University of Potsdam

About: University of Potsdam is a education organization based out in Potsdam, Germany. It is known for research contribution in the topics: Population & Computer science. The organization has 9629 authors who have published 26740 publications receiving 759745 citations. The organization is also known as: Universität Potsdam.

Model-based and model-free decisions in alcohol dependence

Abstract: Background: Human and animal work suggests a shift from goal-directed to habitual decision-making in addiction However, the evidence for this in human alcohol dependence is as yet inconclusive Methods: Twenty-six healthy controls and 26 recently detoxified alcohol-dependent patients underwent behavioral testing with a 2-step task designed to disentangle goal-directed and habitual response patterns Results: Alcohol-dependent patients showed less evidence of goal-directed choices than healthy controls, particularly after losses There was no difference in the strength of the habitual component The group differences did not survive controlling for performance on the Digit Symbol Substitution Task Conclusion: Chronic alcohol use appears to selectively impair goal-directed function, rather than promoting habitual responding It appears to do so particularly after nonrewards, and this may be mediated by the effects of alcohol on more general cognitive functions subserved by the prefrontal cortex

First passages in bounded domains: when is the mean first passage time meaningful?

Abstract: We study the first passage statistics to adsorbing boundaries of a Brownian motion in bounded two-dimensional domains of different shapes and configurations of the adsorbing and reflecting boundaries. From extensive numerical analysis we obtain the probability $P(\ensuremath{\omega})$ distribution of the random variable $\ensuremath{\omega}={\ensuremath{\tau}}_{1}/({\ensuremath{\tau}}_{1}+{\ensuremath{\tau}}_{2})$, which is a measure for how similar the first passage times ${\ensuremath{\tau}}_{1}$ and ${\ensuremath{\tau}}_{2}$ are of two independent realizations of a Brownian walk starting at the same location. We construct a chart for each domain, determining whether $P(\ensuremath{\omega})$ represents a unimodal, bell-shaped form, or a bimodal, M-shaped behavior. While in the former case the mean first passage time (MFPT) is a valid characteristic of the first passage behavior, in the latter case it is an insufficient measure for the process. Strikingly we find a distinct turnover between the two modes of $P(\ensuremath{\omega})$, characteristic for the domain shape and the respective location of absorbing and reflective boundaries. Our results demonstrate that large fluctuations of the first passage times may occur frequently in two-dimensional domains, rendering quite vague the general use of the MFPT as a robust measure of the actual behavior even in bounded domains, in which all moments of the first passage distribution exist.

Prediction of Geochemical Composition from XRF Core Scanner Data: A New Multivariate Approach Including Automatic Selection of Calibration Samples and Quantification of Uncertainties

Abstract: A multivariate log-ratio calibration (MLC) model for XRF-core-scanning devices is presented, based on a combination of basic XRF-spectrometry theory and principles of compositional data analysis. The performance of the MLC model is evaluated in comparison with other empirical calibration procedures for XRF core scanners data using two data sets acquired with two different XRF core scanner. The quality of calibration models is assessed by calculating the uncertainties associated with predicted concentrations using cross-validation techniques. Results show that (1) the commonly used direct linear calibration (DLC) methods, which are based on the questionable assumption of a unique linear relation between intensities and concentrations and do not acknowledge the compositional nature of the calibration problem, give poor results; (2) the univariate log-ratio calibration (ULC) model, which is consistent with the compositional nature of the calibration problem but does not fully incorporate absorption and enhancement effects on intensities, and permits estimation of “relative” concentrations only, is markedly better, and (3) the MLC algorithm introduced in this contribution, which incorporates measurement uncertainties, accommodates absorption and enhancement effects on intensities, and exploits the covariance between and among intensities and concentrations, is the best by far. The predictive power of the MLC model may be further increased by employing automatic sample selection based on the multivariate geometry of intensity measurements in log-ratio space. The precision attained by MLC in conjunction with automatic sample selection is comparable to that attained by conventional XRF analysis of heterogenous materials under laboratory conditions. A solution to the long-standing problem of XRF core scanner calibration implies that high-resolution records of sediment composition with associated uncertainties can now be routinely established, which should increase the range of quantitative applications of XRF-core-scanning devices and strengthen inferences based on analysis of geochemical proxies.