Showing papers by "International School for Advanced Studies published in 1997"
TL;DR: The activity-dependent modulation of mRNA targeting and protein accumulation in the dendrites may provide a mechanism for achieving a selective local regulation of the activity of neurotrophins and their receptors, close to their sites of action.
Abstract: The mechanisms underlying the subcellular localization of neurotrophins and their receptors are poorly understood. We show that in cultured hippocampal neurons, the mRNAs for BDNF and TrkB have a somatodendritic localization, and we quantify the extent of their dendritic mRNA localization. In the dendrites the labeling covers on average the proximal 30% of the total dendritic length. On high potassium depolarization, the labeling of BDNF and TrkB mRNA extends on average to 68% of the dendritic length. This increase does not depend on new RNA synthesis, is inhibited by the Na+ channel blocker tetrodotoxin, and involves the activation of glutamate receptors. Extracellular Ca2+, partly flowing through L-type Ca2+ channels, is absolutely required for this process to occur. At the protein level, a brief stimulation of hippocampal neurons with 10 mM KCl leads to a marked increase of BDNF and TrkB immunofluorescence density in the distal portion of dendrites, which also occurs, even if at lower levels, when transport is inhibited by nocodazole. The protein synthesis inhibitor cycloheximide abolishes this increase. The activity-dependent modulation of mRNA targeting and protein accumulation in the dendrites may provide a mechanism for achieving a selective local regulation of the activity of neurotrophins and their receptors, close to their sites of action.
354 citations
TL;DR: In this paper, the problem of gauge dependence in relativistic perturbation theory has been studied and the Taylor expansion of tensor fields under the action of one-parameter families (not necessarily groups) of diffeomorphisms is studied.
Abstract: We consider in detail the problem of gauge dependence that exists in relativistic perturbation theory, going beyond the linear approximation and treating second- and higher-order perturbations. We first derive some mathematical results concerning the Taylor expansion of tensor fields under the action of one-parameter families (not necessarily groups) of diffeomorphisms. Secondly, we define gauge invariance to an arbitrary order n. Finally, we give a generating formula for the gauge transformation to an arbitrary order and explicit rules to second and third order. This formalism can be used in any field of applied general relativity, such as cosmological and black hole perturbations, as well as in other spacetime theories. As a specific example, we consider here second-order perturbations in cosmology, assuming a flat Robertson - Walker background, giving explicit second-order transformations between the synchronous and the Poisson (generalized longitudinal) gauges.
349 citations
TL;DR: A set of mammalian expression vectors which permit facile and rapid cloning of antibody genes for both transient and stable expression in mammalian cells and all the important elements in the vectors are flanked by unique restriction sites, allowing simple substitution of elements.
349 citations
TL;DR: In this article, the fine properties of functions in, the space of functions with bounded deformation, were analyzed, and it was shown that functions are approximately differentiable in almost every point of their domain.
Abstract: The paper is concerned with the fine properties of functions in , the space of functions with bounded deformation. We analyse the set of Lebesgue points and the set where these functions have one-sided approximate limits. Moreover, following the analogy with , we decompose the symmetric distributional derivative into an absolutely continuous part , a jump part , and a Cantor part . The main result of the paper is a structure theorem for functions, showing that these parts of the derivative can be recovered from the corresponding ones of the one-dimensional sections. Moreover, we prove that functions are approximately differentiable in almost every point of their domain.
283 citations
TL;DR: In this article, a link between the luminosity radiatively dissipated in the central engine of radio-loud Active Galactic Nuclei and the kinetic power in their jets is presented.
Abstract: We present evidence in favour of a link between the luminosity radiatively dissipated in the central engine of radio-loud Active Galactic Nuclei and the kinetic power in their jets. This piece of evidence is based on the relation we find between the luminosity in broad emission lines and the kinetic power in pc-scale radio jets, for a sample of radio-loud quasars for which suitable data are available in the literature. We find that the ionizing luminosity and the kinetic one are of the same order of magnitude, suggesting that the processes responsible for them are somehow related. A strong magnetic field in equipartition with the radiation field could be responsible for regulating both processes. BL Lac objects seem to follow a similar behaviour, but with comparatively fainter broad line emission.
279 citations
TL;DR: In this paper, the spectral energy distributions of the 13 Hubble Deep Field (HDF) galaxies reliably detected by the Infrared Space Observatoiy (ISO) are modelled.
Abstract: We have modelled the spectral energy distributions of the 13 Hubble Deep Field (HDF) galaxies reliably detected by the Infrared Space Observatoiy (ISO). For two galaxies the emission detected by ISO is consistent with being starlight or the infrared 'cirrus' in the galaxies. For the remaining 11 galaxies there is a clear mid-infrared excess, which we interpret as emission from dust associated with a strong starburst. 10 of these galaxies are spirals or interacting pairs, while the remaining one is an elliptical with a prominent nucleus and broad emission lines. We give a new discussion of how the star formation rate can be deduced from the far-infrared luminosity, and derive star formation rates for these galaxies of 8-1000o M? yr-1, where o takes account of the uncertainty in the initial mass function. The HDF galaxies detected by ISO are clearly forming stars at a prodigious rate compared with nearby normal galaxies. We discuss the implications of our detections for the history of star and heavy element formation in the Universe. Although uncertainties in the calibration, reliability of source detection, associations and starburst models remain, it is clear that dust plays an important role in star formation out to redshift 1 at least.
267 citations
TL;DR: It has been shown that it is possible to read, from the firing rates of just a small population of neurons, the code that is used in the macaque temporal lobe visual cortex to distinguish between different faces being looked at, and an exponential increase in the number of stimuli that can be represented has been demonstrated in the brain.
Abstract: It has been shown that it is possible to read, from the firing rates of just a small population of neurons, the code that is used in the macaque temporal lobe visual cortex to distinguish between different faces being looked at. To analyse the information provided by populations of single neurons in the primate temporal cortical visual areas, the responses of a population of 14 neurons to 20 visual stimuli were analysed in a macaque performing a visual fixation task. The population of neurons analysed responded primarily to faces, and the stimuli utilised were all human and monkey faces. Each neuron had its own response profile to the different members of the stimulus set. The mean response of each neuron to each stimulus in the set was calculated from a fraction of the ten trials of data available for every stimulus. From the remaining data, it was possible to calculate, for any population response vector, the relative likelihoods that it had been elicited by each of the stimuli in the set. By comparison with the stimuli actually shown, the mean percentage correct identification was computed and also the mean information about the stimuli, in bits, that the population of neurons carried on a single trial. When the decoding algorithm used for this calculation approximated an optimal, Bayesian estimate of the relative likelihoods, the percentage correct increased from 14 correct (chance was 5 correct) with one neuron to 67 with 14 neurons. The information conveyed by the population of neurons increased approximately linearly from 0.33 bits with one neuron to 2.77 bits with 14 neurons. This leads to the important conclusion that the number of stimuli that can be encoded by a population of neurons in this part of the visual system increases approximately exponentially as the number of cells in the sample increases (in that the log of the number of stimuli increases almost linearly). This is in contrast to a local encoding scheme (of 'grandmother' cells), in which the number of stimuli encoded increases linearly with the number of cells in the sample. Thus one of the potentially important properties of distributed representations, an exponential increase in the number of stimuli that can be represented, has been demonstrated in the brain with this population of neurons. When the algorithm used for estimating stimulus likelihood was as simple as could be easily implemented by neurons receiving the population's output (based on just the dot product between the population response vector and each mean response vector), it was still found that the 14-neuron population produced 66 correct guesses and conveyed 2.30 bits of information, or 83 of the information that could be extracted with the nearly optimal procedure. It was also shown that, although there was some redundancy in the representation (with each neuron contributing to the information carried by the whole population 60 of the information it carried alone, rather than 100), this is due to the fact that the number of stimuli in the set was limited (it was 20). The data are consistent with minimal redundancy for sufficiently large and diverse sets of stimuli. The implication for brain connectivity of the distributed encoding scheme, which was demonstrated here in the case of faces, is that a neuron can receive a great deal of information about what is encoded by a large population of neurons if it is able to receive its inputs from a random subset of these neurons, even of limited numbers (e.g. hundreds).
236 citations
TL;DR: In this paper, two families of models of dusty tori in active galactic nuclei (AGNs; moderately thick and extended versus very thick and compact) are tested against available observations.
Abstract: Two families of models of dusty tori in active galactic nuclei (AGNs; moderately thick and extended versus very thick and compact) are tested against available observations. The confrontation suggests that the former class better explains the infrared (IR) broadband spectra of both broad- and narrow-line AGNs, the anisotropy of the emission deduced by comparing IR properties of Seyfert 1 and 2 nuclei, and the results of IR spectroscopy and those of high spatial resolution observations. There is, however, clear evidence for a broad distribution of optical depths. We also examine the relationship between IR and X-ray emission. The data support a view in which the matter responsible for the X-ray absorption is mostly dust free, lying inside the dust sublimation radius. The consequences of these results for the hard X-ray background as well as IR counts and background are discussed.
231 citations
TL;DR: In this paper, a prototype model of stock market is introduced and studied numerically, in which traders trade according to their own strategy, to accumulate their assets by speculating on the price's fluctuations which are produced by themselves.
Abstract: A prototype model of stock market is introduced and studied numerically. In this self-organized system, we consider only the interaction among traders without external influences. Agents trade according to their own strategy, to accumulate their assets by speculating on the price's fluctuations which are produced by themselves. The model reproduced rather realistic price histories whose statistical properties are also similar to those observed in real markets.
161 citations
TL;DR: In this article, the authors examined (D − 2)-brane solutions in supergravities and showed that they fall into four categories depending on the details of the dilaton coupling.
159 citations
TL;DR: In this paper, a prototype model of stock market is introduced and studied numerically, in which traders trade according to their own strategy, to accumulate their assets by speculating on the price's fluctuations which are produced by themselves.
Abstract: A prototype model of stock market is introduced and studied numerically. In this self-organized system, we consider only the interaction among traders without external influences. Agents trade according to their own strategy, to accumulate his assets by speculating on the price's fluctuations which are produced by themselves. The model reproduced rather realistic price histories whose statistical properties are also similar to those observed in real markets.
TL;DR: The data show that in the developing hippocampus synchronous burst activity is generated in the hilar region by transiently coupled cells, which via its excitatory action may serve a neurotrophic function during the refinement of hippocampal circuitry.
Abstract: The establishment of synaptic connections and their refinement during development require neural activity. Increasing evidence suggests that spontaneous bursts of neural activity within an immature network are mediated by γ-aminobutyric acid via a paradoxical excitatory action. Our data show that in the developing hippocampus such synchronous burst activity is generated in the hilar region by transiently coupled cells. These cells have been identified as neuronal elements because they fire action potentials and they are not positive for the glial fibrillary acidic protein staining. Oscillations in hilar cells are “paced” by a hyperpolarization-activated current, with properties of I h. Coactivated interneurons synchronously release GABA, which via its excitatory action may serve a neurotrophic function during the refinement of hippocampal circuitry.
TL;DR: In this paper, the authors give a topological and geometrical description of focus-focus singularities of integrable Hamiltonian systems and explain why the monodromy around these singularities is non-trivial.
Abstract: We give a topological and geometrical description of focus-focus singularities of integrable Hamiltonian systems. In particular, we explain why the monodromy around these singularities is non-trivial, a result obtained before by J.J. Duistermaat and others for some concrete systems.
TL;DR: In this article, the authors show that the (q1, q2) string spectrum is reproduced by the membrane BPS spectrum, determined using a certain limit, and establish the correspondence between BPS states of type IIB strings on a circle and oscillating states of a fundamental supermembrane wrapped around a 2-torus.
TL;DR: In this article, the group theoretical properties of R-R scalars of string theories in the low-energy supergravity limit are considered and the solvable Lie subalgebra Gs ⊂ U of the U-duality algebra that generates the scalar manifold of the theory: exp[Gs] = U/H.
TL;DR: Constant-pressure, first-principles molecular dynamic simulations were used to investigate the behavior of methane at high pressure and temperature, and suggest that, below 100 gigapascals, methane dissociates into a mixture of hydrocarbons, and it separates into hydrogen and carbon only above 300 gigapascalals.
Abstract: Constant-pressure, first-principles molecular dynamic simulations were used to investigate the behavior of methane at high pressure and temperature. Contrary to the current interpretation of shock-wave experiments, the simulations suggest that, below 100 gigapascals, methane dissociates into a mixture of hydrocarbons, and it separates into hydrogen and carbon only above 300 gigapascals. The simulation conditions (100 to 300 gigapascals; 4000 to 5000 kelvin) were chosen to follow the isentrope in the middle ice layers of Neptune and Uranus. Implications on the physics of these planets are discussed.
TL;DR: The Hamiltonian dynamics of the classical planar Heisenberg model is numerically investigated in two and three dimensions in this paper, and it is conjectured that the phase transition might correspond to a change in the topology of the manifolds whose geodesics are the motions of the system.
Abstract: The Hamiltonian dynamics of the classical planar Heisenberg model is numerically investigated in two and three dimensions. In three dimensions peculiar behaviors are found in the temperature dependence of the largest Lyapunov exponent and of other observables related to the geometrization of the dynamics. On the basis of a heuristic argument it is conjectured that the phase transition might correspond to a change in the topology of the manifolds whose geodesics are the motions of the system.
TL;DR: In this paper, a very good agreement of the mode frequencies and their pressure coefficients yields unambiguous assignment of all observed features, including the $525{\mathrm{cm}}^{\ensuremath{-}1}$ line which is a highly harmonic librational mode of the icosahedron and mainly involves bond bending.
Abstract: The Raman spectrum of icosahedral \ensuremath{\alpha}-boron presents a very sharp peak at $525{\mathrm{cm}}^{\ensuremath{-}1}$ that was consistently rejected from the lattice modes but is still unexplained. New Raman scattering experiments under pressure are compared with ab initio lattice dynamics calculations. The very good agreement of the mode frequencies and their pressure coefficients yields unambiguous assignment of all observed features, including the $525{\mathrm{cm}}^{\ensuremath{-}1}$ line which is a highly harmonic librational mode of the icosahedron and mainly involves bond bending. This mode is also identified in the Raman spectrum of other icosahedral boron-rich solids.
TL;DR: In this paper, a multi-wave-band spectroscopic study of the nearby Seyfert 1 galaxy MCG-6-30-15 is presented, which clearly shows the effects of dust extinction/reddening on both the emission line spectrum and the non-stellar AGN continuum.
Abstract: We present a multiwaveband spectroscopic study of the nearby Seyfert 1 galaxy MCG-6-30-15. New optical spectra from the Anglo-Australian Telescope are presented which clearly show the effects of dust extinction/reddening on both the emission line spectrum and the non-stellar AGN continuum. The reddening is constrained to be in the range E(B-V)=0.61-1.09. Spectroscopy in the X-ray band, with both ROSAT and ASCA, reveal absorption by the warm absorber but little or no neutral absorption expected to accompany the dust responsible for the optical reddening. The dusty warm absorber solution to this discrepancy is discussed and photoionization models of such warm absorbers are constructed. The optical spectrum also displays the relatively strong `coronal' lines of [FeX]6375, [FeXI]7892 and [FeXIV]5303. We show that these lines may plausibly originate from the outer regions of the warm absorber, although better calculations of the collision strengths for these transitions are required in order to conclusively address this issue. We also present new ultraviolet data from the International Ultraviolet Explorer and suggest that much of the observed UV flux is scattered into our line of sight (with a scattering fraction of 1-5 per cent). We conclude with a discussion of the global energetics of this system.
TL;DR: In this article, the magnetization of spin-1/2 Heisenberg ladders with 3 legs is studied and plateaux with magnetization one third of the saturation value are shown to exist.
Abstract: Magnetization processes of spin-1/2 Heisenberg ladders are studied using strong-coupling expansions, numerical diagonalization of finite systems and a bosonization approach. We find that the magnetization exhibits plateaux as a function of the applied field at certain rational fractions of the saturation value. Our main focus are ladders with 3 legs where plateaux with magnetization one third of the saturation value are shown to exist.
TL;DR: In this article, the authors show how toroidally compactified eleven-dimensional supergravity can be consistently truncated to yield a variety of maximally supersymmetric massive supergravities in space-time dimensions D ⩽ 8.
TL;DR: In this paper, the International Ultraviolet Explorer (IUE) was used to observe the BL Lac object PKS 2155-304 for 10 days and 2 days with ASCA and with ground-based radio, infrared, and optical telescopes.
Abstract: In 1994 May, the BL Lac object PKS 2155-304 was observed continuously for ~10 days with the International Ultraviolet Explorer and the Extreme Ultraviolet Explorer and for 2 days with ASCA, as well as with ROSAT and with ground-based radio, infrared, and optical telescopes. The light curves show a well-defined X-ray flare followed by a broader, lower amplitude extreme-ultraviolet flare ~1 day later and a broad, low-amplitude UV flare ~2 days later. X-ray fluxes obtained at three well-separated times the preceding week indicate at least one previous flare of comparable amplitude or perhaps ongoing stochastic X-ray variations, and additional rapid variability was seen at the beginning of the IUE observation, when extremely sharp changes in UV flux occurred. The X-ray flux observed with ASCA flared by a factor of ~2 in about half a day and decayed roughly as fast. In contrast, the subsequent UV flare had an amplitude of only ~35% and lasted longer than 2 days. Assuming that the X-ray, EUV, and UV events are associated, the lags, the decrease of amplitude with wavelength, and the broadening of the temporal profile with wavelength are all qualitatively as expected for synchrotron emission from an inhomogeneous, relativistic jet. Because of the high quality of the data, we can rule out that the observed flares were caused by either a Fermi-type shock acceleration event or a pair cascade in a homogeneous synchrotron-emitting region. A homogeneous region is still possible if there was an instantaneous (t hours) injection of high-energy electrons that emit first at X-ray energies. Alternatively, the data are consistent with a compression wave or other disturbance crossing a region with stratified particle energy distributions. This kind of situation is expected to occur behind a shock front and/or in an inhomogeneous jet. The present light curves are in sharp contrast to the multiwavelength variability observed in 1991 November, when the amplitude was wavelength independent and the UV lagged the X-rays by less than ~3 hr. This means that the origin of rapid multiwavelength variability in this blazar is complex, involving at least two different modes.
TL;DR: In this article, the BL Lac object PKS 2155-304 was observed continuously for ~10 d with IUE and EUVE and for 2 d with ASCA, as well as with ROSAT and with ground-based radio, IR, and optical telescopes.
Abstract: In May 1994 the BL Lac object PKS 2155-304 was observed continuously for ~10 d with IUE and EUVE and for 2 d with ASCA, as well as with ROSAT and with ground- based radio, IR, and optical telescopes. The light curves show a well-defined X-ray flare (x2 increase in 1/2 d, with similar decay) followed by a broader, lower amplitude EUV flare ~1 day later, and a broad, low-amplitude UV flare ~2 d later (amplitude ~35%, duration >2 d). In the preceding week there was at least one X-ray flare of comparable amplitude or perhaps ongoing stochastic X-ray variations. An extremely rapid change in UV flux was seen at the beginning of the IUE observation. Assuming the central X-ray, EUV, and UV events are associated, the lags, the decrease of amplitude with wavelength, and the broadening of the temporal profile with wavelength are all qualitatively as expected for synchrotron emission from an inhomogeneous relativistic jet. We can rule out a Fermi-type shock acceleration event or a pair cascade in a homogeneous synchrotron-emitting region. A homogeneous region is still possible if there was an instantaneous (t<
01 Jan 1997
TL;DR: In this paper, the effects of heavy particle mixing for arbitrary mass splittings were studied for leptogenesis scenarios and a considerable enhancement of the asymmetries was achieved when the masses of the mixed states are comparable.
TL;DR: In this paper, the one-loop corrections to the gauge couplings were shown to be free of any infra-red ambiguity, and the contribution of N = 1 sectors to the unification picture was analyzed.
TL;DR: Spontaneous activity in cultured networks of cortical neurons has been investigated by using a multisite recording technique with planar electrode arrays, and the properties of the network such as the synchronization and the periodicity have been quantified to clarify how variations of intrinsic parameters of thenetwork can induce structural transitions in the neural dynamics.
Abstract: Experimental investigation of the dynamics of biological networks is a fundamental step towards understanding how the nervous system works. Spontaneous activity in cultured networks of cortical neurons has been investigated by using a multisite recording technique with planar electrode arrays. In these networks, the spatiotemporal firing patterns were studied in the presence of different extracellular solutions. Transitions from asynchronous firing dynamics to synchronous firing dynamics were observed when the extracellular Ca2+ concentration was increased from 0.1 mM to 1 mM. Addition of extracellular Mg2+ reduced the spontaneous activity at any Ca2+ concentration, and an increase in the extracellular K+ concentration enhanced the frequency of periodical synchronous bursts. N-methyl-D-aspartate (NMDA) and non-NMDA glutamate receptor antagonists inhibited synchronous activity. A spatiotemporal analysis of the data has been performed, and the properties of the network such as the synchronization and the periodicity have been quantified in order to clarify how variations of intrinsic parameters of the network can induce structural transitions in the neural dynamics. This experimental study is a possible approach to investigate the computational properties of a neuronal network.
TL;DR: In this paper, a simple E{circle_times}e Jahn-Teller model was proposed for non-magnetic insulators with Mott-like properties, with an interesting interplay between electron-electron repulsion and the Jahn Teller effect.
Abstract: Narrow-band conductors may turn insulating and magnetic as a consequence of strong electron-electron correlation. In molecular conductors, the concomitance of a strong Jahn-Teller coupling may give rise to the alternative possibility of a nonmagnetic insulator, with or without a static cooperative Jahn-Teller distortion. In the latter case the insulator has Mott-like properties, with an interesting interplay between electron-electron repulsion and the Jahn-Teller effect, which is dynamical. We study this kind of nonmagnetic insulator in a very simple E{circle_times}e Jahn-Teller model and we discuss its general properties in a more general context, also in connection with the insulating state of K{sub 4}C{sub 60} and Rb{sub 4}C{sub 60}. {copyright} {ital 1997} {ital The American Physical Society}
TL;DR: In this paper, a generic partial supersymmetry breaking of N = 2 supergravity with zero vacuum energy and with surviving unbroken arbitrary gauge groups is presented, where specific examples are given.
TL;DR: In this article, the authors studied some applications of solvable Lie algebras in type IIA, type IIB and M-theories and showed that RR and NS generators find a natural geometric interpretation in this framework.