The perils of thresholding

The thresholding of time series of activity or intensity is frequently used to define and differentiate events. This is either implicit, for example due to resolution limits, or explicit, in order to filter certain small scale physics from the supposed true asymptotic events. Thresholding the birth-death process, however, introduces a scaling region into the event size distribution, which is characterised by an exponent that is unrelated to the actual asymptote and is rather an artefact of thresholding. As a result, numerical fits of simulation data produce a range of exponents, with the true asymptote visible only in the tail of the distribution. This tail is increasingly difficult to sample as the threshold is increased. In the present case, the exponents and the spurious nature of the scaling region can be determined analytically, thus demonstrating the way in which thresholding conceals the true asymptote. The analysis also suggests a procedure for detecting the influence of the threshold by means of a data collapse involving the threshold-imposed scale.Comment: 16 pages, 10 figure

Automatic design of mechanical metamaterial actuators

Mechanical metamaterial actuators achieve pre-determined input\u2013output operations exploiting architectural features encoded within a single 3D printed element, thus removing the need for assembling different structural components. Despite the rapid progress in the field, there is still a need for efficient strategies to optimize metamaterial design for a variety of functions. We present a computational method for the automatic design of mechanical metamaterial actuators that combines a reinforced Monte Carlo method with discrete element simulations. 3D printing of selected mechanical metamaterial actuators shows that the machine-generated structures can reach high efficiency, exceeding human-designed structures. We also show that it is possible to design efficient actuators by training a deep neural network which is then able to predict the efficiency from the image of a structure and to identify its functional regions. The elementary actuators devised here can be combined to produce metamaterial machines of arbitrary complexity for countless engineering applications

Testing the robustness of laws of polysemy and brevity versus frequency

The pioneering research of G.K. Zipf on the relationship between word frequency and other word features led to the formulation of various linguistic laws. Here we focus on a couple of them: the meaning-frequency law, i.e. the tendency of more frequent words to be more polysemous, and the law of abbreviation, i.e. the tendency of more frequent words to be shorter. Here we evaluate the robustness of these laws in contexts where they have not been explored yet to our knowledge. The recovery of the laws again in new conditions provides support for the hypothesis that they originate from abstract mechanisms.Peer ReviewedPostprint (author's final draft

Tau oligomers impair memory and induce synaptic and mitochondrial dysfunction in wild-type mice

<p>Abstract</p> <p>Background</p> <p>The correlation between neurofibrillary tangles of tau and disease progression in the brains of Alzheimer's disease (AD) patients remains an area of contention. Innovative data are emerging from biochemical, cell-based and transgenic mouse studies that suggest that tau oligomers, a pre-filament form of tau, may be the most toxic and pathologically significant tau aggregate.</p> <p>Results</p> <p>Here we report that oligomers of recombinant full-length human tau protein are neurotoxic in vivo after subcortical stereotaxic injection into mice. Tau oligomers impaired memory consolidation, whereas tau fibrils and monomers did not. Additionally, tau oligomers induced synaptic dysfunction by reducing the levels of synaptic vesicle-associated proteins synaptophysin and septin-11. Tau oligomers produced mitochondrial dysfunction by decreasing the levels of NADH-ubiquinone oxidoreductase (electron transport chain complex I), and activated caspase-9, which is related to the apoptotic mitochondrial pathway.</p> <p>Conclusions</p> <p>This study identifies tau oligomers as an acutely toxic tau species in vivo, and suggests that tau oligomers induce neurodegeneration by affecting mitochondrial and synaptic function, both of which are early hallmarks in AD and other tauopathies. These results open new avenues for neuroprotective intervention strategies of tauopathies by targeting tau oligomers.</p

The role of suction thrust in the metachronal paddles of swimming invertebrates

© The Author(s), 2020. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Colin, S. P., Costello, J. H., Sutherland, K. R., Gemmell, B. J., Dabiri, J. O., & Du Clos, K. T. The role of suction thrust in the metachronal paddles of swimming invertebrates. Scientific Reports, 10(1), (2020): 17790, doi:10.1038/s41598-020-74745-y.An abundance of swimming animals have converged upon a common swimming strategy using multiple propulsors coordinated as metachronal waves. The shared kinematics suggest that even morphologically and systematically diverse animals use similar fluid dynamic relationships to generate swimming thrust. We quantified the kinematics and hydrodynamics of a diverse group of small swimming animals who use multiple propulsors, e.g. limbs or ctenes, which move with antiplectic metachronal waves to generate thrust. Here we show that even at these relatively small scales the bending movements of limbs and ctenes conform to the patterns observed for much larger swimming animals. We show that, like other swimming animals, the propulsors of these metachronal swimmers rely on generating negative pressure along their surfaces to generate forward thrust (i.e., suction thrust). Relying on negative pressure, as opposed to high pushing pressure, facilitates metachronal waves and enables these swimmers to exploit readily produced hydrodynamic structures. Understanding the role of negative pressure fields in metachronal swimmers may provide clues about the hydrodynamic traits shared by swimming and flying animals.This work was funded by National Science Foundation (NSF OCE 1829913 to SPC), the Alfred P. Sloan Foundation (to BJG) and the Gordon and Betty Moore Foundation (8835 to KRS). The work was also supported by the Roger Williams Foundation to Promote Scholarship and Teaching

Replica theory for learning curves for Gaussian processes on random graphs

Statistical physics approaches can be used to derive accurate predictions for the performance of inference methods learning from potentially noisy data, as quantified by the learning curve defined as the average error versus number of training examples. We analyse a challenging problem in the area of non-parametric inference where an effectively infinite number of parameters has to be learned, specifically Gaussian process regression. When the inputs are vertices on a random graph and the outputs noisy function values, we show that replica techniques can be used to obtain exact performance predictions in the limit of large graphs. The covariance of the Gaussian process prior is defined by a random walk kernel, the discrete analogue of squared exponential kernels on continuous spaces. Conventionally this kernel is normalised only globally, so that the prior variance can differ between vertices; as a more principled alternative we consider local normalisation, where the prior variance is uniform

Dual-arm dexterous mobile manipulator with new omnidirectional wheels

[ES] Este artículo describe un manipulador móvil, bimanual y con capacidad de manipulación diestra denominado MADAR (de Mobile Anthropomorphic Dual-Arm Robot). Básicamente puede dividirse en dos partes, una base móvil y una estructura superior portando dos brazos en configuración antropomorfa con manos mecánicas diestras equipadas con sensores táctiles. La base, completamente de desarrollo propio, es de forma circular y tiene tres ruedas con un diseño novedoso que permiten una movilidad omnidireccional. La estructura superior integra elementos comerciales, como los brazos, las manos y distintos sensores, que han sido adaptados para su funcionamiento conjunto. El artículo incluye tanto la descripción de los principales elementos del hardware como del software desarrollado para su control y uso.[EN] This article describes a mobile manipulator, equipped with two arms with dexterous capabilities, called MADAR (from Mobile Anthropomorphic Dual-Arm Robot). Basically, the manipulator can be divided into two parts, a mobile base and an upper structure that includes two arms with dexterous hands equipped with tactile sensors. The base, completely self-developed, is circular in shape and has three wheels with a novel design that allow omnidirectional mobility. The upper structure integrates commercial elements, such as the arms, the hands and dierent sensors. The article includes the description of the main elements of the hardware and the software developed for its control and use.Este trabajo ha sido parcialmente financiado por el gobierno español mediante el proyecto DPI2016-80077-R.Suárez, R.; Palomo-Avellaneda, L.; Martínez, J.; Clos, D.; García, N. (2020). Manipulador móvil, bibrazo y diestro con nuevas ruedas omnidireccionales. Revista Iberoamericana de Automática e Informática industrial. 17(1):10-21. https://doi.org/10.4995/riai.2019.11422OJS1021171ABB, Jan. 2018. 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