603 research outputs found

    Generating Coherent Patterns of Activity from Chaotic Neural Networks

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    SummaryNeural circuits display complex activity patterns both spontaneously and when responding to a stimulus or generating a motor output. How are these two forms of activity related? We develop a procedure called FORCE learning for modifying synaptic strengths either external to or within a model neural network to change chaotic spontaneous activity into a wide variety of desired activity patterns. FORCE learning works even though the networks we train are spontaneously chaotic and we leave feedback loops intact and unclamped during learning. Using this approach, we construct networks that produce a wide variety of complex output patterns, input-output transformations that require memory, multiple outputs that can be switched by control inputs, and motor patterns matching human motion capture data. Our results reproduce data on premovement activity in motor and premotor cortex, and suggest that synaptic plasticity may be a more rapid and powerful modulator of network activity than generally appreciated

    A Massive Non-Abelian Vector Model

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    The introduction of a Lagrange multiplier field to ensure that the classical equations of motion are satisfied serves to restrict radiative corrections in a model to being only one loop. The consequences of this for a massive non-Abelian vector model are considered.Comment: 8 pages, LaTeX format; further comments added; accepted for publication at the Canadian Journal of Physic

    Homeostasis and Learning through Spike-Timing Dependent Plasticity

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    Synaptic plasticity is thought to be the neuronal correlate of learning. Moreover, modification of synapses contributes to the activity-dependent homeostatic maintenance of neurons and neural networks. In this chapter, we review theories of synaptic plasticity and show that both homeostatic control of activity and detection of correlations in the presynaptic input can arise from spike-timing dependent plasticity (STDP). Relations to classical rate-based Hebbian learning are discussed

    A Model Behind the Standard Model

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    In spite of its many successes, the Standard Model makes many empirical assumptions in the Higgs and fermion sectors for which a deeper theoretical basis is sought. Starting from the usual gauge symmetry u(1)×su(2)×su(3)u(1) \times su(2) \times su(3) plus the 3 assumptions: (A) scalar fields as vielbeins in internal symmetry space \cite{framevec}, (B) the ``confinement picture'' of symmetry breaking \cite{tHooft,Banovici}, (C) generations as ``dual'' to colour \cite{genmixdsm}, we are led to a scheme which offers: (I) a geometrical significance to scalar fields, (II) a theoretical criterion on what scalar fields are to be introduced, (III) a partial explanation of why su(2)su(2) appears broken while su(3)su(3) confines, (IV) baryon-lepton number (B - L) conservation, (V) the standard electroweak structure, (VI) a 3-valued generation index for leptons and quarks, and (VII) a dynamical system with all the essential features of an earlier phenomenological model \cite{genmixdsm} which gave a good description of the known mass and mixing patterns of quarks and leptons including neutrino oscillations. There are other implications the consistency of which with experiment, however, has not yet been systematically explored. A possible outcome is a whole new branch of particle spectroscopy from su(2)su(2) confinement, potentially as rich in details as that of hadrons from colour confinement, which will be accessible to experiment at high energy.Comment: 66 pages, added new material on phenomenology, and some new reference

    The Sachs-Wolfe Effect: Gauge Independence and a General Expression

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    In this paper we address two points concerning the Sachs-Wolfe effect: (i) the gauge independence of the observable temperature anisotropy, and (ii) a gauge-invariant expression of the effect considering the most general situation of hydrodynamic perturbations. The first result follows because the gauge transformation of the temperature fluctuation at the observation event only contributes to the isotropic temperature change which, in practice, is absorbed into the definition of the background temperature. Thus, we proceed without fixing the gauge condition, and express the Sachs-Wolfe effect using the gauge-invariant variables.Comment: 5 pages, closer to published versio

    Vacuum Bubble in an Inhomogeneous Cosmology

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    We study the propagation of bubbles of new vacuum in a radially inhomogeneous Lemaitre-Tolman-Bondi background that includes a cosmological constant. This exemplifies the classical evolution of a tunneling bubble through a metastable state with curvature inhomogeneities, and will be relevant in the context of the Landscape. We demand that the matter profile in the LTB background satisfy the weak energy condition. For sample profiles that satisfy this restriction, we find that the evolution of the bubble (in terms of the physically relevant coordinates intrinsic to the shell) is largely unaffected by the prsence of local inhomogeneities. Our setup should also be a useful toy model for capturing the effects of ambient inhomogeneities on an inflating region.Comment: 31 pages, 21(!) figures, v2: minor changes, figures re-sized (might require zoom on some systems), references adde

    Basic Gravitational Currents and Killing-Yano Forms

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    It has been shown that for each Killing-Yano (KY)-form accepted by an nn-dimensional (pseudo)Riemannian manifold of arbitrary signature, two basic gravitational currents can be defined. Conservation of the currents are explicitly proved by showing co-exactness of the one and co-closedness of the other. Some general geometrical facts implied by these conservation laws are also elucidated. In particular, the conservation of the one-form currents implies that the scalar curvature of the manifold is a flow invariant for all of its Killing vector fields. It also directly follows that, while all KY-forms and their Hodge duals on a constant curvature manifold are the eigenforms of the Laplace-Beltrami operator, for an Einstein manifold this is certain only for KY 1-forms, (n1)(n-1)-forms and their Hodge duals.Comment: 11 page

    CBR Anisotropy from Primordial Gravitational Waves in Two-Component Inflationary Cosmology

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    We examine stochastic temperature fluctuations of the cosmic background radiation (CBR) arising via the Sachs-Wolfe effect from gravitational wave perturbations produced in the early universe. We consider spatially flat, perturbed FRW models that begin with an inflationary phase, followed by a mixed phase containing both radiation and dust. The scale factor during the mixed phase takes the form a(η)=c1η2+c2η+c3a(\eta)=c_1\eta^2+c_2\eta+c_3, where cic_i are constants. During the mixed phase the universe smoothly transforms from being radiation to dust dominated. We find analytic expressions for the graviton mode function during the mixed phase in terms of spheroidal wave functions. This mode function is used to find an analytic expression for the multipole moments al2\langle a_l^2\rangle of the two-point angular correlation function C(γ)C(\gamma) for the CBR anisotropy. The analytic expression for the multipole moments is written in terms of two integrals, which are evaluated numerically. The results are compared to multipoles calculated for models that are {\it completely} dust dominated at last-scattering. We find that the multipoles al2\langle a_l^2\rangle of the CBR temperature perturbations for l>10l>10 are significantly larger for a universe that contains both radiation and dust at last-scattering. We compare our results with recent, similar numerical work and find good agreement. The spheroidal wave functions may have applications to other problems of cosmological interest.Comment: 28 pgs + 6 postscript figures, RevTe

    Gravitational Waves in a Spatially Closed deSitter Spacetime

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    Perturbation of gravitational fields may be decomposed into scalar,vector and tensor components.In this paper we concern with the evolution of tensor mode perturbations in a spatially closed deSitter background of RW form. It may be thought as gravitional waves in a classical description. The chosen background has the advantage of to be maximally extended and symmetric. The spatially flat models commonly emerge from inflationary scenarios are not completely extended.We first derive the general weak field equations.Then the form of the field equations in two special cases, planar and spherical waves are obtained and their solutions are presented. We conclued with discussing the significance of the results and their implications.Comment: 16 pages,no figure

    Perturbative Expansion around the Gaussian Effective Action: The Background Field Method

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    We develop a systematic method of the perturbative expansion around the Gaussian effective action based on the background field method. We show, by applying the method to the quantum mechanical anharmonic oscillator problem, that even the first non-trivial correction terms greatly improve the Gaussian approximation.Comment: 16 pages, 3 eps figures, uses RevTeX and epsf. Errors in Table 1 are corrected and new references are adde
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