Bifurcation analysis of a normal form for excitable media: Are stable dynamical alternans on a ring possible?

We present a bifurcation analysis of a normal form for travelling waves in one-dimensional excitable media. The normal form which has been recently proposed on phenomenological grounds is given in form of a differential delay equation. The normal form exhibits a symmetry preserving Hopf bifurcation which may coalesce with a saddle-node in a Bogdanov-Takens point, and a symmetry breaking spatially inhomogeneous pitchfork bifurcation. We study here the Hopf bifurcation for the propagation of a single pulse in a ring by means of a center manifold reduction, and for a wave train by means of a multiscale analysis leading to a real Ginzburg-Landau equation as the corresponding amplitude equation. Both, the center manifold reduction and the multiscale analysis show that the Hopf bifurcation is always subcritical independent of the parameters. This may have links to cardiac alternans which have so far been believed to be stable oscillations emanating from a supercritical bifurcation. We discuss the implications for cardiac alternans and revisit the instability in some excitable media where the oscillations had been believed to be stable. In particular, we show that our condition for the onset of the Hopf bifurcation coincides with the well known restitution condition for cardiac alternans.Comment: to be published in Chao

Cultures of caste and rural development in the social network of a south Indian village

Cultures of caste in much of rural India have become entangled with institutions of rural development. In community-driven development, emphasis on “local resource persons” and “community spokespersons” has created new opportunities for brokerage and patronage within some villages, which interact with existing forms of authority and community afforded by caste identity and intra-caste headmanship. In this article, we study how these entangled cultures of caste and development translate into social network structures using data on friendship ties from a south Indian village. We find that although caste continues to be important in shaping community structures and leadership in the village’s network, its influence varies across different communities. This fluidity of caste’s influence on community network structures is argued to be the result of multiple distinct yet partially overlapping cultural-political forces, which include sharedness afforded by caste identity and new forms of difference and inequality effected through rural development

Instability and Spatiotemporal Dynamics of Alternans in Paced Cardiac Tissue

We derive an equation that governs the spatiotemporal dynamics of small amplitude alternans in paced cardiac tissue. We show that a pattern-forming linear instability leads to the spontaneous formation of stationary or traveling waves whose nodes divide the tissue into regions with opposite phase of oscillation of action potential duration. This instability is important because it creates dynamically an heterogeneous electrical substrate for inducing fibrillation if the tissue size exceeds a fraction of the pattern wavelength. We compute this wavelength analytically as a function of three basic length scales characterizing dispersion and inter-cellular electrical coupling.Comment: 4 pages, 3 figures, submitted to PR

A unified model for holistic power usage in cloud datacenter servers

Cloud datacenters are compute facilities formed by hundreds and thousands of heterogeneous servers requiring significant power requirements to operate effectively. Servers are composed by multiple interacting sub-systems including applications, microelectronic processors, and cooling which reflect their respective power profiles via different parameters. What is presently unknown is how to accurately model the holistic power usage of the entire server when including all these sub-systems together. This becomes increasingly challenging when considering diverse utilization patterns, server hardware characteristics, air and liquid cooling techniques, and importantly quantifying the non-electrical energy cost imposed by cooling operation. Such a challenge arises due to the need for multi-disciplinary expertise required to study server operation holistically. This work provides a unified model for capturing holistic power usage within Cloud datacenter servers. Constructed through controlled laboratory experiments, the model captures the relationship of server power usage between software, hardware, and cooling agnostic of architecture and cooling type (air and liquid). An exciting prospect is the ability to quantify the amount of non-electrical power consumed through cooling, allowing for more realistic and accurate server power profiles. This work represents the first empirically supported analysis and modeling of holistic power usage for Cloud datacenter servers, and bridges a significant gap between computer science and mechanical engineering research. Model validation through experiments demonstrates an average standard error of 3% for server power usage within both air and liquid cooled environments

The United States Army Ocular Teleconsultation program 2004 through 2009

PURPOSE: To describe the United States Army Ocular Teleconsultation program and all consultations received from its inception in July 2004 through December 2009. DESIGN: Retrospective, noncomparative, consecutive case series. METHODS: All 301 consecutive ocular teleconsultations received were reviewed. The main outcome measures were differential diagnosis, evacuation recommendations, and origination of consultation. Secondary measures included patient demographics, reason for consultation, and inclusion of clinical images. RESULTS: The average response time was 5 hours and 41 minutes. Most consultations originated from Iraq (58.8%) and Afghanistan (18.6%). Patient care-related requests accounted for 94.7% of consultations; nonphysicians submitted 26.3% of consultations. Most patients (220/285; 77.2%) were United States military personnel; the remainder included local nationals and coalition forces. Children accounted for 23 consultations (8.1%). Anterior segment disease represented the largest grouping of cases (129/285; 45.3%); oculoplastic problems represented nearly one quarter (68/285; 23.9%). Evacuation was recommended in 123 (43.2%) of 285 cases and in 21 (58.3%) of 36 cases associated with trauma. Photographs were included in 38.2%, and use was highest for pediatric and strabismus (83.3%) and oculoplastic (67.6%) consultations. Consultants facilitated evacuation in 87 (70.7%) of 123 consultations where evacuation was recommended and avoided unnecessary evacuations in 28 (17.3%) of 162 consultations. CONCLUSIONS: This teleconsultation program has brought valuable tertiary level support to deployed providers, thereby helping to facilitate appropriate and timely referrals, and in some cases avoiding unnecessary evacuation. Advances in remote diagnostic and imaging technology could further enhance consultant support to distant providers and their patients

The United States Army Ocular Teleconsultation program 2004 through 2009

PURPOSE: To describe the United States Army Ocular Teleconsultation program and all consultations received from its inception in July 2004 through December 2009. DESIGN: Retrospective, noncomparative, consecutive case series. METHODS: All 301 consecutive ocular teleconsultations received were reviewed. The main outcome measures were differential diagnosis, evacuation recommendations, and origination of consultation. Secondary measures included patient demographics, reason for consultation, and inclusion of clinical images. RESULTS: The average response time was 5 hours and 41 minutes. Most consultations originated from Iraq (58.8%) and Afghanistan (18.6%). Patient care-related requests accounted for 94.7% of consultations; nonphysicians submitted 26.3% of consultations. Most patients (220/285; 77.2%) were United States military personnel; the remainder included local nationals and coalition forces. Children accounted for 23 consultations (8.1%). Anterior segment disease represented the largest grouping of cases (129/285; 45.3%); oculoplastic problems represented nearly one quarter (68/285; 23.9%). Evacuation was recommended in 123 (43.2%) of 285 cases and in 21 (58.3%) of 36 cases associated with trauma. Photographs were included in 38.2%, and use was highest for pediatric and strabismus (83.3%) and oculoplastic (67.6%) consultations. Consultants facilitated evacuation in 87 (70.7%) of 123 consultations where evacuation was recommended and avoided unnecessary evacuations in 28 (17.3%) of 162 consultations. CONCLUSIONS: This teleconsultation program has brought valuable tertiary level support to deployed providers, thereby helping to facilitate appropriate and timely referrals, and in some cases avoiding unnecessary evacuation. Advances in remote diagnostic and imaging technology could further enhance consultant support to distant providers and their patients

MPGDs in Compton imaging with liquid-xenon

The interaction of radiation with liquid xenon, inducing both scintillation and ionization signals, is of particular interest for Compton-sequences reconstruction. We report on the development and recent results of a liquid-xenon time-projection chamber, dedicated to a novel nuclear imaging technique named "3 gamma imaging". In a first prototype, the scintillation is detected by a vacuum photomultiplier tube and the charges are collected with a MICROMEGAS structure; both are fully immersed in liquid xenon. In view of the final large-area detector, and with the aim of minimizing dead-zones, we are investigating a gaseous photomultiplier for recording the UV scintillation photons. The prototype concept is presented as well as preliminary results in liquid xenon. We also present soft x-rays test results of a gaseous photomultiplier prototype made of a double Thick Gaseous Electron Multiplier (THGEM) at normal temperature and pressure conditions.Comment: presented at MPGD09, CRETE, June 2009; to be published in JINST Proceedings, PDF, 10 pages, 11 figure