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PubMed1984

What is the meaningful measure of neuronal spike train activity?

Neuronal spike train activity is conventionally viewed in terms of certain measures of central tendency (mean or median of intervals between discrete events or mean rate of such events) and their associated measures of variability (S.D., skew, etc.). Less commonly, investigators will compute the probabilities of occurrence in an Information Theory context. Another rarely considered measure of spike trains is serial ordering. Seldom is more than one of these approaches applied to the same spike train, and we are unaware of any case where all 3 have been applied to the same train. In order to test the inter-relationships among these approaches, we examined the same spike trains with all 3 analytic methods. Measures of central tendency were taken from the original absolute interval values, whereas entropy and serial order (Markov order) were computed from the sequences of patterns of adjacent intervals, expressed in the same non-parametric format. We found that entropy (measure of uncertainty) did not correlate well with the degree of serial ordering (Markov order). Entropy also did not correlate well with measures of central tendency (median or mean interval, or impulses/s) nor with the variability of such measures. An inverse correlation was seen between Markov order and several measures of central tendency (mean interval and rate) as well as with several measures of variability. The implications of these analyses extend beyond the analysis of spike trains to most all biological and physical time series. For neurophysiologists, these analyses may challenge our common assumptions about the most appropriate way to describe and interpret neuronal spike train activity.(ABSTRACT TRUNCATED AT 250 WORDS)

Action Potentials↗
PubMed1994

A rapid algorithm and a computer program for multiple test procedures using logical structures of hypotheses.

It is demonstrated how improvements of general multiple test procedures can be obtained using information about the logical structures among the hypotheses. Based on a procedure of Bergmann and Hommel (B. Bergmann and G. Hommel, Improvements of general multiple test procedures for redundant systems of hypotheses, in Multiple Hypothesenprüfung--Multiple Hypotheses Testing, Eds. P. Bauer, G. Hommel and E. Sonnemann, pp. 100-115 (Springer-Verlag, Berlin, 1988)), a computer program was written by Bernhard (G. Bernhard, Computerunterstützte Durchführung von multiplen Testprozeduren--Algorithmen und Powervergleich, Doctoral thesis (Mainz, 1992)) using this information. It is applicable for a general class of systems of hypotheses which can be expressed in a linear way. By means of a simulation study it is shown that the proposed procedure is often substantially more powerful than other usual multiple test procedures.

Algorithms↗
PubMed1991

Chaos and predicting varix hemorrhage.

Despite a wide array of mechanisms implicated in esophagogastric varix hemorrhage, predicting the onset of bleeding or even identifying the dominant factor in its causation has proved elusive. In this article we re-examine variceal pathophysiology and hypothesize that bleeding is rooted in turbulent portal system flow, a phenomenon embodied in the 'new' science of chaos and the principles of non-linear, rhythmic fluid dynamics. Analogous to forecasting the weather, predicting turbulent flow hinges on defining the initial physical conditions, and, like the field theory of quantum physics, depends on probabilities and instabilities rather than direct proportional analysis. Based on the complex regional and systemic hemodynamic forces and local physical properties underlying formation and perpetuation of esophagogastric varices, we propose, as with climatologic events, that a relatively minor or remote physiologic adjustment may set into motion a sequence of destabilizing splanchnic blood flow kinetics that ultimately causes a varix to erupt. In other words, the onset of varix bleeding derives from a sudden, erratic switch from disorganized but stable to disruptive turbulent variceal blood flow, and as a 'chaotic' phenomenon, is likely to remain for the foreseeable future highly unpredictable.

Biophysical Phenomena↗
PubMed1989

Oxygen toxicity and reactive oxygen metabolites in mammals.

The current hypothesis for the damage caused to mammalian tissues by hyperoxia is that oxygen radicals and related reactive oxygen metabolites are formed at rates which exceed the ability of the cells' natural antioxidant defense mechanisms to detoxify these deleterious products. In this review a very brief description of oxygen pathology is given together with data on the relevance of in vivo tissue pO2 levels. After a short historical account of the biochemistry of oxygen toxicity in the pre-radical days. the evidence for the current status of the radical theory is reviewed. This covers the probable sources of excess reactive oxygen metabolite generation under conditions of increased oxygen tension, and the measurement of the reactive species thought to be important in causing this damage. The large volume of circumstantial evidence, including the production of tolerance, raising or lowering antioxidant defenses and the administration of exogenously produced radicals is considered.

Animals↗
PubMed2004

Reaction rates of heavy metal ions at goethite: relaxation experiments and modeling.

In the present paper we extend our theory that calculates the fastest reaction step observable in suspensions containing charged microcrystals and heavy metal cations. The calculation requires the solution of the nonlinear Poisson-Boltzmann equation for nonsymmetric electrolytes plus the Nernst-Planck equation for transport of ions in electric fields. We find that the diffusional transport of ions to and from the surface is the rate-limiting process for our experimentally observed maximum rates. At low pH and low metal ion concentration the diffusion of metal ions is the rate-limiting step, whereas for high pH and high metal ion concentration the diffusion of the solvated protons controls the overall relaxation rate. The validity of this theory is checked for the reactions of Pb2+ and Cd2+ with goethite by means of pressure jump relaxation experiments over a wide range of temperature and pH. In all cases we observe fast processes (relaxation in the range of 10(3) s(-1)) in quantitative agreement with the theory, followed by slower processes, most probably caused by diffusion into the interior of the porous microcrystals.

Journal Article↗
PubMed2006

Bioturbation: a fresh look at Darwin's last idea.

Bioturbation refers to the biological reworking of soils and sediments, and its importance for soil processes and geomorphology was first realised by Charles Darwin, who devoted his last scientific book to the subject. Here, we review some new insights into the evolutionary and ecological role of bioturbation that would have probably amazed Darwin. In modern ecological theory, bioturbation is now recognised as an archetypal example of 'ecosystem engineering', modifying geochemical gradients, redistributing food resources, viruses, bacteria, resting stages and eggs. From an evolutionary perspective, recent investigations provide evidence that bioturbation had a key role in the evolution of metazoan life at the end of the Precambrian Era.

Animals↗
PubMed1993

Macromolecular diffusion in crowded solutions.

The effects of crowding on the self or tracer diffusion of macromolecules in concentrated solutions is an important but difficult problem, for which, so far, there has been no rigorous treatment. Muramatsu and Minton suggested a simple model to calculate the diffusion coefficient of a hard sphere among other hard spheres. In this treatment, scaled particle theory is used to evaluate the probability that the target volume for a step in a random walk is free of any macromolecules. We have improved this approach by using a more appropriate target volume which also allows the calculation to be extended to the diffusion of a hard sphere among hard spherocylinders. We conclude that, to the extent that proteins can be approximated as hard particles, the hindrance of globular proteins by other proteins is reduced when the background proteins aggregate (the more so the greater the decrease in particle surface area), the hindrance due to rod-shaped background particles is reduced slightly if the rod-like particles are aligned, and the anisotropy of the diffusion of soluble proteins among cytoskeletal proteins will normally be small.

Biophysical Phenomena↗
PubMed2004

Learning at a distance I. Statistical learning of non-adjacent dependencies.

In earlier work we have shown that adults, young children, and infants are capable of computing transitional probabilities among adjacent syllables in rapidly presented streams of speech, and of using these statistics to group adjacent syllables into word-like units. In the present experiments we ask whether adult learners are also capable of such computations when the only available patterns occur in non-adjacent elements. In the first experiment, we present streams of speech in which precisely the same kinds of syllable regularities occur as in our previous studies, except that the patterned relations among syllables occur between non-adjacent syllables (with an intervening syllable that is unrelated). Under these circumstances we do not obtain our previous results: learners are quite poor at acquiring regular relations among non-adjacent syllables, even when the patterns are objectively quite simple. In subsequent experiments we show that learners are, in contrast, quite capable of acquiring patterned relations among non-adjacent segments-both non-adjacent consonants (with an intervening vocalic segment that is unrelated) and non-adjacent vowels (with an intervening consonantal segment that is unrelated). Finally, we discuss why human learners display these strong differences in learning differing types of non-adjacent regularities, and we conclude by suggesting that these contrasts in learnability may account for why human languages display non-adjacent regularities of one type much more widely than non-adjacent regularities of the other type.

Adult↗
PubMed1987

A method for defining steady-state unidirectional fluxes through branched chemical, osmotic and chemiosmotic reactions.

A general mathematical technique is described for deriving analytical expressions and obtaining numerical solutions for the steady-state unidirectional fluxes between two chemical states via any set of intermediate states present within any hypothetical system of unbranched or branched and overlapping elementary processes. The technique is a restricted application of the theory of Markov processes with conditional probabilities being assigned to the chemical state transitions constituting the system of reactions. While, in principle, the technique requires the summation of an infinite power series of a matrix defining the conditional probabilities of single state transitions, the power series is evaluated by means of the Taylor series expansion for matrices. As this technique allows isotopic exchange velocity equations to be derived from systems of reactions in which no distinction between the labelled and unlabelled species is required it provides a distinct and independent alternative to previously proposed methods. The technique is illustrated by application to a mechanism for second-order carrier-mediated transport.

Biological Transport↗
PubMed1975

Androblastoma and germ cell tumor of the same testis: a case report.

The rare case of a testicular androblastoma associated with a complex germ cell tumor of the same testis is reported. Gynecomastia preceded the clinical appearace of testicular swelling by 3 months, suggesting that the 2 tumors developed contemporaneously and that, in theory, the Sertoli cell tumor is probably dependent on the germ cell tumor.

Adult↗
PubMed1990

A probabilistic contrast model of causal induction.

Deviations from the predictions of covariational models of causal attribution have often been reported in the literature. These include a bias against using consensus information, a bias toward attributing effects to a person, and a tendency to make a variety of unpredicted conjunctive attributions. It is contended that these deviations, rather than representing irrational biases, could be due to (a) unspecified information over which causal inferences are computed and (b) the questionable normativeness of the models against which these deviations have been measured. A probabilistic extension of Kelley's analysis-of-variance analogy is proposed. An experiment was performed to assess the above biases and evaluate the proposed model against competing ones. The results indicate that the inference process is unbiased.

Adult↗