888 research outputs found
Logarithmic rate dependence in deforming granular materials
Rate-independence for stresses within a granular material is a basic tenet of
many models for slow dense granular flows. By contrast, logarithmic rate
dependence of stresses is found in solid-on-solid friction, in geological
settings, and elsewhere. In this work, we show that logarithmic rate-dependence
occurs in granular materials for plastic (irreversible) deformations that occur
during shearing but not for elastic (reversible) deformations, such as those
that occur under moderate repetitive compression. Increasing the shearing rate,
\Omega, leads to an increase in the stress and the stress fluctuations that at
least qualitatively resemble what occurs due to an increase in the density.
Increases in \Omega also lead to qualitative changes in the distributions of
stress build-up and relaxation events. If shearing is stopped at t=0, stress
relaxations occur with \sigma(t)/ \sigma(t=0) \simeq A \log(t/t_0). This
collective relaxation of the stress network over logarithmically long times
provides a mechanism for rate-dependent strengthening.Comment: 4 pages, 5 figures. RevTeX
Signatures of granular microstructure in dense shear flows
Granular materials react to shear stresses differently than do ordinary
fluids. Rather than deforming uniformly, materials such as dry sand or
cohesionless powders develop shear bands: narrow zones containing large
relative particle motion leaving adjacent regions essentially rigid[1,2,3,4,5].
Since shear bands mark areas of flow, material failure and energy dissipation,
they play a crucial role for many industrial, civil engineering and geophysical
processes[6]. They also appear in related contexts, such as in lubricating
fluids confined to ultra-thin molecular layers[7]. Detailed information on
motion within a shear band in a three-dimensional geometry, including the
degree of particle rotation and inter-particle slip, is lacking. Similarly,
only little is known about how properties of the individual grains - their
microstructure - affect movement in densely packed material[5]. Combining
magnetic resonance imaging, x-ray tomography, and high-speed video particle
tracking, we obtain the local steady-state particle velocity, rotation and
packing density for shear flow in a three-dimensional Couette geometry. We find
that key characteristics of the granular microstructure determine the shape of
the velocity profile.Comment: 5 pages, incl. 4 figure
Complexity without chaos: Plasticity within random recurrent networks generates robust timing and motor control
It is widely accepted that the complex dynamics characteristic of recurrent
neural circuits contributes in a fundamental manner to brain function. Progress
has been slow in understanding and exploiting the computational power of
recurrent dynamics for two main reasons: nonlinear recurrent networks often
exhibit chaotic behavior and most known learning rules do not work in robust
fashion in recurrent networks. Here we address both these problems by
demonstrating how random recurrent networks (RRN) that initially exhibit
chaotic dynamics can be tuned through a supervised learning rule to generate
locally stable neural patterns of activity that are both complex and robust to
noise. The outcome is a novel neural network regime that exhibits both
transiently stable and chaotic trajectories. We further show that the recurrent
learning rule dramatically increases the ability of RRNs to generate complex
spatiotemporal motor patterns, and accounts for recent experimental data
showing a decrease in neural variability in response to stimulus onset
Resolving the infinitude controversy
A simple inductive argument shows natural languages to have infinitly many sentences, but workers in the field have uncovered clear evidence of a diverse group of ‘exceptional’ languages from Proto-Uralic to Dyirbal and most recently, Pirahã, that appear to lack recursive devices entirely. We argue that in an information-theoretic setting non-recursive natural languages appear neither exceptional nor functionally inferior to the recursive majority
Destabilization of the Dystrophin-Glycoprotein Complex without Functional Deficits in α-Dystrobrevin Null Muscle
α-Dystrobrevin is a component of the dystrophin-glycoprotein complex (DGC) and is thought to have both structural and signaling roles in skeletal muscle. Mice deficient for α-dystrobrevin (adbn−/−) exhibit extensive myofiber degeneration and neuromuscular junction abnormalities. However, the biochemical stability of the DGC and the functional performance of adbn−/− muscle have not been characterized. Here we show that the biochemical association between dystrophin and β-dystroglycan is compromised in adbn−/− skeletal muscle, suggesting that α-dystrobrevin plays a structural role in stabilizing the DGC. However, despite muscle cell death and DGC destabilization, costamere organization and physiological performance is normal in adbn−/− skeletal muscle. Our results demonstrate that myofiber degeneration alone does not cause functional deficits and suggests that more complex pathological factors contribute to the development of muscle weakness in muscular dystrophy
The emerging structure of the Extended Evolutionary Synthesis: where does Evo-Devo fit in?
The Extended Evolutionary Synthesis (EES) debate is gaining ground in contemporary evolutionary biology. In parallel, a number of philosophical standpoints have emerged in an attempt to clarify what exactly is represented by the EES. For Massimo Pigliucci, we are in the wake of the newest instantiation of a persisting Kuhnian paradigm; in contrast, Telmo Pievani has contended that the transition to an EES could be best represented as a progressive reformation of a prior Lakatosian scientific research program, with the extension of its Neo-Darwinian core and the addition of a brand-new protective belt of assumptions and auxiliary hypotheses. Here, we argue that those philosophical vantage points are not the only ways to interpret what current proposals to ‘extend’ the Modern Synthesis-derived ‘standard evolutionary theory’ (SET) entail in terms of theoretical change in evolutionary biology. We specifically propose the image of the emergent EES as a vast network of models and interweaved representations that, instantiated in diverse practices, are connected and related in multiple ways. Under that assumption, the EES could be articulated around a paraconsistent network of evolutionary theories (including some elements of the SET), as well as models, practices and representation systems of contemporary evolutionary biology, with edges and nodes that change their position and centrality as a consequence of the co-construction and stabilization of facts and historical discussions revolving around the epistemic goals of this area of the life sciences. We then critically examine the purported structure of the EES—published by Laland and collaborators in 2015—in light of our own network-based proposal. Finally, we consider which epistemic units of Evo-Devo are present or still missing from the EES, in preparation for further analyses of the topic of explanatory integration in this conceptual framework
Acute and Persistent Mycobacterium tuberculosis Infections Depend on the Thiol Peroxidase TPX
The macrophage is the natural niche of Mycobacterium tuberculosis infection. In order to combat oxidative and nitrosative stresses and persist in macrophages successfully, M. tuberculosis is endowed with a very efficient antioxidant complex. Amongst these antioxidant enzymes, TpX is the only one in M. tuberculosis with sequence homology to thiol peroxidase. Previous reports have demonstrated that the M. tuberculosis TpX protein functions as a peroxidase in vitro. It is the dominant antioxidant which protects M. tuberculosis against oxidative and nitrosative stresses. The level of the protein increases in oxidative stress. To determine the roles of tpx gene in M. tuberculosis survival and virulence in vivo, we constructed an M. tuberculosis strain lacking the gene. The characteristics of the mutant were examined in an in vitro stationary phase model, in response to stresses; in murine bone marrow derived macrophages and in an acute and an immune resistant model of murine tuberculosis. The tpx mutant became sensitive to H2O2 and NO compared to the wild type strain. Enzymatic analysis using bacterial extracts from the WT and the tpx mutant demonstrated that the mutant contains reduced peroxidase activity. As a result of this, the mutant failed to grow and survive in macrophages. The growth deficiency in macrophages became more pronounced after interferon-γ activation. In contrast, its growth was significantly restored in the macrophages of inducible nitric oxide synthase (iNOS or NOS2) knockout mice. Moreover, the tpx mutant was impaired in its ability to initiate an acute infection and to maintain a persistent infection. Its virulence was attenuated. Our results demonstrated that tpx is required for M. tuberculosis to deal with oxidative and nitrosative stresses, to survive in macrophages and to establish acute and persistent infections in animal tuberculosis models
Analytic philosophy for biomedical research: the imperative of applying yesterday's timeless messages to today's impasses
The mantra that "the best way to predict the future is to invent it" (attributed to the computer scientist Alan Kay) exemplifies some of the expectations from the technical and innovative sides of biomedical research at present. However, for technical advancements to make real impacts both on patient health and genuine scientific understanding, quite a number of lingering challenges facing the entire spectrum from protein biology all the way to randomized controlled trials should start to be overcome. The proposal in this chapter is that philosophy is essential in this process. By reviewing select examples from the history of science and philosophy, disciplines which were indistinguishable until the mid-nineteenth century, I argue that progress toward the many impasses in biomedicine can be achieved by emphasizing theoretical work (in the true sense of the word 'theory') as a vital foundation for experimental biology. Furthermore, a philosophical biology program that could provide a framework for theoretical investigations is outlined
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