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Network Dynamics (network + dynamics)

Distribution by Scientific Domains

Business, Economics, Finance and Accounting	41%
Life Sciences	33%

Selected Abstracts

The Neutralizer: a self-configurable failure detector for minimizing distributed storage maintenance cost

CONCURRENCY AND COMPUTATION: PRACTICE & EXPERIENCE, Issue 2 2009
Zhi Yang
Abstract To achieve high data availability or reliability in an efficient manner, distributed storage systems must detect whether an observed node failure is permanent or transient, and if necessary, generate replicas to restore the desired level of replication. Given the unpredictability of network dynamics, however, distinguishing permanent and transient failures is extremely difficult. Though timeout-based detectors can be used to avoid mistaking transient failures as permanent failures, it is unknown how the timeout values should be selected to achieve a better tradeoff between detection latency and accuracy. In this paper, we address this fundamental tradeoff from several perspectives. First, we explore the impact of different timeout values on maintenance cost by examining the probability of their false positives and false negatives. Second, we propose a self-configurable failure detector called the Neutralizer based on the idea of counteracting false positives with false negatives. The Neutralizer could enable the system to maintain a desired replication level on average with the least amount of bandwidth. We conduct extensive simulations using real trace data from a widely deployed peer-to-peer system and synthetic traces based on PlanetLab and Microsoft PCs, showing a significant reduction in aggregate bandwidth usage after applying the Neutralizer (especially in an environment with a low average node availability). Overall, we demonstrate that the Neutralizer closely approximates the performance of a perfect ,oracle' detector in many cases. Copyright © 2008 John Wiley & Sons, Ltd. [source]

Calcium homeostasis and signaling in yeast cells and cardiac myocytes

FEMS YEAST RESEARCH, Issue 8 2009
Jiangjun Cui
Abstract Calcium ions are the most ubiquitous and versatile signaling molecules in eukaryotic cells. Calcium homeostasis and signaling systems are crucial for both the normal growth of the budding yeast Saccharomyces cerevisiae and the intricate working of the mammalian heart. In this paper, we make a detailed comparison between the calcium homeostasis/signaling networks in yeast cells and those in mammalian cardiac myocytes. This comparison covers not only the components, structure and function of the networks but also includes existing knowledge on the measured and simulated network dynamics using mathematical models. Surprisingly, most of the factors known in the yeast calcium homeostasis/signaling network are conserved and operate similarly in mammalian cells, including cardiac myocytes. Moreover, the budding yeast S. cerevisiae is a simple organism that affords powerful genetic and genomic tools. Thus, exploring and understanding the calcium homeostasis/signaling system in yeast can provide a shortcut to help understand calcium homeostasis/signaling systems in mammalian cardiac myocytes. In turn, this knowledge can be used to help treat relevant human diseases such as pathological cardiac hypertrophy and heart failure. [source]

A learning rule for place fields in a cortical model: Theta phase precession as a network effect

HIPPOCAMPUS, Issue 7 2005
Silvia Scarpetta
Abstract We show that a model of the hippocampus introduced recently by Scarpetta et al. (2002, Neural Computation 14(10):2371,2396) explains the theta phase precession phenomena. In our model, the theta phase precession comes out as a consequence of the associative-memory-like network dynamics, i.e., the network's ability to imprint and recall oscillatory patterns, coded both by phases and amplitudes of oscillation. The learning rule used to imprint the oscillatory states is a natural generalization of that used for static patterns in the Hopfield model, and is based on the spike-time-dependent synaptic plasticity, experimentally observed. In agreement with experimental findings, the place cells' activity appears at consistently earlier phases of subsequent cycles of the ongoing theta rhythm during a pass through the place field, while the oscillation amplitude of the place cells' firing rate increases as the animal approaches the center of the place field and decreases as the animal leaves the center. The total phase precession of the place cell is lower than 360°, in agreement with experiments. As the animal enters a receptive field, the place cells' activity comes slightly less than 180° after the phase of maximal pyramidal cell population activity, in agreement with the findings of Skaggs et al. (1996, Hippocampus 6:149,172). Our model predicts that the theta phase is much better correlated with location than with time spent in the receptive field. Finally, in agreement with the recent experimental findings of Zugaro et al. (2005, Nature Neuroscience 9(1):67,71), our model predicts that theta phase precession persists after transient intrahippocampal perturbation. © 2005 Wiley-Liss, Inc. [source]

Brain network dynamics during error commission

HUMAN BRAIN MAPPING, Issue 1 2009
Michael C. Stevens
Abstract Previous studies suggest that the anterior cingulate and other prefrontal brain regions might form a functionally-integrated error detection network in the human brain. This study examined whole brain functional connectivity to both correct and incorrect button presses using independent component analysis (ICA) of functional magnetic resonance imaging (fMRI) data collected from 25 adolescent and 25 adult healthy participants (ages 11,37) performing a visual Go/No-Go task. Correct responses engaged a network comprising left lateral prefrontal cortex, left postcentral gyrus/inferior parietal lobule, striatum, and left cerebellum. In contrast, a similar network was uniquely engaged during errors, but this network was not integrated with activity in regions believed to be engaged for higher-order cognitive control over behavior. A medial/dorsolateral prefrontal-parietal neural network responded to all No-Go stimuli, but with significantly greater activity to errors. ICA analyses also identified a third error-related circuit comprised of anterior temporal lobe, limbic, and pregenual cingulate cortices, possibly representing an affective response to errors. There were developmental differences in error-processing activity within many of these neural circuits, typically reflecting greater hemodynamic activation in adults. These findings characterize the spatial structure of neural networks underlying error commission and identify neurobiological differences between adolescents and adults. Hum Brain Mapp 2009. © 2007 Wiley-Liss, Inc. [source]

Networking and innovation: a systematic review of the evidence

INTERNATIONAL JOURNAL OF MANAGEMENT REVIEWS, Issue 3-4 2004
Luke Pittaway
Recent work on competitiveness has emphasized the importance of business networking for innovativeness. Until recently, insights into the dynamics of this relationship have been fragmented. This paper presents a systematic review of research linking the networking behaviour of firms with their innovative capacity. We find that the principal benefits of networking as identified in the literature include: risk sharing; obtaining access to new markets and technologies; speeding products to market; pooling complementary skills; safeguarding property rights when complete or contingent contracts are not possible; and acting as a key vehicle for obtaining access to external knowledge. The evidence also illustrates that those firms which do not co-operate and which do not formally or informally exchange knowledge limit their knowledge base long term and ultimately reduce their ability to enter into exchange relationships. At an institutional level, national systems of innovation play an important role in the diffusion of innovations in terms of the way in which they shape networking activity. The paper provides evidence suggesting that network relationships with suppliers, customers and intermediaries such as professional and trade associations are important factors affecting innovation performance and productivity. Where networks fail, it is due to inter-firm conflict, displacement, lack of scale, external disruption and lack of infrastructure. The review identifies several gaps in the literature that need to be filled. For instance, there is a need for further exploration of the relationship between networking and different forms of innovation, such as process and organisational innovation. Similarly, we need better understanding of network dynamics and network configurations, as well as the role of third parties such as professional and trade associations. Our study highlights the need for interdisciplinary research in these areas. [source]

A Taxonomy of Supply Networks

JOURNAL OF SUPPLY CHAIN MANAGEMENT, Issue 4 2001
Christine M. Harland
SUMMARY There has been limited research into how different types of supply networks can be created and operated. This article develops a taxonomy of supply networks with a particular focus on managing network creation and operation. The taxonomy is based on a review of network literature from various academic perspectives and extensive empirical data across a variety of industry sectors including automotive, fast-moving consumer goods, electronics, pharmaceuticals, and communications technologies. The main differentiating factors for classifying a matrix of four types of supply network are found to be the degree of supply network dynamics and the degree of focal company supply network influence. Network characteristics and different patterns ofnetworking activities are identified for each type of supply network. [source]

Phase Biaxility in Smectic-A Side-Chain Liquid Crystalline Elastomers

MACROMOLECULAR RAPID COMMUNICATIONS, Issue 8 2009
Rebekka Storz
Abstract 2H NMR investigations on the biaxial phase behavior of smectic-A liquid crystalline side-chain elastomers are presented. Biaxiality parameters were determined by measuring the quadrupolar splitting of two spin probes, namely benzene-d6 and hexamethylbenzene-d18, at various angles between the principal director and the external magnetic field: while for a uniaxial sample the angular dependence can be described by the second Legendre polynomial, an additional asymmetric term needs to be included to fit the data of the two investigated biaxial systems. Two elastomers synthesized from mesogens that differ in the molecular geometry in order to study the molecular origin of biaxiality were compared. Biaxiality is observed for both elastomers when approaching the glass transition, suggesting that the network dynamics dominate the formation of the biaxial phase. [source]

Decentralized control strategies for dynamic routing

OPTIMAL CONTROL APPLICATIONS AND METHODS, Issue 6 2002
ftar
Abstract The routing problem in multi-destination data communication networks is considered. A dynamic model, which can incorporate arbitrary, different, time-varying processing delays at different nodes, is developed to describe the network dynamics. Based on this model, controllers for routing control are proposed. The structures of the proposed controllers are motivated by an optimal control problem. These proposed controllers are completely decentralized in the sense that all necessary on-line computations are done locally at each node. Furthermore, the information needed for these computations is related only to the queue lengths at the present node and the adjacent downstream nodes. Both cases when the controls can be continuously changed and when the controls are updated at discrete time instants are considered. In the latter case the controls at different nodes may be updated at different time instants (i.e. the network is not necessarily synchronous). It is shown that the controllers enjoy many desirable properties; in particular, they clear all the queues of the network in the absence of external message arrivals, in finite time. Furthermore, the controllers do not direct messages around a loop. They also have certain robustness properties. Some simulation results relating to a number of realistic problems are presented to illustrate various features of the controllers. Copyright © 2002 John Wiley & Sons, Ltd. [source]

Innovation alignment and project network dynamics: An integrative model for change

PROJECT MANAGEMENT JOURNAL, Issue 3 2007
John E. Taylor
Abstract Innovation research has predominantly focused on hierarchically organized firms competing within single markets. Recently, however, researchers have debated over whether the increasing use of project networks within and across industries promotes or stifles innovation. This paper discusses a model based on crossnational diffusion data from three technological innovations in three-dimensional computer-aided design (3D CAD) and related implementation data from 82 firms. From the data we induce a set of constructs that form the basis of a two-stage model for understanding innovation in project networks. In the first stage of the model the alignment of an innovation to the existing allocation of work in a project network is ascertained. In the second stage, the implementation success and diffusion outcomes for innovations misaligned with the allocation of work are governed by the relational stability, accrual of interests, boundary permeability, and existence of an agent for project network change. In developing this integrative, two-stage model we resolve the contradiction in the academic literature regarding the degree to which project network dynamics can promote or stifle innovation. [source]

The Tangled Webs of Westminster and Whitehall: The Discourse, Strategy and Practice of Networking Within the British Core Executive

PUBLIC ADMINISTRATION, Issue 1 2000
Colin Hay
In this paper we identify and seek to resolve a certain paradox in the existing litera-ture on networks and networking. Whilst earlier policy network perspectives have tended to emphasize the structural character of networks as durable, dense and relatively static organization forms, the more recent strategic network literature emphasizes the flexible, adaptive and dynamic quality of networking as a social and political practice. However, neither perspective has yet developed a theory of network formation, evolution, transformation and termination. In this paper, we seek to rectify this omission, advancing a ,strategic relational' theory of network dynamics based on a rethinking of the concept of network itself. We illustrate this perspective with respect to the policy process centred in and around Westminster and Whitehall, drawing on a series of semi-structured interviews with ministers and officials from four departments. [source]

Multiple forms of activity-dependent intrinsic plasticity in layer V cortical neurones in vivo

THE JOURNAL OF PHYSIOLOGY, Issue 13 2009
Jeanne T. Paz
Synaptic plasticity is classically considered as the neuronal substrate for learning and memory. However, activity-dependent changes in neuronal intrinsic excitability have been reported in several learning-related brain regions, suggesting that intrinsic plasticity could also participate to information storage. Compared to synaptic plasticity, there has been little exploration of the properties of induction and expression of intrinsic plasticity in an intact brain. Here, by the means of in vivo intracellular recordings in the rat we have examined how the intrinsic excitability of layer V motor cortex pyramidal neurones is altered following brief periods of repeated firing. Changes in membrane excitability were assessed by modifications in the discharge frequency versus injected current (F,I) curves. Most (,64%) conditioned neurones exhibited a long-lasting intrinsic plasticity, which was expressed either by selective changes in the current threshold or in the slope of the F,I curve, or by concomitant changes in both parameters. These modifications in the neuronal input,output relationship led to a global increase or decrease in intrinsic excitability. Passive electrical membrane properties were unaffected by the intracellular conditioning, indicating that intrinsic plasticity resulted from modifications of voltage-gated ion channels. These results demonstrate that neocortical pyramidal neurones can express in vivo a bidirectional use-dependent intrinsic plasticity, modifying their sensitivity to weak inputs and/or the gain of their input,output function. These multiple forms of experience-dependent intrinsic changes, which expand the computational abilities of individual neurones, could shape new network dynamics and thus might participate in the formation of mnemonic motor engrams. [source]