We investigate spontaneous critical dynamics of excitatory and inhibitory (EI) sparsely connected populations of spiking leaky integrate-and-fire neurons with conductance-based synapses. We use a ...bottom-up approach to derive a single neuron gain function and a linear Poisson neuron approximation which we use to study mean-field dynamics of the EI population and its bifurcations. In the low firing rate regime, the quiescent state loses stability due to saddle-node or Hopf bifurcations. In particular, at the Bogdanov-Takens (BT) bifurcation point which is the intersection of the Hopf bifurcation and the saddle-node bifurcation lines of the 2D dynamical system, the network shows avalanche dynamics with power-law avalanche size and duration distributions. This matches the characteristics of low firing spontaneous activity in the cortex. By linearizing gain functions and excitatory and inhibitory nullclines, we can approximate the location of the BT bifurcation point. This point in the control parameter phase space corresponds to the internal balance of excitation and inhibition and a slight excess of external excitatory input to the excitatory population. Due to the tight balance of average excitation and inhibition currents, the firing of the individual cells is fluctuation-driven. Around the BT point, the spiking of neurons is a Poisson process and the population average membrane potential of neurons is approximately at the middle of the operating interval
V
Rest
,
V
th
. Moreover, the EI network is close to both oscillatory and active-inactive phase transition regimes.
Abstract
A common technique for simulating non–Newtonian fluid dynamics, such as snow avalanches, is to solve the Shallow Water Equations (SWE), together with a rheological model describing the ...momentum dissipation by shear stresses. Friction and cohesion terms are commonly modelled using the Voellmy friction model and, recently, the Bartelt cohesion model. Here, an adaptation of the Roe scheme that ensures the balance between the flux and pressure gradients and the friction source term is presented. An upwind scheme was used for the discretisation of the SWE numerical fluxes and the non–velocity-dependent terms of the friction–cohesion model, whereas a centred scheme was used for the velocity-dependent source terms. The model was tested in analytically solvable settings, laboratory experiments and real cases. In all cases, the model performed well, avoiding numerical instabilities and achieving stable and consistent solution even for an avalanche stopping on a sloping terrain.
Dry‐snow slab avalanches release due to the formation of a crack in a weak layer buried below cohesive snow slabs, followed by rapid crack propagation. The onset of rapid crack propagation occurs if ...stresses at the crack tip in the weak layer overcome its strength. In this study, we use the finite element method to evaluate the maximum shear stress τmax induced by a preexisting crack in a weak snow layer allowing for the bending of the overlaying slab. It is shown that τmax increases with increasing crack length, slab thickness, slab density, weak layer elastic modulus, and slope angle. In contrast, τmax decreases with increasing elastic modulus of the slab. Assuming a realistic failure envelope, we computed the critical crack length ac for the onset of crack propagation. The model allows for remote triggering from flat (or low angle) terrain. Yet it shows that the critical crack length decreases with increasing slope angle.
Plain Language Summary
Dry‐snow slab avalanches release due to the formation of a crack in a weak layer buried below cohesive snow slabs, followed by rapid crack propagation. Characterizing conditions for the onset of crack propagation in snow is a great challenge and has been the subject of several investigations. Yet there is still no consensus about the nature of the initial failure in the weak layer, whether it occurs in shear only or if the collapse of the weak layer helps to drive crack propagation. Here, to investigate this question, we employed a numerical model to study stress concentrations in the weak layer in the presence of a preexisting crack, allowing the bending of the overlaying slab. We computed the maximum shear stress close to the crack tip for different system configurations and mechanical properties. We showed that steeper slopes promote crack propagation as predicted by classical shear models. However, the collapse of the weak layer is essential for crack propagation from flat terrain and thus remote avalanche triggering.
Key Points
We evaluate the effect of slab deformation on the onset of crack propagation in buried weak snow layers using the finite element method
The critical crack length for the onset of crack propagation decreases with increasing slope angle
Slab bending, induced by weak layer collapse, is essential for crack propagation from flat terrain and thus remote avalanche triggering
Rapidly growing empirical evidence supports the hypothesis that the cortex operates near criticality. Although the confirmation of this hypothesis would mark a significant advance in fundamental ...understanding of cortical physiology, a natural question arises: What functional benefits are endowed to cortical circuits that operate at criticality? In this review, we first describe an introductory-level thought experiment to provide the reader with an intuitive understanding of criticality. Second, we discuss some practical approaches for investigating criticality. Finally, we review quantitative evidence that three functional properties of the cortex are optimized at criticality: 1) dynamic range, 2) information transmission, and 3) information capacity. We focus on recently reported experimental evidence and briefly discuss the theory and history of these ideas.
Among glacier instabilities, collapses of large parts of low-angle glaciers are a striking, exceptional phenomenon. So far, merely the 2002 collapse of Kolka Glacier in the Caucasus Mountains and the ...2016 twin detachments of the Aru glaciers in western Tibet have been well documented. Here we report on the previously unnoticed collapse of an unnamed cirque glacier in the Central Andes of Argentina in March 2007. Although of much smaller ice volume, this 4.2±0.6Ã106 m.sup.3 collapse in the Andes is similar to the Caucasus and Tibet ones in that the resulting ice avalanche travelled a total distance of â¼2 km over a surprisingly low angle of reach (â¼5.sup.â).
Snow is an important environmental variable and a primary water resource in many areas of the world. Monitoring seasonal snowpack properties is also crucial for properly managing snow-related hazards ...such as snow avalanches and snowmelt floods. Recently, an innovative radar architecture, based on the use of two receivers, has been proposed for snowpack monitoring for the case of dry snow, where the snowpack depth and bulk density can be calculated with one single radar measurement, without any kind of external aid. This article presents the extension of this innovative radar architecture for the case of wet snow. The approach to determine, not only the snowpack depth and bulk density but also the liquid water content, is outlined and discussed in detail, along with the experimental validation of the operating principle for two cases.
Rock avalanches are a form of hazardous long-runout landslide and leave fragmented deposits of complex sedimentology that, if studied in detail, can provide insight into their emplacement processes. ...Complexity arises due to the myriad overlapping factors known to contribute to the final deposit fabric, such as source structures, lithology (i.e. material properties), topographic feedback, substrate interaction and emplacement processes (i.e. internal factors), as well as our reliance on (un)suitable exposures. Herein, we present sedimentological data from two carbonate rock avalanche deposits (Tschirgant in Austria and Flims in Switzerland), where changes in lithology can be eliminated from the causal equation due to their largely mono-mineralic composition. We further eliminated the effects of external influences such as topography or substrate interactions by detailed facies mapping of the deposit interior. Since sedimentary properties locally vary within less than 1-m
2
outcrop area, emplacement processes are the only causes that remain to explain the different fabrics. Characteristic (fractal) grain size distributions of three distinctive sub-facies in the interior of these, and other, rock avalanche deposits—jigsaw-fractured, fragmented, and shear zone facies—can be linked to specific processes acting during emplacement. We suggest that a heterogeneously distributed and progressively increasing particle breakage in the moving granular mass best explains the ranges of fractal dimensions and associated features for the respective sub-facies, from simple breakage along pre-existing planes, through dynamic fragmentation which locally minimises coordination number, to zones of shear concentration. No exotic emplacement mechanisms (such as air-layer lubrication or fluidised substrates) are required to produce these features; continued, heterogeneous degrees of fragmentation of an initially intact source rock best explains the sedimentary record of rock avalanches.
En Europe, 130 personnes périssent chaque année en avalanche, essentiellement lors de la pratique de sports de montagne. Cet article se focalise sur une innovation technique destinée à éviter ...l’ensevelissement lorsqu’une personne est emportée par une coulée : l’airbag d’avalanche. L’objectif est de réaliser un état des connaissances sur ce nouvel équipement, en privilégiant l’étude de son efficacité.
Le protocole de revue systématique mis en oeuvre, vérifiable et reproductible, donne accès aux données de recherche publiées ces vingt dernières années (1996–2016).
Dix-sept publications spécifiquement consacrées à la question ont été recensées. Une première période (1996–2002) est marquée par la mise en oeuvre d’expérimentations relativement fragiles (en termes de conditions comme d’échantillons), conduisant à des conclusions hâtives et flatteuses pour les fabricants d’airbags, notamment en termes de survie potentielle des victimes équipées (alors représentées par des mannequins). Bénéficiant d’informations fiabilisées sur les accidents d’avalanche impliquant des personnes équipées d’airbags, les publications postérieures à 2002, plus rigoureuses sur le plan de la méthode, fournissent des résultats plus précis qui confirment l’efficacité des airbags, tout en la relativisant sur plusieurs plans : diminution du « taux de survie » constaté, évocation croissante des situations et contextes particuliers pouvant altérer l’efficacité du dispositif (mise en avant d’effets pervers), précautions accrues dans l’interprétation des données.
Cette mise au point semble d’autant plus nécessaire que les chiffres avancés par les producteurs et les médias spécialisés constituent la principale influence poussant à investir dans un airbag. Au-delà de l’affinement progressif des études centrées sur l’efficacité des airbags, il reste à investiguer, parallèlement, l’influence de la possession d’un airbag sur le processus décisionnel sur le terrain.
In Europe, 130 people die every year in avalanches, mostly while practicing mountain sports. This paper focuses on a technical innovation aiming to avoid burial when a person is caught in an avalanche: the avalanche airbag. We undertook a literature review focused on the study of the effectiveness of such device.
A systematic review protocol has been implemented. It is reproducible and gave access to 17 publications dealing with this specific topic during the last twenty years (1996–2016).
A first period (1996–2002) is characterized by the implementation of relatively fragile experiments (in terms of conditions as well as samples), leading to hasty and flattering conclusions for airbag manufacturers, especially in terms of survival rates of the equipped victims (then represented by mannequins). Benefiting from reliable information on avalanche accidents involving people equipped with airbags, post-2002 publications, much more rigorous in terms of methods, provide more accurate results that both confirm the effectiveness of airbags, and relativize it on several levels: a decrease in the “survival rate”, a growing evocation of particular situations and contexts that could alter the effectiveness of the system, and increased precautions in the interpretation of data.
This synthesis seems all the more necessary as the figures put forward by the producers and the specialized media constitute the main influencing force to invest in an airbag. Beyond the gradual refining of studies centering on the effectiveness of airbags, the influence of the possession of an airbag on the decision-making process on the field remains to investigate.
Dry-snow slab avalanche release is preceded by a fracture process within the snowpack. Recognizing weak-layer collapse as an integral part of the fracture process is crucial and explains phenomena ...such as whumpf sounds and remote triggering of avalanches from low-angle terrain. In this two-part work we propose a novel closed-form analytical model for a snowpack under skier loading and a mixed-mode failure criterion for the nucleation of weak-layer failure.
Nitrotyrosine is one of the post-translational modifications (PTMs) in proteins that occurs when their tyrosine residue is nitrated. Compared with healthy people, a remarkably increased level of ...nitrotyrosine is detected in those suffering from rheumatoid arthritis, septic shock, and coeliac disease. Given an uncharacterized protein sequence that contains many tyrosine residues, which one of them can be nitrated and which one cannot? This is a challenging problem, not only directly related to in-depth understanding the PTM's mechanism but also to the nitrotyrosine-based drug development. Particularly, with the avalanche of protein sequences generated in the postgenomic age, it is highly desired to develop a high throughput tool in this regard. Here, a new predictor called "iNitro-Tyr" was developed by incorporating the position-specific dipeptide propensity into the general pseudo amino acid composition for discriminating the nitrotyrosine sites from non-nitrotyrosine sites in proteins. It was demonstrated via the rigorous jackknife tests that the new predictor not only can yield higher success rate but also is much more stable and less noisy. A web-server for iNitro-Tyr is accessible to the public at http://app.aporc.org/iNitro-Tyr/. For the convenience of most experimental scientists, we have further provided a protocol of step-by-step guide, by which users can easily get their desired results without the need to follow the complicated mathematics that were presented in this paper just for the integrity of its development process. It has not escaped our notice that the approach presented here can be also used to deal with the other PTM sites in proteins.
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