556 research outputs found
On the molecular mechanism of surface charge amplification and related phenomena at aqueous polyelectrolyte-graphene interfaces
In this communication we illustrate the occurrence of a recently reported new
phenomenon of surface-charge amplification, SCA, (originally dubbed
overcharging, OC), [Jimenez-Angeles F. and Lozada-Cassou M., J. Phys. Chem. B,
2004, 108, 7286] by means of molecular dynamics simulation of aqueous
electrolytes solutions involving multivalent cations in contact with charged
graphene walls and the presence of short-chain lithium polystyrene sulfonates
where the solvent water is described explicitly with a realistic molecular
model. We show that the occurrence of SCA in these systems, in contrast to that
observed in primitive models, involves neither contact co-adsorption of the
negatively charged macroions nor divalent cations with a large size and charge
asymmetry as required in the case of implicit solvents. In fact the SCA
phenomenon hinges around the preferential adsorption of water (over the
hydrated ions) with an average dipolar orientation such that the charges of the
water's hydrogen and oxygen sites induce magnification rather than screening of
the positive-charged graphene surface, within a limited range of surface-charge
density.Comment: 10 pages, 6 figure
A simple conceptual model of abrupt glacial climate events
Here we use a very simple conceptual model in an attempt to reduce essential
parts of the complex nonlinearity of abrupt glacial climate changes (the
so-called Dansgaard-Oeschger events) to a few simple principles, namely (i) a
threshold process, (ii) an overshooting in the stability of the system and
(iii) a millennial-scale relaxation. By comparison with a so-called Earth
system model of intermediate complexity (CLIMBER-2), in which the events
represent oscillations between two climate states corresponding to two
fundamentally different modes of deep-water formation in the North Atlantic, we
demonstrate that the conceptual model captures fundamental aspects of the
nonlinearity of the events in that model. We use the conceptual model in order
to reproduce and reanalyse nonlinear resonance mechanisms that were already
suggested in order to explain the characteristic time scale of
Dansgaard-Oeschger events. In doing so we identify a new form of stochastic
resonance (i.e. an overshooting stochastic resonance) and provide the first
explicitly reported manifestation of ghost resonance in a geosystem, i.e. of a
mechanism which could be relevant for other systems with thresholds and with
multiple states of operation. Our work enables us to explicitly simulate
realistic probability measures of Dansgaard-Oeschger events (e.g. waiting time
distributions, which are a prerequisite for statistical analyses on the
regularity of the events by means of Monte-Carlo simulations). We thus think
that our study is an important advance in order to develop more adequate
methods to test the statistical significance and the origin of the proposed
glacial 1470-year climate cycle
Noise-induced memory in extended excitable systems
We describe a form of memory exhibited by extended excitable systems driven
by stochastic fluctuations. Under such conditions, the system self-organizes
into a state characterized by power-law correlations thus retaining long-term
memory of previous states. The exponents are robust and model-independent. We
discuss novel implications of these results for the functioning of cortical
neurons as well as for networks of neurons.Comment: 4 pages, latex + 5 eps figure
From synaptic interactions to collective dynamics in random neuronal networks models: critical role of eigenvectors and transient behavior
The study of neuronal interactions is currently at the center of several
neuroscience big collaborative projects (including the Human Connectome, the
Blue Brain, the Brainome, etc.) which attempt to obtain a detailed map of the
entire brain matrix. Under certain constraints, mathematical theory can advance
predictions of the expected neural dynamics based solely on the statistical
properties of such synaptic interaction matrix. This work explores the
application of free random variables (FRV) to the study of large synaptic
interaction matrices. Besides recovering in a straightforward way known results
on eigenspectra of neural networks, we extend them to heavy-tailed
distributions of interactions. More importantly, we derive analytically the
behavior of eigenvector overlaps, which determine stability of the spectra. We
observe that upon imposing the neuronal excitation/inhibition balance, although
the eigenvalues remain unchanged, their stability dramatically decreases due to
strong non-orthogonality of associated eigenvectors. It leads us to the
conclusion that the understanding of the temporal evolution of asymmetric
neural networks requires considering the entangled dynamics of both
eigenvectors and eigenvalues, which might bear consequences for learning and
memory processes in these models. Considering the success of FRV analysis in a
wide variety of branches disciplines, we hope that the results presented here
foster additional application of these ideas in the area of brain sciences.Comment: 24 pages + 4 pages of refs, 8 figure
Recommended from our members
A simple conceptual model of abrupt glacial climate events
Here we use a very simple conceptual model in an attempt to reduce essential parts of the complex nonlinearity of abrupt glacial climate changes (the so-called Dansgaard-Oeschger events) to a few simple principles, namely (i) the existence of two different climate states, (ii) a threshold process and (iii) an overshooting in the stability of the system at the start and the end of the events, which is followed by a millennial-scale relaxation. By comparison with a so-called Earth system model of intermediate complexity (CLIMBER-2), in which the events represent oscillations between two climate states corresponding to two fundamentally different modes of deep-water formation in the North Atlantic, we demonstrate that the conceptual model captures fundamental aspects of the nonlinearity of the events in that model. We use the conceptual model in order to reproduce and reanalyse nonlinear resonance mechanisms that were already suggested in order to explain the characteristic time scale of Dansgaard-Oeschger events. In doing so we identify a new form of stochastic resonance (i.e. an overshooting stochastic resonance) and provide the first explicitly reported manifestation of ghost resonance in a geosystem, i.e. of a mechanism which could be relevant for other systems with thresholds and with multiple states of operation. Our work enables us to explicitly simulate realistic probability measures of Dansgaard-Oeschger events (e.g. waiting time distributions, which are a prerequisite for statistical analyses on the regularity of the events by means of Monte-Carlo simulations). We thus think that our study is an important advance in order to develop more adequate methods to test the statistical significance and the origin of the proposed glacial 1470-year climate cycle
How we move is universal: scaling in the average shape of human activity
Human motor activity is constrained by the rhythmicity of the 24 hours
circadian cycle, including the usual 12-15 hours sleep-wake cycle. However,
activity fluctuations also appear over a wide range of temporal scales, from
days to a few seconds, resulting from the concatenation of a myriad of
individual smaller motor events. Furthermore, individuals present different
propensity to wakefulness and thus to motor activity throughout the circadian
cycle. Are activity fluctuations across temporal scales intrinsically
different, or is there a universal description encompassing them? Is this
description also universal across individuals, considering the aforementioned
variability? Here we establish the presence of universality in motor activity
fluctuations based on the empirical study of a month of continuous wristwatch
accelerometer recordings. We study the scaling of average fluctuations across
temporal scales and determine a universal law characterized by critical
exponents , and . Results are highly reminiscent of the
universality described for the average shape of avalanches in systems
exhibiting crackling noise. Beyond its theoretical relevance, the present
results can be important for developing objective markers of healthy as well as
pathological human motor behavior.Comment: Communicated to the Granada Seminar, "Physics Meets the Social
Sciences: Emergent cooperative phenomena, from bacterial to human group
behavior". June 14-19, 2015. La Herradura, Spai
Spared nerve injury rats exhibit thermal hyperalgesia on an automated operant dynamic thermal escape Task
Well-established methods are available to measure thermal and mechanical sensitivity in awake behaving rats. However, they require experimenter manipulations and tend to emphasize reflexive behaviors. Here we introduce a new behavioral test, with which we examine thermal sensitivity of rats with neuropathic injury. We contrast thermal hyperalgesia between spared nerve injury and chronic constriction injury rats. This device is a fully automated thermal sensitivity assessment tool designed to emphasize integrated learned responses to thermal painful and non-painful stimuli that are applied dynamically to a surface on which the animal is standing. It documents escape behavior in awake, unrestrained animals to innocuous and noxious heating of the floor where the animal is located. Animals learn to minimize pain by escaping to the opposite non-heated side; escape latency is recorded. On this device, thermal stimulus-response curves showed > 6°C leftward shift in both groups of neuropathic rats. In contrast, when these animals were tested on hotplate the stimulus-response shift was < 2°C. Spared nerve injury rats showed even less evidence for thermal hyperalgesia when thermal sensitivity was tested by measuring paw withdrawal to infrared heating, plantar test. The implications of test dependent magnitude of thermal hyperalgesia are discussed from the viewpoint of the tests used, as well as the animal models studied. It is argued that the dynamic thermal operant task reveals the relevance of the neuropathic injury associated pain-like behavior in relation to the whole organism
- …