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PubMed1977

[Disturbances of the human instinct behaviour in hypothalamic lesions (author's transl)].

Since the findings of W.R. Hess, who systematically stimulated the hypothalamus of the cat, motivational systems for instinct behaviour have been postulated in the hypothalamus and the surrounding limbic brain. It has been asked, if hypothalamic lesions in man would cause alterations in instinct behaviour, which could be attributed to the instinct motivational systems. Therefore 19 patients with hypothalamic lesions have been examined in respect of their instinct behaviour. They showed no qualitative alteration in their instinct behaviour, but quantitative diminution and rarefication of their gain instincts (nutritional, sexual and social instinct). Mainly reduced was the sex instinct together with hormonal deficiency. Inversely the safety instinct (the only avoidance instinct with variable threshold) was more often activated. The other avoidance instincts (for excretion, thermo-regulation, bodycare and painavoidance) remained intact. It is concluded that the pacemaker for the threshold of the gain instincts and of the safety instinct is disturbed and not the motivation system in its qualitative functioning. As this pacemaker is so characteristically different in each instinct, the theory of different specific motivational systems is more probable than the theory of one unspecific system with different functions.

Brain Diseases↗
PubMed2005

Neural systems responding to degrees of uncertainty in human decision-making.

Much is known about how people make decisions under varying levels of probability (risk). Less is known about the neural basis of decision-making when probabilities are uncertain because of missing information (ambiguity). In decision theory, ambiguity about probabilities should not affect choices. Using functional brain imaging, we show that the level of ambiguity in choices correlates positively with activation in the amygdala and orbitofrontal cortex, and negatively with a striatal system. Moreover, striatal activity correlates positively with expected reward. Neurological subjects with orbitofrontal lesions were insensitive to the level of ambiguity and risk in behavioral choices. These data suggest a general neural circuit responding to degrees of uncertainty, contrary to decision theory.

Adult↗
PubMed1977

[Probability neuronal ensembles from the position of the theory of fuzzy sets].

The questions of organization and functioning of the probability neuronal ensembles in the cerebral central analysatory structures were regarded from the standpoints of the washed out multitudes theory. The possibility of formalization of structure of a neuronal ensemble (the washed out multitude of neurons) was shown, examples of determining the function of a neuron belonging to a neuronal ensemble were presented, the questions of transition from washed out multitudes to unwashed those were considered.

Mathematics↗
PubMed2002

Variational perturbation theory for Markov processes.

We develop a convergent variational perturbation theory for conditional probability densities of Markov processes. The power of the theory is illustrated by applying it to the diffusion of a particle in an anharmonic potential.

Journal Article↗
PubMed1992

Toward normative expert systems: Part I. The Pathfinder project.

Pathfinder is an expert system that assists surgical pathologists with the diagnosis of lymph-node diseases. The program is one of a growing number of normative expert systems that use probability and decision theory to acquire, represent, manipulate, and explain uncertain medical knowledge. In this article, we describe Pathfinder and our research in uncertain-reasoning paradigms that was stimulated by the development of the program. We discuss limitations with early decision-theoretic methods for reasoning under uncertainty and our initial attempts to use non-decision-theoretic methods. Then, we describe experimental and theoretical results that directed us to return to reasoning methods based in probability and decision theory.

Artificial Intelligence↗
PubMed2004

Thinking about low-probability events. An Exemplar-Cuing theory.

The way people respond to the chance that an unlikely event will occur depends on how the event is described. We propose that people attach more weight to unlikely events when they can easily generate or imagine examples in which the event has occurred or will occur than when they cannot. We tested this idea in two experiments with mock jurors using written murder scenarios. The results suggested that jurors attach more weight to the defendant's claim that an incriminating DNA match is merely coincidental when it is easy for them to imagine other individuals whose DNA would also match than when it is not easy for them to imagine such individuals. We manipulated the difficulty of imagining such examples by varying the description of the DNA-match statistic. Some of the variations that influenced the jurors were normatively irrelevant.

Crime↗
PubMed2006

Statistical mechanical theory for steady state systems. V. Nonequilibrium probability density.

The phase space probability density for steady heat flow is given. This generalizes the Boltzmann distribution to a nonequilibrium system. The expression includes the nonequilibrium partition function, which is a generating function for statistical averages and which can be related to a nonequilibrium free energy. The probability density is shown to give the Green-Kubo formula in the linear regime. A Monte Carlo algorithm is developed based upon a Metropolis sampling of the probability distribution using an umbrella weight. The nonequilibrium simulation scheme is shown to be much more efficient for the thermal conductivity of a Lennard-Jones fluid than the Green-Kubo equilibrium fluctuation method. The theory for heat flow is generalized to give the generic nonequilibrium probability densities for hydrodynamic transport, for time-dependent mechanical work, and for nonequilibrium quantum statistical mechanics.

Journal Article↗
PubMed2001

Probabilities of encounters between objects in biological systems: meta-observer view.

Encounters between objects play a crucial role in creating specific patterns of organization in many biological systems. This paper explores a general, quantitative principle of encounter probabilities, applicable over various levels of organization, as a fundamental step toward a comprehensive theory of biological probability ranging from cells through organisms to ecological communities. Based on the cognizers-system model, a general description of encounter probabilities in a finite position space is derived as a function of the number, or density, of objects with cognitive, selectivity properties incorporated as parameters. There is a prevailing wisdom, as supposed in many scientific thoughts, that the per-unit-time number of encounters between focal and target objects is given as a linear function of the density of target objects. One result shows that this idea does not describe the real nature of encounter probabilities or rates, but only a special case or approximation of a fundamental description. The analysis also explicates conditions for ignoring the presence of non-target objects, and those for the aggregation of two or more different objects into the same type sharing the same cognitive properties in the description of the probabilities.

Animals↗
PubMed1976

Probability analysis of the interaction of antibodies with multideterminant antigens in radioimmunoassay: application to the amino terminus of the beta chain of hemoglobin S.

A simple theory, based on probability, is developed for the analysis of the interaction of multideterminant antigens with multispecific antisera in radioimmunoassays. The theory is completely general except for the assumptions that the determinants be unique and bind antibodies independently of one another. The analysis shows that the shape of the curve of bound/free as a function of the antigen concentration is very sensitive to the multiplicity of determinants. The predictive ability of the theory is illustrated for the case of antibodies to subregions of the N-terminal third of the beta chain of sickle hemoglobin, studied using antisera fractionated on affinity chromatographic columns of synthetic peptides. The implications for obtaining quantitative binding data by radioimmunoassay for natural antigens, which almost universally have more than one antigenic determinant on the same molecule, are discussed.

Amino Acid Sequence↗
PubMed2005

Long-term stability from fixation probabilities in finite populations: new perspectives for ESS theory.

For mixed strategies in finite populations, long-term stability is defined with respect to the probability of fixation of a mutant. Under weak selection, necessary and sufficient conditions are obtained using a diffusion approximation of the Wright-Fisher model or exact solutions for the Moran model. These differ from the usual ESS conditions if the strategies affect fertility instead of viability, leading to a game matrix depending on the population size, or if the mutant mixed strategy uses a new pure strategy. In this case, the mutant deviation must not exceed some threshold value depending on the population size. In a diploid population, long-term stability may not occur unless there is partial dominance. In the case of sex allocation, continuous stability of an even sex ratio is ascertained. If sex allocation is random, an evolutionary decrease of the variance is predicted.

Animals↗
PubMed1995

Theory of the mobility-shift assay of nonspecific protein-DNA complexes governed by conditional probabilities: the HU:DNA complex.

Complexes of an 88 bp DNA and the HU protein were studied by both experimental and theoretical electrophoretic mobility-shift analyses. Experimental analysis defined the stoichiometry of binding and estimated an apparent intrinsic dissociation constant (Kd = 1 to 3 x 10(-7) M) for the HU:DNA complexes. The theory of conditional probabilities was applied to the binding of HU to DNA in order to fix the initial equilibrium composition of mixtures to be assayed theoretically by the mobility-shift procedure. Electrophoretic mobility-shift patterns were obtained by numerical solution of a set of simultaneous transport-reaction equations, in which the chemical kinetic term is formulated in terms of dissociation of the different DNA:HU complexes in gel cages. The computed patterns simulated the experimental patterns describing the titration of a fixed concentration of an 88 bp DNA fragment with dimeric HU. These insightful results provide guidelines for interpretation of the electrophoretic behavior of systems in which a ligand binds nonspecifically to DNA. In particular, the narrow unresolved zone observed both experimentally and theoretically beyond 50-60% saturation is a reaction zone characteristic of noncooperative ligand-binding governed by conditional probabilities. The discrepancy between the theoretically assigned and experimental values of the intrinsic binding constant is attributed to an HU-induced change in the conformation of DNA.

Bacterial Proteins↗
PubMed1997

Probabilistic Choice as a Consequence of Nonlinear (Sub) Optimization

Several researchers have convincingly argued that much probabilistic choice could be the result of (attempted) maximization of some utility function that is nonlinear in the choice probabilities. Others researchers have developed descriptive utility theories that are nonlinear in event probabilities. The present work combines these two viewpoints and motivates utility theories that can be nonlinear in either one or both of choice and event probabilities. The presented theories are based on the idea that most choice situations involve multiple incompatible goals which have to somehow be combined in reaching a decision. In particular, I assume that the decision maker is faced with a vector optimization problem which is converted to a scalar optimization problem, the latter being solved to yield a noninferior solution to the original vector optimization problem. I show that in certain special cases this approach yields the same utility representations as certain other recent nonlinear utility theories. Copyright 1997 Academic Press. Copyright 1997 Academic Press

Journal Article↗
PubMed2000

Statistical theory of combinatorial libraries of folding proteins: energetic discrimination of a target structure.

A self-consistent theory is presented that can be used to estimate the number and composition of sequences satisfying a predetermined set of constraints. The theory is formulated so as to examine the features of sequences having a particular value of Delta=E(f)- (u), where E(f) is the energy of sequences when in a target structure and (u) is an average energy of non-target structures. The theory yields the probabilities w(i)(alpha) that each position i in the sequence is occupied by a particular monomer type alpha. The theory is applied to a simple lattice model of proteins. Excellent agreement is observed between the theory and the results of exact enumerations. The theory provides a quantitative framework for the design and interpretation of combinatorial experiments involving proteins, where a library of amino acid sequences is searched for sequences that fold to a desired structure.

Amino Acid Sequence↗
PubMedDate not supplied

A physical statistics theory for detectability of target signals in noisy images. I. Mathematical background, empirical review, and development of theory.

This paper addresses the physical probability of finding, by random chance of noise fluctuations, false-positive events in the background field of view of a noisy image. The signal levels of these random events are characterized by a general noise power spectrum; if they are comparable in magnitude with that of a true target signal, averaged over an area equal to that of the true target signal, then the false-positive events obscure the confidence of the true signal identification. The theory shows that the statistics of this phenomenon depend very strongly, and with distinct threshold behavior, upon the generalized power-signal-to-noise ratio of the true target signal; the behavior depends only weakly upon other factors, e.g., the background field of view area examined with respect to the true target area. Evaluation of even the simplest model of detection confidence, based upon this theory, yields several immediate results. The predicted SNR at threshold is approximately 3.5, with a variation of +/- 0.5 from 10%-95% confidence-of-detection levels. The theory is applied also to systems where display or intrinsic signal detection properties, rather than system inputs, limit the statistics. In this case, the theory predicts, in agreement with experiments, contrast-limiting effects of 1% contrast for the human eye vision system and 5% contrast for typical TV displays of scintigraphic images. When experimental detectability studies of numerous investigators all are converted to a uniform specification of the output power-average-signal-to-noise ratio, then the theoretical predictions here give an excellent description of all major aspects of the empirical results.

False Positive Reactions↗
PubMed2002

Electrostatics explains the shift in VDAC gating with salt activity gradient.

We have analyzed voltage-dependent anion-selective channel (VDAC) gating on the assumption that the states occupied by the channel are determined mainly by their electrostatic energy. The voltage dependence of VDAC gating both in the presence and in the absence of a salt activity gradient was explained just by invoking electrostatic interactions. A model describing this energy in the main VDAC states has been developed. On the basis of the model, we have considered how external factors cause the redistribution of the channels among their conformational states. We propose that there is a difference in the electrostatic interaction between the voltage sensor and fixed charge within the channel when the former is located in the cis side of membrane as opposed to the trans. This could be the main cause of the shift in the probability curve. The theory describes satisfactorily the experimental data (Zizi et al., Biophys. J. 1998. 75:704-713) and explains some peculiarities of VDAC gating. The asymmetry of the probability curve was related to the apparent location of the VDAC voltage sensor in the open state. By analyzing published experimental data, we concluded that this apparent location is influenced by the diffusion potential. Also discussed is the possibility that VDAC gating at high voltage may be better described by assuming that the mobile charge consists of two parts that have to overcome different energetic barriers in the channel-closing process.

Biophysical Phenomena↗
PubMed2005

A Turing test of a timing theory.

A quantitative theory of timing or conditioning can be evaluated with a Turing test in which the behavioral results of an experiment can be compared with the predicted results from the theory. An example is described based upon an experiment in which 12 rats were trained on three fixed-interval schedules of reinforcement, and a simulation of the predicted results from a packet theory of timing. An objective classification rule was used to determine whether a sample from the data or a sample from the theory was more similar to another sample from the theory. With an ideal theory, the expected probability of a correct classification would be 0.5. The observed probability of a correct classification was 0.6, which was slightly, but reliably, greater than 0.5. A Turing test provides a graded metric for the evaluation of a quantitative theory.

Animals↗