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Logistic Growth (logistic + growth)
Selected AbstractsMathematical analysis of the cone ERG photopic hill: Clinical applicationsACTA OPHTHALMOLOGICA, Issue 2007P LACHAPELLE Purpose: With brighter stimuli, the photopic ERG b-wave increases to a maximal value and then decreases to a plateau, a feature known as the Photopic Hill (PH). Recently, a mathematical model combining a Gaussian (GF) and a Logistic Growth (LGF) functions was developed to fit the PH (Hamilton et al., Vision Research, in press). We examined if this equation could help us sort out selected retinopathies. Methods: We compared PHs (background: 30 cd.m-2; intensities: -0.8 to 2.84 log cd.sec.m-2) obtained from normals (N=40) and patients (N=20) affected with Congenital Stationary Night Blindness (CSNB), Congenital Postreceptoral Cone Pathway Anomaly (CPCPA) and Retinitis Pigmentosa (RP) with the GL ratio [GL= Gb / (Gb+Vbmax)] were Gb and Vbmax represent the amplitude of the Gaussian and logistic (Vbmax) functions respectively. Results: The normal GL ratio is 0.60 ± 0.08 (mean ± 1SD) compared to ,1.0 in CSNB (almost pure GF) and 0.32±0.08 in CPCP [reduced GF (p<.05) and normal LF (p>.05)] patients. Six of the 8 RP patients had a GL ratio above 0.5 (mean GL= 0.70 ± 0.19) and 2 below (0.28 and 0.41). Of interest, while in some retinopathies, a decline in Gb and Vbmax occurred with disease progression (longitudinal and transversal comparisons), it did not always modify the GL ratio. Conclusions: Human PH can be dissected into two distinct and concomitant phenomena each represented by its own equation. Altghough the retinal origin of the GF and LGF awaits to be confirmed, use of this mathematical approach appears to add valuable information that will further refine the diagnosis of retinal disorders affecting the photopic (cone) pathway. Supported by CIHR and Réseau Vision. [source] Population trends and habitat use of Harlequin Ducks in Rhode IslandJOURNAL OF FIELD ORNITHOLOGY, Issue 3 2007Christine M. Caron ABSTRACT To assess population trends of Harlequin Ducks (Histrionicus histrionicus) in Rhode Island (U.S.A.), we analyzed Christmas Bird Counts and other historical surveys and also conducted surveys during the winter of 2005,2006. We estimated sex and age ratios, evaluated the effects of tidal regime and time of day on survey precision, and quantified habitat use. The population in Rhode Island experienced logistic growth from 1976 to 2004, with approximately 150 birds now wintering at three primary sites in the state. We estimated that the current ratio of males to females in the region was 1.6:1 (62% males) and that 13% of males were first-winter birds. Most Harlequin Ducks were observed in rocky habitats within 50 m of the shore or offshore islands. We detected the greatest numbers of birds, with the least amount of variation, during morning surveys at low tide, suggesting that this may be the most appropriate time for population monitoring. Increases in the Rhode Island population and male-biased sex ratios may indicate a local population recovery resulting from a hunting ban initiated in 1990. Although most Harlequin Ducks in eastern North America winter in Maine, the population in Rhode Island represents one of the largest in the southern part of their range. SINOPSIS Para determinar las tendencias poblacionales de arlequines (Histrionicus histrionicus), en Rhode Island, analizamos los conteos navideños, otros censos históricos y además llevamos a cabos censos durante el invierno de 2005,2006. Cuantificamos el uso de habitat, estimamos la tasa de edad y sexos y evaluamos el efecto del régimen de mareas y la hora del dia en la presición de los censos. La población de patos arlequines experimentó un crecimiento logístico de 1976,2004, con unos 150 patos pasando el invierno en tres localidades primarias, del estado. Estimamos una tasa de 1.6:1 de machos a hembras (62% de machos), siendo un 13% de los machos individuos de primer invierno. La mayoría de las aves fueron observadas en hábitats rocosos a unos 50 m de la playa o en islas cerca de la playa. Detectamos el mayor número de aves, con la menor variación, en los censos que se llevaron a cabo durante la marea baja, lo que sugiere que este es el momento más apropiado para monitorear sus poblaciones. El aumento de estas aves en Rhode Island, el sesgo en su sexo, hacia mayor cantidad de machos, pudiera indicar, de un recobro poblacional como resultado la prohibición de su cacería a partir de 1990. Aunque la mayoría de los arlequines de la parte este de los EUA pasan el invierno en Maine, la población de Rhode Island representa una de las mayores en la parte sur de su distribución. [source] Radiotherapy for hepatocellular carcinoma: Systematic review of radiobiology and modeling projections indicate reconsideration of its useJOURNAL OF GASTROENTEROLOGY AND HEPATOLOGY, Issue 4 2010Alan J Wigg Abstract Background and Aims:, External beam radiotherapy currently has a limited role in the treatment of hepatocellular carcinoma (HCC). The purpose of this article was to review available radiobiological data on HCC and normal liver and incorporate these data into radiobiological models that may be used to explain and improve treatment. Methods:, Volume doubling times of HCC were described and used to demonstrate growth of HCC with time, assuming both exponential and logistic growth. Radiosensitivity of HCC was described and used to demonstrate the probability of uncomplicated tumor control as tumor size increases. The relationship between tolerance of liver to irradiation and volume irradiated was examined. Results:, The median volume doubling time for untreated HCC was 130 days. HCC have a long period of subclinical growth. Radiosensitivity of HCC lies within the range of other tumors commonly treated with radiotherapy. When treating small volumes of normal liver, relatively high doses may be used with low risk of late radiation damage. There is a high probability of sterilizing subclinical disease and small HCC with tolerable radiation doses. Conclusion:, New radiobiological data, modeling, emerging clinical data and the advantages offered by standard external beam radiotherapy techniques suggest the need for reconsidering the use of radiotherapy and for new trials. [source] CONSTANT EFFORT AND CONSTANT QUOTAFISHING POLICIES WITH CUT-OFFS IN A RANDOM ENVIRONMENTNATURAL RESOURCE MODELING, Issue 2 2001CARLOS A. BRAUMANN ABSTRACT. Consider a population subjected to constant effort or constant quota fishing with a generaldensity-dependence population growth function (because that function is poorly known). Consider environmental random fluctuations that either affect an intrinsic growth parameter or birth/death rates, thus resulting in two stochastic differential equations models. From previous results of ours, we obtain conditions for non-extinction and for existence of a population size stationary density. Constant quota (which always leads to extinction in random environments) and constant effort policies are studied; they are hard to implement for extreme population sizes. Introducing cut-offs circumvents these drawbacks. In a deterministic environment, for a wide range of values, cutting-off does not affect the steady-state yield. This is not so in a random environment and we will give expressions showing how steady-state average yield and population size distribution vary as functions of cut-off choices. We illustrate these general results with function plots for the particular case of logistic growth. [source] Robust parameter estimation during logistic modeling of batch and fed-batch culture kineticsBIOTECHNOLOGY PROGRESS, Issue 3 2009Chetan T. Goudar Abstract Methods for robust logistic modeling of batch and fed-batch mammalian cell cultures are presented in this study. Linearized forms of the logistic growth, logistic decline, and generalized logistic equation were derived to obtain initial estimates of the parameters by linear least squares. These initial estimates facilitated subsequent determination of refined values by nonlinear optimization using three different algorithms. Data from BHK, CHO, and hybridoma cells in batch or fed-batch cultures at volumes ranging from 100 mL,300 L were tested with the above approach and solution convergence was obtained for all three nonlinear optimization approaches for all data sets. This result, despite the sensitivity of logistic equations to parameter variation because of their exponential nature, demonstrated that robust estimation of logistic parameters was possible by this combination of linearization followed by nonlinear optimization. The approach is relatively simple and can be implemented in a spreadsheet to robustly model mammalian cell culture batch or fed-batch data. © 2009 American Institute of Chemical Engineers Biotechnol. Prog., 2009 [source] | |||||||||||||