14,898 research outputs found
Remark about Non-BPS Dp-Brane at the Tachyon Vacuum Moving in Curved Background
This paper is devoted to the study of the dynamics of a non-BPS Dp-brane at
the tachyon vacuum that moves in the curved background.Comment: 20 page
An oscillatory interference model of grid cell firing
We expand upon our proposal that the oscillatory interference mechanism proposed for the phase precession effect in place cells underlies the grid-like firing pattern of dorsomedial entorhinal grid cells (O'Keefe and Burgess (2005) Hippocampus 15:853-866). The original one-dimensional interference model is generalized to an appropriate two-dimensional mechanism. Specifically, dendritic subunits of layer 11 medial entorhinal stellate cells provide multiple linear interference patterns along different directions, with their product determining the firing of the cell. Connection of appropriate speed- and direction- dependent inputs onto dendritic subunits could result from an unsupervised learning rule which maximizes postsynaptic firing (e.g. competitive learning). These inputs cause the intrinsic oscillation of subunit membrane potential to. increase above theta frequency by an amount proportional to the animal's speed of running in the "preferred" direction. The phase difference between this oscillation and a somatic input at theta-frequency essentially integrates velocity so that the interference of the two oscillations reflects distance traveled in the preferred direction. The overall grid pattern is maintained in environmental location by phase reset of the grid cell by place cells receiving sensory input from the environment, and environmental boundaries in particular. We also outline possible variations on the basic model, including the generation of grid-like firing via the interaction of multiple cells rather than via multiple dendritic subunits. Predictions of the interference model are given for the frequency composition of EEG power spectra and temporal autocorrelograms of grid cell firing as functions of the speed and direction of running and the novelty of the environment. (C) 2007 Wiley-Liss, Inc
Scaling Solutions to 6D Gauged Chiral Supergravity
We construct explicitly time-dependent exact solutions to the field equations
of 6D gauged chiral supergravity, compactified to 4D in the presence of up to
two 3-branes situated within the extra dimensions. The solutions we find are
scaling solutions, and are plausibly attractors which represent the late-time
evolution of a broad class of initial conditions. By matching their near-brane
boundary conditions to physical brane properties we argue that these solutions
(together with the known maximally-symmetric solutions and a new class of
non-Lorentz-invariant static solutions, which we also present here) describe
the bulk geometry between a pair of 3-branes with non-trivial on-brane
equations of state.Comment: Contribution to the New Journal of Physics focus issue on Dark
Energy; 28 page
Theta-modulated place-by-direction cells in the hippocampal formation in the rat
We report the spatial and temporal properties of a class of cells termed theta-modulated place-by-direction (TPD) cells recorded from the presubicular and parasubicular cortices of the rat. The firing characteristics of TPD cells in open-field enclosures were compared with those of the following two other well characterized cell classes in the hippocampal formation: place and head-direction cells. Unlike place cells, which code only for the animal's location, or head-direction cells, which code only for the animal's directional heading, TPD cells code for both the location and the head direction of the animal. Their firing is also strongly theta modulated, firing primarily at the negative-to-positive phase of the locally recorded theta wave. TPD theta modulation is significantly stronger than that of place cells. In contrast, the firing of head-direction cells is not modulated by theta at all. In repeated exposures to the same environment, the locational and directional signals of TPD cells are stable. When recorded in different environments, TPD locational and directional fields can uncouple, with the locational field shifting unpredictably ("remapping"), whereas the directional preference remains similar across environments
ON GAUGINO CONDENSATION WITH FIELD-DEPENDENT GAUGE COUPLINGS
We study in detail gaugino condensation in globally and locally
supersymmetric Yang-Mills theories. We focus on models for which gauge-neutral
matter couples to the gauge bosons only through nonminimal gauge kinetic terms,
for the cases of one and several condensing gauge groups. Using only symmetry
arguments, the low-energy expansion, and general properties of supersymmetry,
we compute the low energy Wilson action, as well as the (2PI) effective action
for the composite {\it classical} superfield U\equiv\langle \Tr\WW \rangle,
with the supersymmetric gauge field strength. The 2PI effective
action provides a firmer foundation for the approach of Veneziano and
Yankielowicz, who treated the composite superfield, , as a quantum degree of
freedom. We show how to rederive the Wilson action by minimizing the 2PI action
with respect to . We determine, in both formulations and for global and
local supersymmetry, the effective superpotential, , the non-perturbative
contributions to the low-energy K\"ahler potential , and the leading higher
supercovariant derivative terms in an expansion in inverse powers of the
condensation scale. As an application of our results we include the string
moduli dependence of the super- and K\"ahler potentials for simple orbifold
models.Comment: 54 pages, plain te
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