hydrogen bonds and a second combining a bifurcated hydrogen bond with a water-water hydrogen bond. Clusters based on each of these structural motifs were obtained for all of the n = 3-10 systems, and the effect of solvation on the possible decay pathways was explored. The calculations reveal that solvation stabilizes IrCl(6)(3-) with respect to both electron detachment decay and ionic fragmentation, with the magnitude of the repulsive Coulomb barrier for ionic fragmentation increasing smoothly with sequential solvation. This study is the first to compare the propensity for electron detachment versus ionic fragmentation decay for a sequentially solvated triply charged anion.

Journal Article↗
PubMed2007

Asymptotic theory for the Cox semi-Markov illness-death model.

Irreversible illness-death models are used to model disease processes and in cancer studies to model disease recovery. In most applications, a Markov model is assumed for the multistate model. When there are covariates, a Cox (1972, J Roy Stat Soc Ser B 34:187-220) model is used to model the effect of covariates on each transition intensity. Andersen et al. (2000, Stat Med 19:587-599) proposed a Cox semi-Markov model for this problem. In this paper, we study the large sample theory for that model and provide the asymptotic variances of various probabilities of interest. A Monte Carlo study is conducted to investigate the robustness and efficiency of Markov/Semi-Markov estimators. A real data example from the PROVA (1991, Hepatology 14:1016-1024) trial is used to illustrate the theory.

Biometry↗
PubMed2000

Probability summation for multiple patches of luminance modulation.

When components of a compound pattern stimulate different visual mechanisms, psychophysical performance typically improves by a small amount consistent with probability summation amongst independent detectors. Here we extend previous summation experiments by (i) plotting full psychometric functions; and (ii) using compound stimuli with components that varied in up to three stimulus dimensions: spatial frequency (1, 4, 5 or 11 c/deg), orientation (0 degrees, +/-45 degrees ), and position. Stimulus components were isolated circular sine-phase patches of grating centred on up to four corners of an imaginary square surrounding a fixation-point. Combinations of component patches produced compound stimuli made from up to 16 components that differed in various combinations of the three stimulus dimensions. Other than when the spatial frequency was 11 c/deg, results were well described using: (i) probabilistic summation of individual psychometric functions; (ii) the Quick pooling formula; and (iii) the signal detection analysis for 2IFC developed by Tyler and Chen (2000) [Signal detection theory in the 2AFC paradigm: attention, channel uncertainty and probability summation (under review)]. We conclude that in general, nonlinear spatial summation is consistent with probabilistic summation across independent detecting mechanisms that vary in spatial frequency (a range of at least 1-5 c/deg), orientation (a range of 90 degrees ) and position (a range of at least 24 cycles at 4 c/deg). In further experiments, results were found to be consistent with probability summation for pairs of orthogonally oriented step-edge stimuli and a matrix of randomly oriented 11 c/deg sine-wave patches. This casts doubt on the generality of a recent suggestion that local interactions between colinearly oriented detectors within a spatial neighbourhood of around four cycles may contribute to nonlinear spatial summation [Bonneh & Sagi, 1998; Vision Research, 38, 3541-3553].

Contrast Sensitivity↗
PubMed1973

Propagation speed in myelinated nerve. II. Theoretical dependence on external Na and on temperature.

The Hodgkin-Huxley (H.H.) equations modified by Dodge for Rana pipiens myelinated nerve have been solved to determine how well the theory predicts the effects of changes of temperature and [Na(+)](0) on propagation. Conduction speed theta was found to have an approximately exponential dependence on temperature as was found experimentally, but the theoretical temperature coefficient (Q(10)) was low; 1.5 compared with the experimental finding of 2.95. theta was found to be a linear function of log ([Na(+)](0)) in contrast to the experimental finding of a square root dependence on [Na(+)](0). theta is 50% greater at one-fourth normal [Na(+)](0) than the theory predicts. The difference between the theoretical theta([Na(+)](0)) and the experimental theta([Na(+)](0)) is probably due to an imprecisely known variation of parameters and not to a fundamental inadequacy of the theory.

Action Potentials↗
PubMed2006

A critical test of the failure-to-engage theory of task switching.

When people switch between two tasks, their performance on each is worse than when they perform that task in isolation. One theory of this "switch cost" is the failure-to-engage (FTE) theory, which posits that observed responses are a simple mixture of prepared and unprepared response strategies. The probability that participants use prepared processes can be manipulated experimentally (e.g., by changing preparation time). The FTE theory is a binary mixture model and therefore makes a strong prediction about the existence of fixed points in response time distributions. We found evidence contradicting this prediction, using data from 20 participants in a standard task-switching paradigm. In this article, we examine reasons for the failure of the FTE theory, and we demonstrate that a generalized version of FTE theory accommodates our data.

Adolescent↗
PubMed1984

Quantitative theories for clinical decision making.

Clinical work involves a great many decisions in situations characterized by uncertainty and by the need for value judgments. Formal, mostly quantitative approaches to medical decision making have been developed over the last thirty years. The best known theories are discriminant analysis or supervised pattern recognition, decision analysis and, more recently, artificial intelligence. Discriminant analysis is used for estimating diagnostic probabilities on the basis of diagnostic data. Outcome, action, state, probability, utility, and choice are the main concepts of decision analysis. Problems of diagnosis and therapy choice can be structured within the framework of decision analysis. The quality of a decision analysis largely depends on a good structuring of the problem, and on the quality of the probability and utility assessments made during the analysis. Decision analysis is basically a prescriptive theory for decision making in individual patient management. Other areas of application include basic and advanced teaching, formulation, explication and analysis of clinical problems, detection of gaps in clinical knowledge, development of clinical protocols, etc.

Decision Making↗