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At least 127 records · Page 7Linked to original sources
PubMed2000

Threshold modeling of autonomic control of heart rate variability.

Even in the absence of external perturbation to the human cardiovascular system, measures of cardiac function, such as heart rate, vary with time in normal physiology. The primary source of the variation is constant regulation by a complex control system which modulates cardiac function through the autonomic nervous system. Here, we present methods of characterizing the statistical properties of the underlying processes that result in variations in ECG R-wave event times within the framework of an integrate-and-fire model. We first present techniques for characterizing the noise processes that result in heart rate variability even in the absence of autonomic input. A relationship is derived that relates the spectrum of R-R intervals to the spectrum of the underlying noise process. We then develop a technique for the characterization of the dynamic nature of autonomically related variability resulting from exogenous inputs, such as respiratory-related modulation. A method is presented for the estimation of the transfer function that relates the respiratory-related input to the variations in R-wave event times. The result is a very direct analysis of autonomic control of heart rate variability through noninvasive measures, which provides a method for assessing autonomic function in normal and pathological states.

Autonomic Nervous System↗
PubMed2001

Threshold model for misclassified binary responses with applications to animal breeding.

A Bayesian procedure for misclassified binary data was developed. An animal breeding simulation indicated that, when error of classification was ignored, the variance between clusters was inferred incorrectly. Data were reanalyzed assuming that the probability of misclassification was either known or unknown. In the first case, input parameter values were recovered in the analysis. When the probability was unknown, there was a slight bias; the true probability of misclassification and the true number of miscoded observations appeared within high credibility regions. An analysis of fertility in dairy cows is presented.

Animals↗
PubMed1980

An empirical test of the two-threshold model of disease transmission.

Recent research on diabetes mellitus has revealed the existence of at least two independent subtypes. The multifactorial analysis method suggested by Reich & co-workers (1972) for testing the independence of transmission between subtypes of a complex disorder agrees with this distinction. These techniques were also sensitive to the non-unitary nature of the mental retardation within a sample of subjects drawn from the data set of Reed & Reed (1965). Therefore, we believe that this analysis could be a valuable adjunct to the limited repertoire of methods currently available to psychiatric researchers for dealing with issues of differential diagnosis.

Adult↗
PubMed2005

Development of combined imbibition and hydrothermal threshold models to simulate maize (Zea mays) and chickpea (Cicer arietinum) seed germination in variable environments.

* The ability of hydrothermal time (HTT) and virtual osmotic potential (VOP) models to describe the kinetics of maize (Zea mays) and chickpea (Cicer arietinum) seed germination under variable conditions of water potential was investigated with a view to gaining an improved understanding of the impact of on-farm seed priming on seedling establishment through simulation. * Germination and/or imbibition time courses were recorded over a wide range of constant temperatures and water potentials and simple stepwise changes in water potential. * Both models adequately described germination under constant environmental conditions, but not conditions of water potential that varied. To test the hypothesis that this inaccuracy resulted from the use of ambient water potential, a parsimonious model of seed imbibition was developed to calibrate the HTT and VOP models (IHTT and IVOP) and drive them with estimates of seed water potential. * The IHTT and IVOP models described germination during stepwise changes in water potential more accurately than the conventional models, and should contribute to improved predictions of germination time in the field.

Cicer↗
PubMed2005

Categorization of neural excitability using threshold models.

A classification of spiking neurons according to the transition from quiescence to periodic firing of action potentials is commonly used. Nonbursting neurons are classified into two types, type I and type II excitability. We use simple phenomenological spiking neuron models to derive a criterion for the determination of the neural excitability based on the afterpotential following a spike. The crucial characteristic is the existence for type II model of a positive overshoot, that is, a delayed after depolarization, during the recovery process of the membrane potential. Our prediction is numerically tested using well-known type I and type II models including the Connor, Walter, & McKown (1977) model and the Hodgkin-Huxley (1952) model.

Action Potentials↗
PubMed1983

The transmission of schizophrenia under a multifactorial threshold model.

Family studies of schizophrenia have reported elevated rates of both definite and definite-plus-probable schizophrenia among the relatives of definite schizophrenics. These elevated rates imply a strong association between the two forms of diagnosis and suggest some form of familial transmission. Here we have used recently developed maximum likelihood methods to investigate this association and characterize the nature of the familial transmission. Results indicated that although the two forms of diagnosis were strongly related, they could not be considered alternative manifestations of a single liability distribution. Heritability estimates for either form of diagnosis were comparable (h2 = .668 +/- .052 and c2 = .191 +/- .038 for definite while h2 = .628 +/- .073 and c2 = .236 +/- .106 for definite-plus-probable), although cultural transmission (i.e., c2) was statistically significant only for definite-plus-probable. For either form of diagnosis, residual twin resemblance was statistically significant and could not be explained in terms of the effects of genetic dominance. These results are comparable to those of an earlier analysis based upon a similar data set. Finally, the statistical correction used to adjust for between-study heterogeneity in morbidity risk figures did not noticeably alter the parameter estimates.

Adult↗