180 research outputs found
Scale-free networks are not robust under neutral evolution
Recently it has been shown that a large variety of different networks have
power-law (scale-free) distributions of connectivities. We investigate the
robustness of such a distribution in discrete threshold networks under neutral
evolution. The guiding principle for this is robustness in the resulting
phenotype. The numerical results show that a power-law distribution is not
stable under such an evolution, and the network approaches a homogeneous form
where the overall distribution of connectivities is given by a Poisson
distribution.Comment: Submitted for publicatio
Self-organization of heterogeneous topology and symmetry breaking in networks with adaptive thresholds and rewiring
We study an evolutionary algorithm that locally adapts thresholds and wiring
in Random Threshold Networks, based on measurements of a dynamical order
parameter. A control parameter determines the probability of threshold
adaptations vs. link rewiring. For any , we find spontaneous symmetry
breaking into a new class of self-organized networks, characterized by a much
higher average connectivity than networks without threshold
adaptation (). While and evolved out-degree distributions
are independent from for , in-degree distributions become broader
when , approaching a power-law. In this limit, time scale separation
between threshold adaptions and rewiring also leads to strong correlations
between thresholds and in-degree. Finally, evidence is presented that networks
converge to self-organized criticality for large .Comment: 4 pages revtex, 6 figure
Self-organized critical neural networks
A mechanism for self-organization of the degree of connectivity in model
neural networks is studied. Network connectivity is regulated locally on the
basis of an order parameter of the global dynamics which is estimated from an
observable at the single synapse level. This principle is studied in a
two-dimensional neural network with randomly wired asymmetric weights. In this
class of networks, network connectivity is closely related to a phase
transition between ordered and disordered dynamics. A slow topology change is
imposed on the network through a local rewiring rule motivated by
activity-dependent synaptic development: Neighbor neurons whose activity is
correlated, on average develop a new connection while uncorrelated neighbors
tend to disconnect. As a result, robust self-organization of the network
towards the order disorder transition occurs. Convergence is independent of
initial conditions, robust against thermal noise, and does not require fine
tuning of parameters.Comment: 5 pages RevTeX, 7 figures PostScrip
The radiative effect of ion-induced inorganic nucleation in the free troposphere
To determine the effect of cosmic rays on the Earth's climate via ion-induced nucleation, a parametrisation of inorganic nucleation was formulated based on experiments at the CERN CLOUD experiment. The parametrisation was implemented in the GLOMAP aerosol microphysics model and used to estimate the radiative effect of the change in ionisation experienced over an 11-year solar cycle
Topological Evolution of Dynamical Networks: Global Criticality from Local Dynamics
We evolve network topology of an asymmetrically connected threshold network
by a simple local rewiring rule: quiet nodes grow links, active nodes lose
links. This leads to convergence of the average connectivity of the network
towards the critical value in the limit of large system size . How
this principle could generate self-organization in natural complex systems is
discussed for two examples: neural networks and regulatory networks in the
genome.Comment: 4 pages RevTeX, 4 figures PostScript, revised versio
Secondary organic aerosol formation from photooxidation of naphthalene and alkylnaphthalenes: implications for oxidation of intermediate volatility organic compounds (IVOCs)
Current atmospheric models do not include secondary
organic aerosol (SOA) production from gas-phase reactions
of polycyclic aromatic hydrocarbons (PAHs). Recent
studies have shown that primary emissions undergo oxidation
in the gas phase, leading to SOA formation. This
opens the possibility that low-volatility gas-phase precursors
are a potentially large source of SOA. In this work,
SOA formation from gas-phase photooxidation of naphthalene,
1-methylnaphthalene (1-MN), 2-methylnaphthalene (2-
MN), and 1,2-dimethylnaphthalene (1,2-DMN) is studied in
the Caltech dual 28-m^3 chambers. Under high-NO_x conditions
and aerosol mass loadings between 10 and 40μgm^(−3),
the SOA yields (mass of SOA per mass of hydrocarbon reacted)
ranged from 0.19 to 0.30 for naphthalene, 0.19 to 0.39
for 1-MN, 0.26 to 0.45 for 2-MN, and constant at 0.31 for
1,2-DMN. Under low-NO_x conditions, the SOA yields were
measured to be 0.73, 0.68, and 0.58, for naphthalene, 1-
MN, and 2-MN, respectively. The SOA was observed to be
semivolatile under high-NO_x conditions and essentially nonvolatile
under low-NO_x conditions, owing to the higher fraction
of ring-retaining products formed under low-NO_x conditions.
When applying these measured yields to estimate
SOA formation from primary emissions of diesel engines
and wood burning, PAHs are estimated to yield 3–5 times
more SOA than light aromatic compounds over photooxidation
timescales of less than 12 h. PAHs can also account for
up to 54% of the total SOA from oxidation of diesel emissions,
representing a potentially large source of urban SOA
Análisis del potencial a circuito abierto durante la síntesis de nanopartículas de oro
Se describen estudios sobre mezclas de reacción a las que se adiciona NaBH4 para la síntesis de nanopartículas metálicas. Se midió el potencial a circuito abierto de las soluciones utilizando diferentes electrodos y composiciones de las mezclas. Estos estudios permitieron determinar cuáles fenómenos pueden dificultar la reducción total de especies metálicas solubles, un aspecto de relevancia que ha sido informado. Se determinó que la síntesis debe llevarse utilizando (i) medios ligeramente alcalinos de pH regulado, para evitar la hidrólisis de los aniones borohidruro; (ii) soluciones con baja concentración de oxígeno molecular, de manera de evitar un consumo del reductor por parte del O2; (iii) fuerzas iónicas moderadas, para evitar la aglomeración de las nanopartículas formadas.Facultad de Ciencias Exacta
Surface spin-flop transition in a uniaxial antiferromagnetic Fe/Cr superlattice induced by a magnetic field of arbitrary direction
We studied the transition between the antiferromagnetic and the surface
spin-flop phases of a uniaxial antiferromagnetic [Fe(14 \AA)/Cr(11 \AA] superlattice. For external fields applied parallel to the in-plane easy
axis, the layer-by-layer configuration, calculated in the framework of a
mean-field one-dimensional model, was benchmarked against published polarized
neutron reflectivity data. For an in-plane field applied at an angle with the easy axis, magnetometry shows that the magnetization
vanishes at H=0, then increases slowly with increasing . At a critical value
of , a finite jump in is observed for , while a
smooth increase of is found for . A dramatic
increase in the full width at half maximum of the magnetic susceptibility is
observed for . The phase diagram obtained from
micromagnetic calculations displays a first-order transition to a surface
spin-flop phase for low values, while the transition becomes continuous
for greater than a critical angle, . This is in fair agreement with the experimentally observed results.Comment: 24 pages, 7 figure
Comparison of chemical characteristics of 495 biomass burning plumes intercepted by the NASA DC-8 aircraft during the ARCTAS/CARB-2008 field campaign
This paper compares measurements of gaseous and particulate emissions from a wide range of biomass-burning plumes intercepted by the NASA DC-8 research aircraft during the three phases of the ARCTAS-2008 experiment: ARCTAS-A, based out of Fairbanks, Alaska, USA (3 April to 19 April 2008); ARCTAS-B based out of Cold Lake, Alberta, Canada (29 June to 13 July 2008); and ARCTAS-CARB, based out of Palmdale, California, USA (18 June to 24 June 2008). Approximately 500 smoke plumes from biomass burning emissions that varied in age from minutes to days were segregated by fire source region and urban emission influences. The normalized excess mixing ratios (NEMR) of gaseous (carbon dioxide, acetonitrile, hydrogen cyanide, toluene, benzene, methane, oxides of nitrogen and ozone) and fine aerosol particulate components (nitrate, sulfate, ammonium, chloride, organic aerosols and water soluble organic carbon) of these plumes were compared. A detailed statistical analysis of the different plume categories for different gaseous and aerosol species is presented in this paper.
The comparison of NEMR values showed that CH4 concentrations were higher in air-masses that were influenced by urban emissions. Fresh biomass burning plumes mixed with urban emissions showed a higher degree of oxidative processing in comparison with fresh biomass burning only plumes. This was evident in higher concentrations of inorganic aerosol components such as sulfate, nitrate and ammonium, but not reflected in the organic components. Lower NOx NEMRs combined with high sulfate, nitrate and ammonium NEMRs in aerosols of plumes subject to long-range transport, when comparing all plume categories, provided evidence of advanced processing of these plumes
Impaired Autonomic Responses to Emotional Stimuli in Autoimmune Limbic Encephalitis
Limbic encephalitis (LE) is an autoimmune-mediated disorder that affects structures of the limbic system, in particular the amygdala. The amygdala constitutes a brain area substantial for processing of emotional, especially fear-related signals. The amygdala is also involved in neuroendocrine and autonomic functions, including skin conductance responses (SCRs) to emotionally arousing stimuli. This study investigates behavioral and autonomic responses to discrete emotion-evoking and neutral film clips in a patient suffering from LE associated with contactin-associated protein-2 (CASPR2)-antibodies as compared to a healthy control group. Results show a lack of SCRs in the patient while watching the film clips, with significant differences compared to healthy controls in the case of fear-inducing videos. There was no comparable impairment in behavioral data (emotion report, valence and arousal ratings). The results point to a defective modulation of sympathetic responses during emotional stimulation in patients with LE, probably due to impaired functioning of the amygdala
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