6,601 research outputs found
Invariance of visual operations at the level of receptive fields
Receptive field profiles registered by cell recordings have shown that
mammalian vision has developed receptive fields tuned to different sizes and
orientations in the image domain as well as to different image velocities in
space-time. This article presents a theoretical model by which families of
idealized receptive field profiles can be derived mathematically from a small
set of basic assumptions that correspond to structural properties of the
environment. The article also presents a theory for how basic invariance
properties to variations in scale, viewing direction and relative motion can be
obtained from the output of such receptive fields, using complementary
selection mechanisms that operate over the output of families of receptive
fields tuned to different parameters. Thereby, the theory shows how basic
invariance properties of a visual system can be obtained already at the level
of receptive fields, and we can explain the different shapes of receptive field
profiles found in biological vision from a requirement that the visual system
should be invariant to the natural types of image transformations that occur in
its environment.Comment: 40 pages, 17 figure
Label-driven weakly-supervised learning for multimodal deformable image registration
Spatially aligning medical images from different modalities remains a
challenging task, especially for intraoperative applications that require fast
and robust algorithms. We propose a weakly-supervised, label-driven formulation
for learning 3D voxel correspondence from higher-level label correspondence,
thereby bypassing classical intensity-based image similarity measures. During
training, a convolutional neural network is optimised by outputting a dense
displacement field (DDF) that warps a set of available anatomical labels from
the moving image to match their corresponding counterparts in the fixed image.
These label pairs, including solid organs, ducts, vessels, point landmarks and
other ad hoc structures, are only required at training time and can be
spatially aligned by minimising a cross-entropy function of the warped moving
label and the fixed label. During inference, the trained network takes a new
image pair to predict an optimal DDF, resulting in a fully-automatic,
label-free, real-time and deformable registration. For interventional
applications where large global transformation prevails, we also propose a
neural network architecture to jointly optimise the global- and local
displacements. Experiment results are presented based on cross-validating
registrations of 111 pairs of T2-weighted magnetic resonance images and 3D
transrectal ultrasound images from prostate cancer patients with a total of
over 4000 anatomical labels, yielding a median target registration error of 4.2
mm on landmark centroids and a median Dice of 0.88 on prostate glands.Comment: Accepted to ISBI 201
Physics of Deformed Special Relativity: Relativity Principle revisited
In many different ways, Deformed Special Relativity (DSR) has been argued to
provide an effective limit of quantum gravity in almost-flat regime. Some
experiments will soon be able to test some low energy effects of quantum
gravity, and DSR is a very promising candidate to describe these latter.
Unfortunately DSR is up to now plagued by many conceptual problems (in
particular how it describes macroscopic objects) which forbids a definitive
physical interpretation and clear predictions. Here we propose a consistent
framework to interpret DSR. We extend the principle of relativity: the same way
that Special Relativity showed us that the definition of a reference frame
requires to specify its speed, we show that DSR implies that we must also take
into account its mass. We further advocate a 5-dimensional point of view on DSR
physics and the extension of the kinematical symmetry from the Poincare group
to the Poincare-de Sitter group (ISO(4,1)). This leads us to introduce the
concept of a pentamomentum and to take into account the renormalization of the
DSR deformation parameter kappa. This allows the resolution of the "soccer ball
problem" (definition of many-particle-states) and provides a physical
interpretation of the non-commutativity and non-associativity of the addition
the relativistic quadrimomentum. In particular, the coproduct of the
kappa-Poincare algebra is interpreted as defining the law of change of
reference frames and not the law of scattering. This point of view places DSR
as a theory, half-way between Special Relativity and General Relativity,
effectively implementing the Schwarzschild mass bound in a flat relativistic
context.Comment: 24 pages, Revtex
Structural Aspects Of Gravitational Dynamics And The Emergent Perspective Of Gravity
I describe several conceptual aspects of a particular paradigm which treats
the field equations of gravity as emergent. These aspects are related to the
features of classical gravitational theories which defy explanation within the
conventional perspective. The alternative interpretation throws light on these
features and could provide better insights into possible description of quantum
structure of spacetime. This review complements the discussion in
arXiv:1207.0505, which describes space itself as emergent in the cosmological
context.Comment: Updated version of talks given at: (a) Sixth International School on
Field Theory and Gravitation - 2012, Petropolis, Brazil; (b) Colloquium at
Institute of Astrophysics, Paris, 2012 and (c) Discussion meeting on String
Theory, International Centre for Theoretical Sciences, Bangalore, 201
A Liouville String Approach to Microscopic Time and Cosmology
In the non-critical string framework that we have proposed recently, the time
is identified with a dynamical local renormalization group scale, the
Liouville mode, and behaves as a statistical evolution parameter, flowing
irreversibly from an infrared fixed point - which we conjecture to be a
topological string phase - to an ultraviolet one - which corresponds to a
static critical string vacuum. When applied to a toy two-dimensional model of
space-time singularities, this formalism yields an apparent renormalization of
the velocity of light, and a -dependent form of the uncertainty relation for
position and momentum of a test string. We speculate within this framework on a
stringy alternative to conventional field-theoretical inflation, and the decay
towards zero of the cosmological constant in a maximally-symmetric space.Comment: Latex 23 pages, no figures, CERN-TH.7000/93, CTP-TAMU-66/9
Holographic Micro Thermofield Geometries of BTZ Black Holes
We find general deformations of BTZ spacetime and identify the corresponding
thermofield initial states of the dual CFT. We deform the geometry by
introducing bulk fields dual to primary operators and find the back-reacted
gravity solutions to the quadratic order of the deformation parameter. The dual
thermofield initial states can be deformed by inserting arbitrary linear
combination of operators at the mid-point of the Euclidean time evolution that
appears in the construction of the thermofield initial states. The deformed
geometries are dual to thermofield states without deforming the boundary
Hamiltonians in the CFT side. We explicitly demonstrate that the AdS/CFT
correspondence is not a linear correspondence in the sense that the linear
structure of Hilbert space of the underlying CFT is realized nonlinearly in the
gravity side. We also find that their Penrose diagrams are no longer a square
but elongated horizontally due to deformation. These geometries describe a
relaxation of generic initial perturbation of thermal system while fixing the
total energy of the system. The coarse-grained entropy grows and the relaxation
time scale is of order . We clarify that the gravity description
involves coarse-graining inevitably missing some information of nonperurbative
degrees.Comment: 28 pages, 7 figures, further typos corrected, reference adde
Noncommutative geometry of the quantum clock
We introduce a model of noncommutative geometry that gives rise to the
uncertainty relations recently derived from the discussion of a quantum clock.
We investigate the dynamics of a free particle in this model from the point of
view of doubly special relativity and discuss the geodesic motion in a
Schwarzschild background.Comment: 7 pages, version accepted for publication on Phys. Lett. A. A
discussion of the motion of a particle in a Schwarzschild background has been
adde
Equilibrium Configurations of Synchronous Binaries: Numerical Solutions and Application to Kuiper-Belt Binary 2001 QG298
We present numerical computations of the equilibrium configurations of
tidally-locked homogeneous binaries, rotating in circular orbits. Unlike the
classical Roche approximations, we self-consistently account for the tidal and
rotational deformations of both components, and relax the assumptions of
ellipsoidal configurations and Keplerian rotation. We find numerical solutions
for mass ratios q between 1e-3 and 1, starting at a small angular velocity for
which tidal and rotational deformations are small, and following a sequence of
increasing angular velocities. Each series terminates at an appropriate ``Roche
limit'', above which no equilibrium solution can be found. Even though the
Roche limit is crossed before the ``Roche lobe'' is filled, any further
increase in the angular velocity will result in mass-loss. For close,
comparable-mass binaries, we find that local deviations from ellipsoidal forms
may be as large as 10-20%, and departures from Keplerian rotation are
significant. We compute the light curves that arise from our equilibrium
configurations, assuming their distance is >>1 AU (e.g. in the Kuiper Belt). We
consider both backscatter (proportional to the projected area) and diffuse
(Lambert) reflections. Backscatter reflection always yields two minima of equal
depths. Diffuse reflection, which is sensitive to the surface curvature,
generally gives rise to unequal minima. We find detectable intensity
differences of up to 10% between our light curves and those arising from the
Roche approximations. Finally, we apply our models to Kuiper Belt binary 2001
QG298, and find a nearly edge-on binary with a mass ratio q = 0.93
^{+0.07}_{-0.03}, angular velocity Omega^2/G rho = 0.333+/-0.001 (statistical
errors only), and pure diffuse reflection. For the observed period of 2001
QG298, these parameters imply a bulk density, rho = 0.72 +/- 0.04 g cm^-3.Comment: Accepted to Ap
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On Eulerian and Lagrangean objectivity in continuum mechanics
In continuum mechanics the commonly—used
definition of objectivity (or frame-indifference) of a tensor field does not distinguish between Eulerian, Lagrangean and two—point tensor fields. This paper highlights the distinction and provides a definition of objectivity which reflects the different transformation rules for Eulerian, Lagrangean and two- point tensor fields under an observer transformation. The notion of induced objectivity is introduced and its implications examined
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