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Search Results: 1 - 10 of 471236 matches for " Stephen A. Billings "
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An Orthogonal Least Squares Based Approach to FIR Designs
Xiao-Feng Wu,Zi-Qiang Lang,Stephen A Billings,

国际自动化与计算杂志 , 2005,
Abstract: This paper is concerned with the application of forward Orthogonal Least Squares (OLS) algorithm to the design of Finite Impulse Response (FIR) filters. The focus of this study is a new FIR filter design procedure and to compare this with traditional methods known as the fir2() routine provided by MATLAB.
Prepatterning in the Stem Cell Compartment
Peter D. Tonge,Victor Olariu,Daniel Coca,Visakan Kadirkamanathan,Kelly E. Burrell,Stephen A. Billings,Peter W. Andrews
PLOS ONE , 2012, DOI: 10.1371/journal.pone.0010901
Abstract: The mechanism by which an apparently uniform population of cells can generate a heterogeneous population of differentiated derivatives is a fundamental aspect of pluripotent and multipotent stem cell behaviour. One possibility is that the environment and the differentiation cues to which the cells are exposed are not uniform. An alternative, but not mutually exclusive possibility is that the observed heterogeneity arises from the stem cells themselves through the existence of different interconvertible substates that pre-exist before the cells commit to differentiate. We have tested this hypothesis in the case of apparently homogeneous pluripotent human embryonal carcinoma (EC) stem cells, which do not follow a uniform pattern of differentiation when exposed to retinoic acid. Instead, they produce differentiated progeny that include both neuronal and non-neural phenotypes. Our results suggest that pluripotent NTERA2 stem cells oscillate between functionally distinct substates that are primed to select distinct lineages when differentiation is induced.
Drift-Diffusion Analysis of Neutrophil Migration during Inflammation Resolution in a Zebrafish Model
Geoffrey R. Holmes,Giles Dixon,Sean R. Anderson,Constantino Carlos Reyes-Aldasoro,Philip M. Elks,Stephen A. Billings,Moira K. B. Whyte,Visakan Kadirkamanathan,Stephen A. Renshaw
Advances in Hematology , 2012, DOI: 10.1155/2012/792163
Abstract: Neutrophils must be removed from inflammatory sites for inflammation to resolve. Recent work in zebrafish has shown neutrophils can migrate away from inflammatory sites, as well as die in situ. The signals regulating the process of reverse migration are of considerable interest, but remain unknown. We wished to study the behaviour of neutrophils during reverse migration, to see whether they moved away from inflamed sites in a directed fashion in the same way as they are recruited or whether the inherent random component of their migration was enough to account for this behaviour. Using neutrophil-driven photoconvertible Kaede protein in transgenic zebrafish larvae, we were able to specifically label neutrophils at an inflammatory site generated by tailfin transection. The locations of these neutrophils over time were observed and fitted using regression methods with two separate models: pure-diffusion and drift-diffusion equations. While a model hypothesis test (the F-test) suggested that the datapoints could be fitted by the drift-diffusion model, implying a fugetaxis process, dynamic simulation of the models suggested that migration of neutrophils away from a wound is better described by a zero-drift, “diffusion” process. This has implications for understanding the mechanisms of reverse migration and, by extension, neutrophil retention at inflammatory sites. 1. Introduction The fate of neutrophils following completion of the inflammatory programme is of critical importance for the outcome of episodes of acute inflammation and can determine whether there is prompt healing of a wound or the development of chronic inflammation and tissue injury. Neutrophils recruited to sites of inflammation may leave the site or die in situ [1]. The most widely accepted mechanism of neutrophil disposal is the programmed cell death or apoptosis, of the neutrophil followed by macrophage uptake and clearance (reviewed in [2]). Recently, other routes have been proposed; neutrophils may move away from the inflamed site into the bloodstream (“reverse transmigration” [3]), by migration through other tissues (“retrograde chemotaxis” or “reverse migration” [4–6]), or be lost into the inflammatory exudate [7, 8]. Current understanding of the process of reverse migration is reviewed elsewhere [9]. The uncertainty as to the in vivo fates of individual cells relates in part to the difficulty in following individual cells during inflammation resolution in vivo. The transgenic zebrafish model is emerging as a key model for the study of vertebrate immunity [10] and allows direct
The Neutrophil's Eye-View: Inference and Visualisation of the Chemoattractant Field Driving Cell Chemotaxis In Vivo
Visakan Kadirkamanathan, Sean R. Anderson, Stephen A. Billings, Xiliang Zhang, Geoffrey R. Holmes, Constantino C. Reyes-Aldasoro, Philip M. Elks, Stephen A. Renshaw
PLOS ONE , 2012, DOI: 10.1371/journal.pone.0035182
Abstract: As we begin to understand the signals that drive chemotaxis in vivo, it is becoming clear that there is a complex interplay of chemotactic factors, which changes over time as the inflammatory response evolves. New animal models such as transgenic lines of zebrafish, which are near transparent and where the neutrophils express a green fluorescent protein, have the potential to greatly increase our understanding of the chemotactic process under conditions of wounding and infection from video microscopy data. Measurement of the chemoattractants over space (and their evolution over time) is a key objective for understanding the signals driving neutrophil chemotaxis. However, it is not possible to measure and visualise the most important contributors to in vivo chemotaxis, and in fact the understanding of the main contributors at any particular time is incomplete. The key insight that we make in this investigation is that the neutrophils themselves are sensing the underlying field that is driving their action and we can use the observations of neutrophil movement to infer the hidden net chemoattractant field by use of a novel computational framework. We apply the methodology to multiple in vivo neutrophil recruitment data sets to demonstrate this new technique and find that the method provides consistent estimates of the chemoattractant field across the majority of experiments. The framework that we derive represents an important new methodology for cell biologists investigating the signalling processes driving cell chemotaxis, which we label the neutrophils eye-view of the chemoattractant field.
An efficient nonlinear cardinal B-spline model for high tide forecasts at the Venice Lagoon
H. L. Wei ,S. A. Billings
Nonlinear Processes in Geophysics (NPG) , 2006,
Abstract: An efficient class of nonlinear models, constructed using cardinal B-spline (CBS) basis functions, are proposed for high tide forecasts at the Venice lagoon. Accurate short term predictions of high tides in the lagoon can easily be calculated using the proposed CBS models.
Signatures of exchange correlations in the thermopower of quantum dots
Gabriel Billings,A. Douglas Stone,Y. Alhassid
Physics , 2010, DOI: 10.1103/PhysRevB.81.205303
Abstract: We use a many-body rate-equation approach to calculate the thermopower of a quantum dot in the presence of an exchange interaction. At temperatures much smaller than the single-particle level spacing, the known quantum jumps (discontinuities) in the thermopower are split by the exchange interaction. The origin and nature of the splitting are elucidated with a simple physical argument based on the nature of the intermediate excited state in the sequential tunneling approach. We show that this splitting is sensitive to the number parity of electrons in the dot and the dot's ground-state spin. These effects are suppressed when cotunneling dominates the electrical and thermal conductances. We calculate the thermopower in the presence of elastic cotunneling, and show that some signatures of exchange correlations should still be observed with current experimental methods. In particular, we propose a method to determine the strength of the exchange interaction from measurements of the thermopower.
Analysis of Thyroid Response Element Activity during Retinal Development
Nathan A. Billings,Mark M. Emerson,Constance L. Cepko
PLOS ONE , 2012, DOI: 10.1371/journal.pone.0013739
Abstract: Thyroid hormone (TH) signaling components are expressed during retinal development in dynamic spatial and temporal patterns. To probe the competence of retinal cells to mount a transcriptional response to TH, reporters that included thyroid response elements (TREs) were introduced into developing retinal tissue. The TREs were placed upstream of a minimal TATA-box and two reporter genes, green fluorescent protein (GFP) and human placental alkaline phosphatase (PLAP). Six of the seven tested TREs were first tested in vitro where they were shown to drive TH-dependent expression. However, when introduced into the developing retina, the TREs reported in different cell types in both a TH-dependent and TH-independent manner, as well as revealed specific spatial patterns in their expression. The role of the known thyroid receptors (TR), TRα and TRβ, was probed using shRNAs, which were co-electroporated into the retina with the TREs. Some TREs were positively activated by TR+TH in the developing outer nuclear layer (ONL), where photoreceptors reside, as well as in the outer neuroblastic layer (ONBL) where cycling progenitor cells are located. Other TREs were actively repressed by TR+TH in cells of the ONBL. These data demonstrate that non-TRs can activate some TREs in a spatially regulated manner, whereas other TREs respond only to the known TRs, which also read out activity in a spatially regulated manner. The transcriptional response to even simple TREs provides a starting point for understanding the regulation of genes by TH, and highlights the complexity of transcriptional regulation within developing tissue.
Analysis of the geomagnetic activity of the Dst index and self-affine fractals using wavelet transforms
H. L. Wei, S. A. Billings,M. Balikhin
Nonlinear Processes in Geophysics (NPG) , 2004,
Abstract: The geomagnetic activity of the Dst index is analyzed using wavelet transforms and it is shown that the Dst index possesses properties associated with self-affine fractals. For example, the power spectral density obeys a power-law dependence on frequency, and therefore the Dst index can be viewed as a self-affine fractal dynamic process. In fact, the behaviour of the Dst index, with a Hurst exponent H≈0.5 (power-law exponent β≈2) at high frequency, is similar to that of Brownian motion. Therefore, the dynamical invariants of the Dst index may be described by a potential Brownian motion model. Characterization of the geomagnetic activity has been studied by analysing the geomagnetic field using a wavelet covariance technique. The wavelet covariance exponent provides a direct effective measure of the strength of persistence of the Dst index. One of the advantages of wavelet analysis is that many inherent problems encountered in Fourier transform methods, such as windowing and detrending, are not necessary.
Data derived NARMAX Dst model
R. J. Boynton, M. A. Balikhin, S. A. Billings, A. S. Sharma,O. A. Amariutei
Annales Geophysicae (ANGEO) , 2011,
Abstract: The NARMAX OLS-ERR methodology is applied to identify a mathematical model for the dynamics of the Dst index. The NARMAX OLS-ERR algorithm, which is widely used in the field of system identification, is able to identify a mathematical model for a wide class of nonlinear systems using input and output data. Solar wind-magnetosphere coupling functions, derived from analytical or data based methods, are employed as the inputs to such models and the outputs are geomagnetic indices. The newly deduced coupling function, p1/2V4/3BTsin6(θ/2), has been implemented as an input to model the Dst dynamics. It was shown that the identified model has a very good forecasting ability, especially with the geomagnetic storms.
Culture and Organizational Improvisation in UK Financial Services  [PDF]
Stephen A. Leybourne
Journal of Service Science and Management (JSSM) , 2009, DOI: 10.4236/jssm.2009.24029
Abstract: This paper considers certain aspects of a four-year program of research, and addresses the changing cultural requirements to support the rise of improvisational working practices within the UK financial services sector. Specifically, it reports on some of the outcomes of a study encompassing over 100 hours of interviews, together with a variety of other primary and secondary data. The outcomes of the full study are documented elsewhere, and they identify a number of key factors that contribute to the successful use and control of improvisational working practices. One of these factors is a supportive organizational culture, and this specific area is dealt with in this paper. A particular focus is how the sample of organizations has attempted to identify and create supportive cultural conditions for improvisational work to take place. In order to bring clarity to the outcomes of this study, a matrix of the case study organizations is also offered, which segregates those organizations according to their cultural support for improvisation and apparent improvisation effectiveness. Some comment on the current difficulties in the Financial Services sector has also been included, as it could be argued that improvisation may have contributed to shortcomings in control processes by members of that sector.
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