Efficient event-driven simulation of large networks of spiking neurons and dynamical synapses

A simulation procedure is described for making feasible large-scale simulations of recurrent neural networks of spiking neurons and plastic synapses. The procedure is applicable if the dynamic variables of both neurons and synapses evolve deterministically between any two successive spikes. Spikes introduce jumps in these variables, and since spike trains are typically noisy, spikes introduce stochasticity into both dynamics. Since all events in the simulation are guided by the arrival of spikes, at neurons or synapses, we name this procedure event-driven. The procedure is described in detail, and its logic and performance are compared with conventional (synchronous) simulations. The main impact of the new approach is a drastic reduction of the computational load incurred upon introduction of dynamic synaptic efficacies, which vary organically as a function of the activities of the pre- and postsynaptic neurons. In fact, the computational load per neuron in the presence of the synaptic dynamics grows linearly with the number of neurons and is only about 6% more than the load with fixed synapses. Even the latter is handled quite efficiently by the algorithm. We illustrate the operation of the algorithm in a specific case with integrate-and-fire neurons and specific spike-driven synaptic dynamics. Both dynamical elements have been found to be naturally implementable in VLSI. This network is simulated to show the effects on the synaptic structure of the presentation of stimuli, as well as the stability of the generated matrix to the neural activity it induces.     

Collective behavior of networks with linear (VLSI) integrate-and-fire neurons

We analyze in detail the statistical properties of the spike emission process of a canonical integrate-and-fire neuron, with a linear integrator and a lower bound for the depolarization, as often used in VLSI implementations (Mead, 1989). The spike statistics of such neurons appear to be qualitatively similar to conventional (exponential) integrate-and-fire neurons, which exhibit a wide variety of characteristics observed in cortical recordings. We also show that, contrary to current opinion, the dynamics of a network composed of such neurons has two stable fixed points, even in the purely excitatory network, corresponding to two different states of reverberating activity. The analytical results are compared with numerical simulations and are found to be in good agreement.     

Lupus anticoagulant, anticardiolipin antibodies and hepatitis C virus infection in thalassaemia

Anticardiolipin antibodies (ACA) and lupus anticoagulant (LA) have been detected in patients with hepatitis C virus (HCV) infection and have been associated in autoimmune diseases (i.e. systemic lupus erythematosus) with an increased risk of thromboembolic events. Because of the high prevalence of HCV infection and the thrombotic risk described in thalassaemia we decided to investigate the prevalence of ACA and LA in a cohort of 68 thalassaemia patients. We found a high prevalence (34%) of beta2-glycoprotein I independent ACA in our thalassaemia patients which was related to HCV infection. None of patients developed any complications related to antiphospholipid antibodies (APL); therefore the clinical significance of positivity for APL in patients with HCV infection is at present unclear. In conclusion, the results of our study indicate that ACA in the serum of HCV-infected thalassaemic patients exhibit the characteristics of natural autoantibodies rather than those of the pathogenic autoantibodies that are found in patients with systemic lupus erythematosus.     

De-nickelification of 70/30 Cupronickel Tubing in a Cooling Heat Exchanger

A cupronickel heat exchanger failed after 10?years of service. The cooling heat exchanger was in service in a marine environment, with water flowing along shellside and natural gas along tubeside. Many tubes showed leakages during a regular maintenance test. A comprehensive failure analysis of the heat exchanger revealed the de-nickelification process on the external side of the damaged tubes (shellside). The de-nickelification originated from a large deposit of sea water scale, over tube lengths, which completely covered the area just outside the internal portion of the tubesheet, and occurred because of a failure of the regular cleaning operation. Stagnant water and possible hot spot formations on the tubes probably caused the phenomenon.     

Comparative evaluation of the olive oil given by a new processing system

To evaluate the level of quality of the olive oil given by a new processing system we compared it with the percolation oil, which is known to be of high quality. The new product was characterized by the following: (i) higher contents of tocopherols, tyrosol- and hydroxytyrosol-aglycones, volatile aromatic substances, sterols, triterpene alcohols and lipochromes, and higher values of 1,2-diglycerides/1,3-diglycerides and campesterol/stigmasterol ratios, colour indices and turbidity; (ii) lower contents of waxes and triterpene dialcohols, and lower values for spectrophotometric indices, carbonyl index, Wolff's ratio (K₂₃₂/K₂₇₀), acidity, and peroxide number; (iii) similar contents of phenols, o- diphenols, free tyrosol, free hydroxytyrosol, triglycerides, stigmastadienes, and aliphatic alcohols, similar sensory score, and similar value of resistance to autoxidation, alcohol index, overall quality indices, and fatty acid composition.     

Diagnostic accuracy of a rural live video telepathology system

Accuracy of diagnoses rendered using a live video telepathology network was assessed for permanent sections of surgical pathology specimens. To determine accuracy, telepathology diagnoses were compared with those obtained by directly viewing the glass slide using a standard microscope. A total of 294 cases were read via both telepathology and glass slide by attending pathologists at a tertiary care medical center. Overall accuracy was defined as exact concordance between diagnoses. Clinically insignificant differences in diagnoses were excluded to determine clinically significant accuracy. For the 285 cases with complete data, the overall accuracy for telepathology was 0.912 (95% confidence interval [CI], 0.872-0.941), whereas the overall accuracy for glass slide readings was 0.968 (95% CI, 0.939-0.985). This difference is statistically significant (p = 0.009). When focusing on clinically significant discrepancies, where the difference in diagnosis might affect therapeutic decisions, the video accuracy was only slightly less than the glass slide accuracy (0.965 [95% CI, 0.934-0.982] vs. 0.982 [95% CI, 0.957-0.994], respectively), but this difference is not statistically significant (p = 0.302). Most of the cases with clinically significant differences involved lesions with inherently high interobserver variation. Certainty of diagnosis did not differ between video and glass slide readings (p = 0.911), but there was an association between certainty of diagnosis and diagnostic accuracy for video (p = 0.003 for clinically significant accuracies). Based on these findings, we recommend when using this telepathology system that only preliminary diagnoses should be given in the following situations: for diagnostic areas with known high interobserver variability; when the consultant has any degree of uncertainty about the presence or absence of the lesion in question; and when there is insufficient experience using telepathology as a diagnostic medium.     

Impact of Ge-Sb-Te compound engineering on the set operation performance in phase-change memories

The phase-change memory (PCM) technology is considered as one of the most attractive non-volatile memory concepts for next generation data storage. It relies on the ability of a chalcogenide material belonging to the Ge-Sb-Te compound system to reversibly change its phase between two stable states, namely the poly-crystalline low-resistive state and the amorphous high-resistive state, allowing the storage of the logical bit. A careful study of the phase-change material properties in terms of the set operation performance, the program window and the electrical switching parameters as a function of composition is very attractive in order to enlarge the possible PCM application spectrum. Concerning the set performance, a crystallization kinetics based interpretation of the observed behavior measured on different Ge-Sb-Te compounds is provided, allowing a physics-based comprehension of the reset-to-set transition.     

Shape optimization for drag reduction in linked bodies using evolution strategies

We present results from the shape optimization of linked bodies for drag reduction in simulations of incompressible flow at moderate Reynolds numbers. The optimization relies on the covariance matrix adaptation evolution strategy (CMA-ES) and the flow simulations use vortex methods with the Brinkman penalization to enforce boundary conditions in complex bodies. We exploit the inherent parallelism of CMA-ES, by implementing a multi-host framework which allows for the distribution of the expensive cost function evaluations across parallel architectures, without being limited to one computing facility. This study repeats in silico for the first time Ingo Rechenberg’s pioneering wind tunnel experiments for drag reduction that led to the inception of evolution strategies. The simulations confirm that the results of these experimental studies indicate a flat plate is not the optimal solution for drag reduction in linked bodies. We present the vorticity field of the flow and identify the governing mechanisms for this drag reduction by the slightly corrugated linked plate configuration.     

Dark matter in satellite galaxies: Will future Iimaging Air Cherenkov Telescopes play a role?

Addressing the question about the importance of future Imaging Air Cherenkov Telescopes (IACTs) for indirect detection of Dark Matter (DM), I will review the results of a paper by Bringmann et al. (2008) in which the authors computed the gamma-ray flux from DM annihilation associated to the halos of two reference dwarf Spheroidal galaxies, Draco and Willman 1. The previously neglected contribution of Virtual Internal Bremsstrahlung is also taken into account, which produces, when present, a bump in the higher part of the energy spectrum. The boost factor is the quantity used to describe how far the experiment is from probing interesting DM scenarios and it is computed for the next generation of IACTs, namely MAGIC-II and CTA. Due to the presence of spectral features, the boost factors can vary, becoming dependent to the characteristics of the particular DM candidate, obstacling a clear statement about prospects of detection. However, at least for the case of Willman 1 observed by CTA, boost factors may be below 10, even without considering the effect of substructures or the Sommerfeld enhancement. This, added to the comparison with the prospects of detection by the Fermi satellite, clearly demonstrate that the answer to the question in the title is an affirmative one.