Biomedical subjects

T Eakin

Publications and source records attributed to T Eakin.

7 recordsLinked to original sources

PubMed2005

Quantification of manual force control and tremor.

Isometric impulse frequencies associated with active tremor and force regulation were examined in 10 patients with idiopathic Parkinson's disease (PD) and in 10 older adults (OAs) who performed an isometric tracing task. The authors decoupled and analyzed the data to determine whether PD-related tremor in the thumb and in the index finger during isometric force control are related and whether PD impairs the performance of volitional force control beyond the errors contributed by tremor. After decoupling, there were clear and robust differences in PD patients' control of isometric force that could not be attributed to action-tremor error. Those errors, which occurred in the absence of movement, suggest impairment in coordinated recruitment and derecruitment of motor units during a fine-motor task.

Adult↗

PubMed1995

A gerontological distance metric for analysis of survival dynamics.

A metric for quantifying a gerontological mapping 'distance' or displacement consistent with the historical concept of velocity of aging and with the more recent concept of acceleration of aging, is introduced using the paradigm of a simple linear dynamics system of elementary physics. This analysis is extended to recent analytical methods utilizing intrinsic or internal time scaling so that biological or gerontological similarity can be distinguished from chronological age similarity, not only among various intraspecies populations but also among interspecies populations which may not even have the same underlying mechanisms of senescence or survival distributions. Illustrative examples are provided and discussed. Also, applications involving the comparison of an individual from one population to an individual from another population, when both can be assessed with respect to their respective group properties, are considered.

Age Factors↗

PubMed1995

Estimating parametric survival model parameters in gerontological aging studies: methodological problems and insights.

Studies of the biology of aging (both experimental and evolutionary) frequently involve the estimation of parameters arising in various multi-parameter survival models such as the Gompertz or Weibull distribution. Standard parameter estimation methodologies, such as maximum likelihood estimation (MLE) or nonlinear regression (NLR), require knowledge of the actual life spans or their explicit algebraic equivalents in order to provide reliable parameter estimates. Many fundamental biological discussions and conclusions are highly dependent upon accurate estimates of these survival parameters (this has historically been the case in the study of genetic and environmental effects on longevity and the evolutionary biology of aging). In this article, we examine some of the issues arising in the estimation of gerontologic survival model parameters. We not only address issues of accuracy when the original life-span data are unknown, we consider the accuracy of the estimates even when the exact life spans are known. We examine these issues as applied to known experimental data on diet restriction and we fit the frequently used, two-parameter Gompertzian survival distribution to these experimental data. Consequences of methodological misuse are demonstrated and subsequently related to the values of the final parameter estimates and their associated errors. These results generalize to other multiparametric distributions such as the Weibull, Makeham, and logistic survival distributions.

Aging↗

PubMed1994

Intrinsic time scaling in survival analysis: application to biological populations.

A method of dimensionless time-scaling based on extrinsic expectation of life at birth but intrinsic to a system generating a survival distribution is introduced. Such scaling allows the survival fraction function and its associated mortality function to serve as Green's functions for their generalized equivalents, i.e., a "population" function and a "death" function. The analytical mechanics of utilizing these concepts are formulated, applied to the classical Gompertz and Weibull survival models, and discussed with respect to biological relevance.

Animals↗

PubMed1994

Transitions of latency time and oscillation phase on parameter surfaces from models of intracellular calcium ion dynamics.

The dynamics of two classical elementary compartmental models stimulating intracellular calcium ion oscillatory behavior are examined in terms of parameter surfaces. It has been found that, along certain lines of instability on surfaces defined by model parameters, the highly non-linear nature of these models produces sharp transitions in the latency time which determines the phase of oscillations once they commence. This sensitivity to initial conditions in deterministic models, along with the stochastic variance inevitably present in actual biological systems, illustrates how two seemingly identical cells activated by identical synchronous stimulation can exhibit oscillatory responses which are out of phase with respect to each other.

Calcium↗

PubMedDate not supplied

How square is the survival curve of a given species?

An investigator-independent parameter, the prolate rectangularity index kappa for describing the so-called rectangularity of biological population survival curves, is introduced, developed, and applied to realworld survival datasets. This new rectangularity parameter is constructed using an intrinsic time scaling that places the intrinsic inflection point time at a value of unity so that species populations may be compared independently of their extrinsic life span distributions. The analytical expressions for the prolate rectangularity index of the theoretical Gompertz and Weibull continuous models are obtained, as are numerical values of this index for discrete experimental population survival data sets from two dissimilar species with orders of magnitude difference in extrinsic life span range. The values of the parameter are also compared for populations of a single species having differing dietary regimens, and for human demographic populations at decade intervals in extrinsic chronological time during the current century. It is found that scaling time, using the survival inflection point, appreciably collapses extrinsic survival profile dispersion among similar populations and allows a more meaningful comparison of profiles among dissimilar populations. Using this method of scaling, demographic populations within the United States are seen to have rectangularity parameter values that have been slowly drifting during this century toward values indicating a higher degree of rectangularity. In recent decades, however, the trend appears to be stabilizing with kappa values indicating no approach towards the theoretical maximum rectangularity. This apparent submaximal stabilization of kappa supports a hypothesis of no genetically pre-determined maximum life span in human populations. Or, if such a maximum exists, we are not currently near it.

Animals↗

PubMedDate not supplied

Multiphasic models of survival: analysis of mortality rate change regions and the issue of finite species lifespan.

Recent large-scale experimental population studies have allowed us to probe the dynamics of survival in the "oldest old" of a small number of biological species. The results of these studies add strong support to the validity of a multiphasic survival/mortality model for population survival as has been previously proposed by a number of investigators. In this paper we briefly review some of the problems with the Gompertz survival model, and the issue of locating the region in which the mortality rate might change. We derive some rigorous formulae for bounding the mortality rate change regions and compare the predictions to the available experimental data. We demonstrate that the mortality cut point parameter appears to be directly related to topological properties of the species survival curve, in particular, the inflection time. We conclude by addressing some of the pitfalls of using a mortality cut point model and propose an alternative formulation for a mortality model involving multiphasic mortality dynamics and incorporating a nonspecified upperbound on species lifespan.

Linear Models↗