Dynamic range of hypercubic stochastic excitable media

We study the response properties of d-dimensional hypercubic excitable networks to a stochastic stimulus. Each site, modelled either by a three-state stochastic susceptible-infected-recovered-susceptible system or by the probabilistic Greenberg-Hastings cellular automaton, is continuously and independently stimulated by an external Poisson rate h. The response function (mean density of active sites rho versus h) is obtained via simulations (for d=1, 2, 3, 4) and mean field approximations at the single-site and pair levels (for all d). In any dimension, the dynamic range of the response function is maximized precisely at the nonequilibrium phase transition to self-sustained activity, in agreement with a reasoning recently proposed. Moreover, the maximum dynamic range attained at a given dimension d is a decreasing function of d.Comment: 7 pages, 4 figure

Anticipated Synchronization in a Biologically Plausible Model of Neuronal Motifs

Two identical autonomous dynamical systems coupled in a master-slave configuration can exhibit anticipated synchronization (AS) if the slave also receives a delayed negative self-feedback. Recently, AS was shown to occur in systems of simplified neuron models, requiring the coupling of the neuronal membrane potential with its delayed value. However, this coupling has no obvious biological correlate. Here we propose a canonical neuronal microcircuit with standard chemical synapses, where the delayed inhibition is provided by an interneuron. In this biologically plausible scenario, a smooth transition from delayed synchronization (DS) to AS typically occurs when the inhibitory synaptic conductance is increased. The phenomenon is shown to be robust when model parameters are varied within physiological range. Since the DS-AS transition amounts to an inversion in the timing of the pre- and post-synaptic spikes, our results could have a bearing on spike-timing-dependent-plasticity models

Detailed reconstruction and safety analysis of a pre–Seveso accident

Industrial safety has been a topic of growing interest during the last decades, mainly because of the increased awareness and knowledge about safety issues. In this framework, the detailed reconstruction of the dynamics of an explosion (1 killed and 8 injured) occurred, on the 26th of June 1971, at Noury Italy (a plant dedicated to the production of chemicals for hardening plastics) is worth of interest and it could be used to improve actual safety guidelines related to the storage of peroxides. The accident happened before whatever Seveso Directive release. Therefore, root-causes reconstruction and related risk assessment were carried out making a comparison between a hypothetical plant layout at that time and a modern plant layout implemented with minimum safety systems, such as acoustic alarms and adequate bypass lines. The accident reconstruction was carried out by doing a deep literature research, mainly based on newspaper clippings of the time, to both remodel the accident at best and draw the most likely layout of the plant. The latter is of fundamental importance to carry out a risk assessment procedure by applying the Recursive Operability Analysis (ROA), which allows for a direct generation of the fault trees that can provide an easy estimation of the probability of occurrence of all unwanted events. This method was applied to the Noury Italy case study to show the criticalities of the storage equipment also underlining the possible improvements which could be implemented also in the ‘70s, therefore preventing the fatal explosion

Recursive operability analysis as a tool for ATEX classification in plants managing explosive dusts

Safety and prevention in workplaces are important issues, especially regards to risks with serious consequences for health and infrastructures, such as dust explosions, which have caused several industrial accidents during the last centuries and, actually, represent a critical issue in the industrial framework. The current European legislation, referred to as ATEX directive, identifies ATEX zones as parts of the plant where explosive atmospheres can be generated. In this work, a modified version of the classic Recursive Operability Analysis method, specifically tailored to define with an automatic procedure the ATEX zones related to flammable dust clouds, is proposed. The method is fast and effective, allowing for an automatic generation of fault trees from which the probability of occurrence defining the specific ATEX zone type can be estimated. This technique was successfully implemented in a chemical plant dedicated to the mixing of inert powders with a stearate powder, a hazardous dust classified as strongly explosible. The extent of all the ATEX zones identified within the plant was simulated with the ALOHA software, treating the dispersed dust cloud of stearate as a dense gas cloud. From the results, it was possible to identify not only type and extension of all the ATEX zones but also either the most critical parts of the plant or the most dangerous activities (e.g. human errors in the use of the forklift was found to account for about 97.7% to explosion probability in this type of plant)

Response of electrically coupled spiking neurons: a cellular automaton approach

Experimental data suggest that some classes of spiking neurons in the first layers of sensory systems are electrically coupled via gap junctions or ephaptic interactions. When the electrical coupling is removed, the response function (firing rate {\it vs.} stimulus intensity) of the uncoupled neurons typically shows a decrease in dynamic range and sensitivity. In order to assess the effect of electrical coupling in the sensory periphery, we calculate the response to a Poisson stimulus of a chain of excitable neurons modeled by $n$-state Greenberg-Hastings cellular automata in two approximation levels. The single-site mean field approximation is shown to give poor results, failing to predict the absorbing state of the lattice, while the results for the pair approximation are in good agreement with computer simulations in the whole stimulus range. In particular, the dynamic range is substantially enlarged due to the propagation of excitable waves, which suggests a functional role for lateral electrical coupling. For probabilistic spike propagation the Hill exponent of the response function is $\alpha=1$, while for deterministic spike propagation we obtain $\alpha=1/2$, which is close to the experimental values of the psychophysical Stevens exponents for odor and light intensities. Our calculations are in qualitative agreement with experimental response functions of ganglion cells in the mammalian retina.Comment: 11 pages, 8 figures, to appear in the Phys. Rev.

Emergence of Hierarchy on a Network of Complementary Agents

Complementarity is one of the main features underlying the interactions in biological and biochemical systems. Inspired by those systems we propose a model for the dynamical evolution of a system composed by agents that interact due to their complementary attributes rather than their similarities. Each agent is represented by a bit-string and has an activity associated to it; the coupling among complementary peers depends on their activity. The connectivity of the system changes in time respecting the constraint of complementarity. We observe the formation of a network of active agents whose stability depends on the rate at which activity diffuses in the system. The model exhibits a non-equilibrium phase transition between the ordered phase, where a stable network is generated, and a disordered phase characterized by the absence of correlation among the agents. The ordered phase exhibits multi-modal distributions of connectivity and activity, indicating a hierarchy of interaction among different populations characterized by different degrees of activity. This model may be used to study the hierarchy observed in social organizations as well as in business and other networks.Comment: 13 pages, 4 figures, submitte

Safe optimization of potentially runaway processes using topology based tools and software

In chemical industries, fast and strongly exothermic reactions are often to be carried out to synthesize a number of intermediates and final desired products. Such processes can exhibit a phenomenon known as \u201cthermal runaway\u201d that consists in a reactor temperature loss of control. During the course of the years, lots of methods, aimed to detect the set of operating parameters (e.g., dosing times, initial reactor temperature, coolant temperature, etc..) at which such a dangerous phenomenon can occur, have been developed. Moreover, in the last few years, the attention has been posed on safe process optimization, that is how to compute the set of operating parameters able to ensure high reactor productivity and, contextually, safe conditions. To achieve this goal, with particular reference to industrial semibatch synthesis carried out using both isothermal and isoperibolic temperature control mode, a dedicated optimization software has been implemented. Such a software identifies the optimum set of operating parameters using a topological criterion able to bind the so-called \u201cQFS region\u201d (where reactants accumulation is low and all the heat released is readily removed by the cooling equipment) and, then, iteratively searching for the constrained system optimum. To manage the software, only a few experimental parameters are needed; essentially: heat(s) of reaction, apparent system kinetics (Arrhenius law), threshold temperature(s) above which unwanted side reactions, decompositions or boiling phenomena are triggered, heat transfer coefficients and reactants heat capacities. Such parameters can be obtained using simple calorimetric techniques (DSC, ARC, RC1, etc..). Over the optimization section, the software posses a simulation section where both normal and upset operating conditions (such as pumps failure and external fire) can be tested

A comprehensive approach to establish the impact of worksites air emissions

Worksite activities are time-limited events associated with continuous releases of airborne pollutants, such as carbon monoxide, particulate matter, and NOx, and they impact potentially vast areas. The side-effects on the environment can be severe, and they are subject of literature studies, with the final aim of proposing solutions that may improve the management of air emissions. No general assessment method or approach is yet available to estimate their effects on the environment and workers’ health. In this work, a general procedure that can be potentially applied to every type of worksite is proposed (i.e., construction sites, upgrading of chemical plants, road sites, etc..). The approach involves a detailed assessment of emissions and their expected pollutant concentrations. A dedicated mathematical model has been defined to assess pollutant emissions over time, consistent with all the different phases of foreseen activities. Emissions are defined on base of the GANTT descriptions of the activities and air pollutant dispersion is simulated with a dedicated model. Finally, the obtained results are evaluated against air quality thresholds as defined by laws and conditioning the human health risks for workers and citizens potentially exposed to pollutants

Study of the performance of disinfection with sodium hypochlorite on a full-scale sewage treatment plant

A full-scale sewage treatment plant was investigated to assess the performance of the disinfection stage. Sodium hypochlorite was used as a disinfectant agent and the process efficiency was evaluated by E.coli removal. The research took place over a period of two years in order to evaluate the effect of retention time (t) and residual chlorine (Cr) under different seasonal conditions. The effectiveness of E.coli removal with sodium hypochlorite proved to be strictly dependent on the factor CR t (product of residual chlorine with the contact time). The regression line of the experimental points was, on the whole, well comparable with the model proposed by Collins, especially in the field of CRt lower than 30 mg L-1 min