Showing papers by "Frankfurt Institute for Advanced Studies published in 2007"

Modulation of neuronal interactions through neuronal synchronization

Abstract: Brain processing depends on the interactions between neuronal groups. Those interactions are governed by the pattern of anatomical connections and by yet unknown mechanisms that modulate the effective strength of a given connection. We found that the mutual influence among neuronal groups depends on the phase relation between rhythmic activities within the groups. Phase relations supporting interactions between the groups preceded those interactions by a few milliseconds, consistent with a mechanistic role. These effects were specific in time, frequency, and space, and we therefore propose that the pattern of synchronization flexibly determines the pattern of neuronal interactions.

Vibronic transitions in large molecular systems: Rigorous prescreening conditions for Franck-Condon factors

Abstract: In this work, rigorous prescreening conditions for the calculation of Franck-Condon factors (FCFs) are derived and implemented. These factors play an important role in numerous applications including the prediction and simulation of vibronic spectra, electron transfer rates, and nonradiative transition probabilities. For larger systems it is crucial to calculate only relevant FCFs, as the computational burden becomes otherwise prohibitive due to the sheer number of Franck-Condon integrals. By exploiting rigorous prescreening criteria one can significantly reduce the computational effort and systematically refine results to the desired target accuracy. In this work, such criteria are derived via the use of sum rules obtained through a coherent state generating function for the FCFs in the harmonic oscillator approximation, following the prescription of Doktorov et al. These sum rules allow efficient and rigorous prescreening prior to the calculation of entire batches of Franck-Condon integrals, reducing the subsequent computational burden. To illustrate the benefit of employing such conditions, they are applied in this work to calculate FC profiles for vibronic spectra of formic acid, thymine, anthracene, and a polycyclic aromatic hydrocarbon derivative containing about 450 vibrational degrees of freedom. Since the prescreening step gives stringent upper and lower bounds for the loss of FC intensity in these spectra arising from the neglected FCFs, the present approach renders most previous a priori selection schemes obsolete and has the potential to complement or even replace other approximate treatments.

Polarization probes of vorticity in heavy ion collisions

Abstract: We discuss the information that can be deduced from a measurement of hadron (hyperon or vector meson) polarization in ultrarelativistic nuclear collisions. We describe the sensitivity of polarization to initial conditions, hydrodynamic evolution, and mean free path and find that the polarization observable is sensitive to all details and stages of the system's evolution. We suggest that an experimental investigation covering production plane and reaction plane polarizations, as well as the polarization of jet-associated particles in the plane defined by the jet and particle direction, can help in disentangling the factors contributing to this observable. Scans of polarization in energy and rapidity might also point to a change in the system's properties.

Synergies Between Intrinsic and Synaptic Plasticity Mechanisms

Abstract: We propose a model of intrinsic plasticity for a continuous activation model neuron based on information theory. We then show how intrinsic and synaptic plasticity mechanisms interact and allow the neuron to discover heavy-tailed directions in the input. We also demonstrate that intrinsic plasticity may be an alternative explanation for the sliding threshold postulated in the BCM theory of synaptic plasticity. We present a theoretical analysis of the interaction of intrinsic plasticity with different Hebbian learning rules for the case of clustered inputs. Finally, we perform experiments on the “bars” problem, a popular nonlinear independent component analysis problem.

Effects of fluctuations on the initial eccentricity from the color glass condensate in heavy ion collisions

Abstract: We introduce a modified form of the Kharzeev-Levin-Nardi (KLN) approach for nuclear collisions. The new ansatz for the unintegrated gluon distribution function preserves factorization, and the saturation scale is bound from below by that for a single nucleon. It also reproduces the correct scaling with the number of collisions at high transverse momentum. The corresponding Monte Carlo implementation allows us to account for fluctuations of the hard sources (nucleons) in the transverse plane. We compute various definitions of the eccentricity within the new approach, which are relevant for the interpretation of the elliptic flow. Our approach predicts breaking of the scaling of the eccentricity with the Glauber eccentricity at the level of about 30%.

Centrality dependence of elliptic flow, the hydrodynamic limit, and the viscosity of hot QCD

Abstract: We show that the centrality and system-size dependence of elliptic flow measured at the BNL Relativistic Heavy Ion Collider (RHIC) are fully described by a simple model based on eccentricity scaling and incomplete thermalization. We argue that the elliptic flow is at least 25% below the (ideal) ``hydrodynamic limit,'' even for the most central Au-Au collisions. This lack of perfect equilibration allows for estimates of the effective parton cross section in the quark-gluon plasma and of its viscosity to entropy density ratio. We also show how the initial conditions affect the transport coefficients and thermodynamic quantities extracted from the data, in particular, the viscosity and the speed of sound.

Color-flavor locked superconductor in a magnetic field

Abstract: We study the effects of moderately strong magnetic fields on the properties of color-flavor locked color superconducting quark matter in the framework of the Nambu-Jona-Lasinio model. We find that the energy gaps, which describe the color superconducting pairing as well as the magnetization, are oscillating functions of the magnetic field. Also, we observe that the oscillations of the magnetization can be so strong that homogeneous quark matter becomes metastable for a range of parameters. We suggest that this points to the possibility of magnetic domains or other types of magnetic inhomogeneities in the quark cores of magnetars.

Low-energy collisions of heavy nuclei : dynamics of sticking, mass transfer and fusion

Abstract: The dynamics of heavy-ion low-energy collisions is studied within the realistic model based on multi-dimensional Langevin equations. Interplay of strongly coupled deep inelastic scattering, quasi-fission and fusion-fission processes is discussed. Collisions of very heavy nuclei ( 238 U+ 238 U, 232 Th+ 250 Cf and 238 U+ 448 Cm) are investigated as an alternative way for the production of super-heavy elements with increasing neutron number. Large charge and mass transfer were found in these reactions due to the inverse (anti-symmetrizing) quasi-fission process leading to the formation of surviving super-heavy long-lived neutron-rich nuclei. In many events the lifetime of the composite giant system consisting of two touching nuclei turns out to be rather long (≥10 -20 s), sufficient for observing line structure in spontaneous positron emission from super-strong electric fields, a fundamental QED process.

Eccentricity fluctuations from the color glass condensate in ultrarelativistic heavy ion collisions

Abstract: In this Rapid Communication, we determine the fluctuations of the initial eccentricity in heavy-ion collisions caused by fluctuations of the nucleon configurations. This is done via a Monte Carlo implementation of a color glass condensate ${k}_{t}$-factorization approach. The eccentricity fluctuations are found to nearly saturate elliptic flow fluctuations measured recently at RHIC. Extrapolations to LHC energies are shown.

Eccentricity fluctuations from the Color Glass Condensate at RHIC and LHC

Abstract: In this brief note, we determine the fluctuations of the initial eccentricity in heavy-ion collisions caused by fluctuations of the nucleon configurations. This is done via a Monte-Carlo implementation of a Color Glass Condensate $k_t$-factorization approach. The eccentricity fluctuations are found to nearly saturate elliptic flow fluctuations measured recently at RHIC. Extrapolations to LHC energies are shown.

Morphology of bile salt micelles as studied by computer simulation methods.

Abstract: The relative arrangement of the neighboring bile ions and the shape of the hydrophobic and hydrogen-bonded primary micelles as well of the large secondary micelles formed by these ions are analyzed in detail on the basis of molecular dynamics computer simulations of 30 and 300 mM sodium cholate and sodium deoxycholate solutions. In the lower concentration considered, the systems only contain primary micelles, whereas in both of the 300 mM systems secondary micelles are also present. The simulations performed were long enough that the systems reached thermodynamic equilibrium. It is found that the neighboring cholate ions prefer alignments in which their quasi-planar tetracyclic ring systems are parallel with each other, whereas for deoxycholate an opening of the angle between these planes is observed. The shape of the micelles is characterized by the ratio of their three principal moments of inertia. The primary deoxycholate micelles are found to be rather spherical, whereas in the case of cholate somewhat flattened, disklike or oblate shaped ellipsoidal primary micelles are found, irrespective of whether these micelles are kept together by hydrogen bonds or are of hydrophobic origin. Finally, the secondary micelles are found to exhibit a large variety of shapes, ranging from flattened oblates to rodlike objects through various different irregular shapes, characterized by markedly different values of the three principal moments of inertia. The observed preferences of the relative arrangement of the neighboring ions and of the aggregate shapes as well as the differences observed in the behavior of the two bile ions studied in these respects are traced back to the molecular structure of these ions.

Shell effects in damped collisions: a new way to superheavies

Abstract: The dynamics of heavy-ion low-energy damped collisions is studied within the model based on the Langevin-type equations. Shell effects on the multidimensional potential energy surface play an important role in these reactions. An enhanced yield of nuclides far from the projectile and target masses was found in multi-nucleon transfer reactions due to the shell effects. Our theoretical predictions need experimental confirmation.

Potential Energy of a Heavy Nuclear System in Fusion-Fission Processes

Abstract: We discuss the problem of description of low-energy nuclear dynamics and the derivation of a multi-dimensional potential energy surface that depends on several collective degrees of freedom and allows a unified analysis of deep inelastic scattering, fusion, and fission processes. A unified description is required due to the strong coupling and significant overlapping of these reaction channels in heavy nuclear systems, which are used, in particular, for synthesis of superheavy elements. The multidimensional adiabatic potential is derived based on an extended versio of the two-center shell model. This model leads to a correct asymptotic value and height of the Coulomb barrier in the entrance channel (fusion), and appropriate behavior in the exit channel, giving the required mass and energy distributions of reaction products and fission fragments. The derived driving potential is proposed to be applied in a consistent dynamic analysis of low-energy interactions of heavy nuclei.

Sub-barrier fusion of neutron-rich nuclei and its astrophysical consequences

Abstract: Near-barrier fusion of neutron-rich nuclei was studied within the semiempirical channel coupling model for intermediate neutron rearrangement and within the time-dependent three-body Schr\"odinger equation The possibility of neutron transfer with positive $Q$ values considerably increases the barrier penetrability A huge enhancement of deep sub-barrier fusion probability was found for light neutron-rich weakly bound nuclei (such as $^{6}\mathrm{He}$) This may be quite important for astrophysical primordial and supernova nucleosynthesis

Resonance or Integration? Self-Sustained Dynamics and Excitability of Neural Microcircuits

Abstract: We investigated spontaneous activity and excitability in large networks of artificial spiking neurons. We compared three different spiking neuron models: integrate-and-fire (IF), regular-spiking (RS), and resonator (RES). First, we show that different models have different frequency-dependent response properties, yielding large differences in excitability. Then, we investigate the responsiveness of these models to a single afferent inhibitory/excitatory spike and calibrate the total synaptic drive such that they would exhibit similar peaks of the postsynaptic potentials (PSP). Based on the synaptic calibration, we build large microcircuits of IF, RS, and RES neurons and show that the resonance property favors homeostasis and self-sustainability of the network activity. On the other hand, integration produces instability while it endows the network with other useful properties, such as responsiveness to external inputs. We also investigate other potential sources of stable self-sustained activity and their relation to the membrane properties of neurons. We conclude that resonance and integration at the neuron level might interact in the brain to promote stability as well as flexibility and responsiveness to external input and that membrane properties, in general, are essential for determining the behavior of large networks of neurons.

Multiplicity fluctuations in relativistic nuclear collisions : Statistical model versus experimental data

Abstract: The multiplicity distributions of hadrons produced in central nucleus-nucleus collisions are studied within the hadron-resonance gas model in the large-volume limit. The microscopic correlator method is used to enforce conservation of three charges---baryon number, electric charge, and strangeness---in the canonical ensemble. In addition, in the microcanonical ensemble energy conservation is included. An analytical method is used to account for resonance decays. The multiplicity distributions and the scaled variances for negatively, positively, and all charged hadrons are calculated along the chemical freeze-out line of central $\mathrm{Pb}+\mathrm{Pb}$ ($\mathrm{Au}+\mathrm{Au}$) collisions from GSI Schwerionen Synchrotron to CERN Large Hadron Collider energies. Predictions obtained within different statistical ensembles are compared with the preliminary NA49 experimental results on central $\mathrm{Pb}+\mathrm{Pb}$ collisions in the SPS energy range. The measured fluctuations are significantly narrower than the Poisson ones and clearly favor expectations for the microcanonical ensemble. Thus this is the first observation of the recently predicted suppression of the multiplicity fluctuations in relativistic gases in the thermodynamical limit owing to conservation laws.

Color Opponency in Synaesthetic Experiences

Abstract: Grapheme-color synaesthesia is a rare condition in which perception of a letter or a digit is associated with concurrent perception of a color. Synaesthetes report that these color experiences are vivid and realistic. We used a Stroop task to show that synaesthetically induced color, like real color, is processed in color-opponent channels (red-green or blue-yellow). Synaesthetic color produced maximal interference with the perception and naming of the real color of a grapheme if the real color was opponent to the synaesthetic color. Interference was reduced considerably if the synaesthetic and real colors engaged different color channels (e.g., synaesthetic blue and real red). No dependence on color opponency was found for semantic conflicts between shape and color (e.g., a blue lemon). Thus, the neural representation of synaesthetic colors closely resembles that of real colors. This suggests involvement of early stages of visual processing in color synaesthesia and explains the vivid and realistic natur...

Interplay of Electrostatic and van der Waals Forces in Coronene Dimer

Abstract: Interplay of electrostatic and van der Waals forces in the coronene dimer is studied. Our results show that the lowest-energy configuration of the dimer is not necessarily a stack, as it might had been expected a priori. This is a surprising result for the dimer of such a large polycyclic aromatic hydrocarbon (PAH) as coronene (C24H12). The energy of the T-shaped configuration at all highest feasible levels of density functional theory (DFT) (B3LYP, PBE/6-31G(d), D95, cc-pVDZ, cc-pVTZ) is lower than the energies of the three plausible stack configurations. To get a better description of the van der Waals interaction, the DFT results were corrected by a phenomenological van der Waals-type term. This correction gives a slight edge to the parallel-displaced stack configuration. However, the magnitude of the correction is somewhat arbitrary, depending on the set of parameters used. This makes the definitive conclusion impossible at the currently achievable level of theory. A simple model is proposed that is useful for the qualitative understanding of possible geometries of the coronene dimer and larger coronene clusters. The model represents the coronene dimer as two sets of charged rings interacting via Coulomb and Lennard-Jones potentials. The model provides an intuitively clear explanation as to why the T-shaped dimers can be of importance even for some of moderately large PAHs such as coronene, and perhaps for circumcoronene as well. The unexpectedly strong competitiveness of the T-shaped

Transitions between multistable states as a model of epileptic seizure dynamics.

Abstract: Epileptic seizures are generally considered to result from excess and synchronized neural activity. Additionally, changes in amplitude and frequency are often seen in local field potential or electroencephalogram recordings during a seizure event. To investigate how seizures initiate, and how dynamical changes occur during seizure progression, we develop a neocortical network model based on a model suggested by Wilson [J. Theor. Biol. 200, 375 (1999)]. We propose a possible mechanism for seizure initiation as a bifurcation, and suggest that experimentally observed changes in field potential amplitude and frequency during the course of a seizure may be explained by noise-induced transitions among multistable states.

Feasibility of an electron-based crystalline undulator.

Abstract: The feasibility to generate powerful monochromatic radiation of the undulator type in the gamma region of the spectrum by means of planar channeling of ultrarelativistic electrons in a periodically bent crystal is proven. It is shown that to overcome the restriction due to the smallness of the dechanneling length, an electron-based crystalline undulator must operate in the regime of higher beam energies than a positron-based one does. A numerical analysis is performed for a 50 GeV electron channeling in Si along the (111) crystallographic planes.

Emergence of Mirror Neurons in a Model of Gaze Following

Abstract: Gaze following is the ability to redirect one's gaze to the location where another agent is looking We present a computational model of how human infants or other agents may acquire gaze following by learning to predict the locations of interesting sights from the looking behavior of other agents through reinforcement learning The model accounts for many findings about the development of gaze following in human infants During learning, the model develops pre-motor representations that exhibit many properties characteristic of mirror neurons, but they are specific to looking behaviors The existence of such a new class of mirror neurons is the main prediction of our model The model also offers a parsimonious account of how these and possibly other mirror neurons may acquire their special response properties In this account, visual representations of other agents' actions become associated with pre-motor neurons that represent the intention to perform corresponding actions The model also demonstrates how this development may be obstructed in autism spectrum disorder, giving rise to specific physiological and anatomical differences in the mirror system

Bulk viscosity of strange quark matter: Urca versus nonleptonic processes

Abstract: A general formalism for calculating the bulk viscosity of strange quark matter is developed. Contrary to the common belief that the nonleptonic processes alone give the dominant contribution to the bulk viscosity, the inclusion of the Urca processes is shown to play an important role at intermediate densities when the characteristic r-mode oscillation frequencies are not too high. The interplay of nonleptonic and Urca processes is analyzed in detail.