138 research outputs found
Place, space and memory cells
SummarySelf-localization and navigation are critical functions for the survival of mobile animals. By processing sensory information such as landmarks and environmental features, as well as keeping track of the path they have taken, animals are able to remain oriented as they explore the world, learning what resources are where and planning how to reach them. This ancient capacity for self-orientation has, through evolution, become intimately entwined with the ability to remember the events of daily life — an ability known as episodic memory. Spatial and episodic memory involve the interaction of many cognitive faculties and brain circuits, and so are fascinating subjects for study that reveal much about how the brain works
Non-Gaussianities due to Relativistic Corrections to the Observed Galaxy Bispectrum
High-precision constraints on primordial non-Gaussianity (PNG) will
significantly improve our understanding of the physics of the early universe.
Among all the subtleties in using large scale structure observables to
constrain PNG, accounting for relativistic corrections to the clustering
statistics is particularly important for the upcoming galaxy surveys covering
progressively larger fraction of the sky. We focus on relativistic projection
effects due to the fact that we observe the galaxies through the light that
reaches the telescope on perturbed geodesics. These projection effects can give
rise to an effective that can be misinterpreted as the primordial
non-Gaussianity signal and hence is a systematic to be carefully computed and
accounted for in modelling of the bispectrum. We develop the technique to
properly account for relativistic effects in terms of purely observable
quantities, namely angles and redshifts. We give some examples by applying this
approach to a subset of the contributions to the tree-level bispectrum of the
observed galaxy number counts calculated within perturbation theory and
estimate the corresponding non-Gaussianity parameter, , for the
local, equilateral and orthogonal shapes. For the local shape, we also compute
the local non-Gaussianity resulting from terms obtained using the consistency
relation for observed number counts. Our goal here is not to give a precise
estimate of for each shape but rather we aim to provide a scheme
to compute the non-Gaussian contamination due to relativistic projection
effects. For the terms considered in this work, we obtain contamination of
.Comment: 31 pages, 6 figures, Typos corrected to match the published version
in JCA
Global gravitational instability of FLRW backgrounds - interpreting the dark sectors
The standard model of cosmology is based on homogeneous-isotropic solutions
of Einstein's equations. These solutions are known to be gravitationally
unstable to local inhomogeneous perturbations, commonly described as evolving
on a background given by the same solutions. In this picture, the FLRW
backgrounds are taken to describe the average over inhomogeneous perturbations
for all times. We study in the present article the (in)stability of FLRW dust
backgrounds within a class of averaged inhomogeneous cosmologies. We examine
the phase portraits of the latter, discuss their fixed points and orbital
structure and provide detailed illustrations. We show that FLRW cosmologies are
unstable in some relevant cases: averaged models are driven away from them
through structure formation and accelerated expansion. We find support for the
proposal that the dark components of the FLRW framework may be associated to
these instability sectors. Our conclusion is that FLRW cosmologies have to be
considered critically as for their role to serve as reliable models for the
physical background.Comment: 15 pages, 13 figures, 1 table. Matches published version in CQ
Purely translational realignment in grid cell firing patterns following nonmetric context change
Grid cells in entorhinal and parahippocampal cortices contribute to a network, centered on the hippocampal place cell system, that constructs a representation of spatial context for use in navigation and memory. In doing so, they use metric cues such as the distance and direction of nearby boundaries to position and orient their firing field arrays (grids). The present study investigated whether they also use purely nonmetric “context” information such as color and odor of the environment. We found that, indeed, purely nonmetric cues—sufficiently salient to cause changes in place cell firing patterns—can regulate grid positioning; they do so independently of orientation, and thus interact with linear but not directional spatial inputs. Grid cells responded homogeneously to context changes. We suggest that the grid and place cell networks receive context information directly and also from each other; the information is used by place cells to compute the final decision of the spatial system about which context the animal is in, and by grid cells to help inform the system about where the animal is within it
Place field repetition and purely local remapping in a multicompartment environment
Hippocampal place cells support spatial memory using sensory information from the environment and self-motion information to localize their firing fields. Currently, there is disagreement about whether CA1 place cells can use pure self-motion information to disambiguate different compartments in environments containing multiple visually identical compartments. Some studies report that place cells can disambiguate different compartments, while others report that they do not. Furthermore, while numerous studies have examined remapping, there has been little examination of remapping in different subregions of a single environment. Is remapping purely local or do place fields in neighboring, unaffected, regions detect the change? We recorded place cells as rats foraged across a 4-compartment environment and report 3 new findings. First, we find that, unlike studies in which rats foraged in 2 compartments, place fields showed a high degree of spatial repetition with a slight degree of rate-based discrimination. Second, this repetition does not diminish with extended experience. Third, remapping was found to be purely local for both geometric change and contextual change. Our results reveal the limited capacity of the path integrator to drive pattern separation in hippocampal representations, and suggest that doorways may play a privileged role in segmenting the neural representation of space
Energy-Momentum Tensor of Cosmological Fluctuations during Inflation
We study the renormalized energy-momentum tensor (EMT) of cosmological scalar
fluctuations during the slow-rollover regime for chaotic inflation with a
quadratic potential and find that it is characterized by a negative energy
density which grows during slow-rollover. We also approach the back-reaction
problem as a second-order calculation in perturbation theory finding no
evidence that the back-reaction of cosmological fluctuations is a gauge
artifact. In agreement with the results on the EMT, the average expansion rate
is decreased by the back-reaction of cosmological fluctuations.Comment: 19 pages, no figures.An appendix and references added, conclusions
unchanged, version accepted for publication in PR
Energy-Momentum Tensor of Field Fluctuations in Massive Chaotic Inflation
We study the renormalized energy-momentum tensor (EMT) of the inflaton
fluctuations in rigid space-times during the slow-rollover regime for chaotic
inflation with a mass term. We use dimensional regularization with adiabatic
subtraction and introduce a novel analytic approximation for the inflaton
fluctuations which is valid during the slow-rollover regime. Using this
approximation we find a scale invariant spectrum for the inflaton fluctuations
in a rigid space-time, and we confirm this result by numerical methods. The
resulting renormalized EMT is covariantly conserved and agrees with the
Allen-Folacci result in the de Sitter limit, when the expansion is exactly
linearly exponential in time. We analytically show that the EMT tensor of the
inflaton fluctuations grows initially in time, but saturates to the value H^2
H(0)^2, where H is the Hubble parameter and H(0) is its value when inflation
has started. This result also implies that the quantum production of light
scalar fields (with mass smaller or equal to the inflaton mass) in this model
of chaotic inflation depends on the duration of inflation and is larger than
the usual result extrapolated from the de Sitter result.Comment: revtex style, 24 pages, 6 eps figures Numerical checks added and
moduli section improve
One-loop corrections to a scalar field during inflation
The leading quantum correction to the power spectrum of a
gravitationally-coupled light scalar field is calculated, assuming that it is
generated during a phase of single-field, slow-roll inflation.Comment: 33 pages, uses feynmp.sty and ioplatex journal style. v2: matches
version published in JCAP. v3: corrects sign error in Eq. (58). Corrects
final coefficient of the logarithm in Eq. (105). Small corrections to
discussion of divergences in 1-point function. Minor improvements to
discussion of UV behaviour in Sec. 4.
Impact of post-Born lensing on the CMB
Lensing of the CMB is affected by post-Born lensing, producing corrections to the convergence power spectrum and introducing field rotation. We show numerically that the lensing convergence power spectrum is affected at the lesssim 0.2% level on accessible scales, and that this correction and the field rotation are negligible for observations with arcminute beam and noise levels gsim 1 μK arcmin. The field rotation generates ~ 2.5% of the total lensing B-mode polarization amplitude (0.2% in power on small scales), but has a blue spectrum on large scales, making it highly subdominant to the convergence B modes on scales where they are a source of confusion for the signal from primordial gravitational waves. Since the post-Born signal is non-linear, it also generates a bispectrum with the convergence. We show that the post-Born contributions to the bispectrum substantially change the shape predicted from large-scale structure non-linearities alone, and hence must be included to estimate the expected total signal and impact of bispectrum biases on CMB lensing reconstruction quadratic estimators and other observables. The field-rotation power spectrum only becomes potentially detectable for noise levels Lt 1 μK arcmin, but its bispectrum with the convergence may be observable at ~ 3σ with Stage IV observations. Rotation-induced and convergence-induced B modes are slightly correlated by the bispectrum, and the bispectrum also produces additional contributions to the lensed BB power spectrum
Back-reaction and effective acceleration in generic LTB dust models
We provide a thorough examination of the conditions for the existence of
back-reaction and an "effective" acceleration (in the context of Buchert's
averaging formalism) in regular generic spherically symmetric
Lemaitre-Tolman-Bondi (LTB) dust models. By considering arbitrary spherical
comoving domains, we verify rigorously the fulfillment of these conditions
expressed in terms of suitable scalar variables that are evaluated at the
boundary of every domain. Effective deceleration necessarily occurs in all
domains in: (a) the asymptotic radial range of models converging to a FLRW
background, (b) the asymptotic time range of non-vacuum hyperbolic models, (c)
LTB self-similar solutions and (d) near a simultaneous big bang. Accelerating
domains are proven to exist in the following scenarios: (i) central vacuum
regions, (ii) central (non-vacuum) density voids, (iii) the intermediate radial
range of models converging to a FLRW background, (iv) the asymptotic radial
range of models converging to a Minkowski vacuum and (v) domains near and/or
intersecting a non-simultaneous big bang. All these scenarios occur in
hyperbolic models with negative averaged and local spatial curvature, though
scenarios (iv) and (v) are also possible in low density regions of a class of
elliptic models in which local spatial curvature is negative but its average is
positive. Rough numerical estimates between -0.003 and -0.5 were found for the
effective deceleration parameter. While the existence of accelerating domains
cannot be ruled out in models converging to an Einstein de Sitter background
and in domains undergoing gravitational collapse, the conditions for this are
very restrictive. The results obtained may provide important theoretical clues
on the effects of back-reaction and averaging in more general non-spherical
models.Comment: Final version accepted for publication in Classical and Quantum
Gravity. 47 pages in IOP LaTeX macros, 12 pdf figure
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