103 research outputs found
Cholinergic and GABAergic pathways in fly motion vision
BACKGROUND: The fly visual system is a highly ordered brain structure with well-established physiological and behavioral functions. A large number of interneurons in the posterior part of the third visual neuropil, the lobula plate tangential cells (LPTCs), respond to visual motion stimuli. In these cells the mechanism of motion detection has been studied in great detail. Nevertheless, the cellular computations leading to their directionally selective responses are not yet fully understood. Earlier studies addressed the neuropharmacological basis of the motion response in lobula plate interneurons. In the present study we investigated the distribution of the respective neurotransmitter receptors in the fly visual system, namely nicotinic acetylcholine receptors (nAChRs) and GABA receptors (GABARs) demonstrated by antibody labeling. RESULTS: The medulla shows a laminar distribution of both nAChRs and GABARs. Both receptor types are present in layers that participate in motion processing. The lobula also shows a characteristic layering of immunoreactivity for either receptor in its posterior portion. Furthermore, immunostaining for nAChRs and GABARs can be observed in close vicinity of lobula plate tangential cells. Immunostaining of GABAergic fibers suggests that inhibitory inputs from the medulla are relayed through the lobula to the lobula plate rather than through direct connections between medulla and lobula plate. CONCLUSIONS: The interaction of excitatory and inhibitory pathways is essential for the computation of visual motion responses and discussed in the context of the Reichardt model for motion detection
Quantum general relativity and Hawking radiation
In a previous paper we have set up the Wheeler-DeWitt equation which
describes the quantum general relativistic collapse of a spherical dust cloud.
In the present paper we specialize this equation to the case of matter
perturbations around a black hole, and show that in the WKB approximation, the
wave-functional describes an eternal black hole in equilibrium with a thermal
bath at Hawking temperature.Comment: 13 pages, minor revisions in: (i) para 5 of Introduction, (ii) para
following Eqn. (10). Revised version to appear in Phys. Rev.
Semiclassical Black Hole States and Entropy
We discuss semiclassical states in quantum gravity corresponding to
Schwarzschild as well as Reissner Nordstr\"om black holes. We show that reduced
quantisation of these models is equivalent to Wheeler-DeWitt quantisation with
a particular factor ordering. We then demonstrate how the entropy of black
holes can be consistently calculated from these states. While this leads to the
Bekenstein-Hawking entropy in the Schwarzschild and non-extreme
Reissner-Nordstr\"om cases, the entropy for the extreme Reissner-Nordstr\"om
case turns out to be zero.Comment: Revtex, 15 pages, some clarifying comments and additional references
included, to appear in Phys. Rev.
Microcanonical statistics of black holes and bootstrap condition
The microcanonical statistics of the Schwarzschild black holes as well as the
Reissner-Nordstrm black holes are analyzed. In both cases we set
up the inequalities in the microcanonical density of states.
These are then used to show that the most probable configuration in the gases
of black holes is that one black hole acquires all of the mass and all of the
charge at high energy limit. Thus the black holes obey the statistical
bootstrap condition and, in contrast to the other investigation, we see that
U(1) charge does not break the bootstrap property.Comment: 16 pages. late
On Modular Invariance and 3D Gravitational Instantons
We study the modular transformation properties of Euclidean solutions of 3D
gravity whose asymptotic geometry has the topology of a torus. These solutions
represent saddle points of the grand canonical partition function with an
important example being the BTZ black hole, and their properties under modular
transformations are inherited from the boundary conformal field theory encoding
the asymptotic dynamics. Within the Chern Simons formulation, classical
solutions are characterised by specific holonomies describing the wrapping of
the gauge field around cycles of the torus. We find that provided these
holonomies transform in an appropriate manner, there exists an associated
modular invariant grand canonical partition function and that the spectrum of
saddle points naturally includes a thermal bath in as discussed by
Maldacena and Strominger. Indeed, certain modular transformations can naturally
be described within classical bulk dynamics as mapping between different
foliations with a "time" coordinate along different cycles of the asymptotic
torus.Comment: 14 pages, RevTeX, typos corrected, to appear in Phys. Rev.
A Quantum Mechanical Model of the Reissner-Nordstrom Black Hole
We consider a Hamiltonian quantum theory of spherically symmetric,
asymptotically flat electrovacuum spacetimes. The physical phase space of such
spacetimes is spanned by the mass and the charge parameters and of the
Reissner-Nordstr\"{o}m black hole, together with the corresponding canonical
momenta. In this four-dimensional phase space, we perform a canonical
transformation such that the resulting configuration variables describe the
dynamical properties of Reissner-Nordstr\"{o}m black holes in a natural manner.
The classical Hamiltonian written in terms of these variables and their
conjugate momenta is replaced by the corresponding self-adjoint Hamiltonian
operator, and an eigenvalue equation for the ADM mass of the hole, from the
point of view of a distant observer at rest, is obtained. Our eigenvalue
equation implies that the ADM mass and the electric charge spectra of the hole
are discrete, and the mass spectrum is bounded below. Moreover, the spectrum of
the quantity is strictly positive when an appropriate self-adjoint
extension is chosen. The WKB analysis yields the result that the large
eigenvalues of the quantity are of the form , where
is an integer. It turns out that this result is closely related to
Bekenstein's proposal on the discrete horizon area spectrum of black holes.Comment: 37 pages, Plain TeX, no figure
Topology Change in Canonical Quantum Cosmology
We develop the canonical quantization of a midisuperspace model which
contains, as a subspace, a minisuperspace constituted of a
Friedman-Lema\^{\i}tre-Robertson-Walker Universe filled with homogeneous scalar
and dust fields, where the sign of the intrinsic curvature of the spacelike
hypersurfaces of homogeneity is not specified, allowing the study of topology
change in these hypersurfaces. We solve the Wheeler-DeWitt equation of the
midisuperspace model restricted to this minisuperspace subspace in the
semi-classical approximation. Adopting the conditional probability
interpretation, we find that some of the solutions present change of topology
of the homogeneous hypersurfaces. However, this result depends crucially on the
interpretation we adopt: using the usual probabilistic interpretation, we find
selection rules which forbid some of these topology changes.Comment: 23 pages, LaTex file. We added in the conclusion some comments about
path integral formalism and corrected litle misprinting
Must Quantum Spacetimes Be Euclidean?
The Bohm-de Broglie interpretation of quantum mechanics is applied to
canonical quantum cosmology. It is shown that, irrespective of any
regularization or choice of factor ordering of the Wheeler-DeWitt equation, the
unique relevant quantum effect which does not break spacetime is the change of
its signature from lorentzian to euclidean. The other quantum effects are
either trivial or break the four-geometry of spacetime. A Bohm-de Broglie
picture of a quantum geometrodynamics is constructed, which allows the
investigation of these latter structures. For instance, it is shown that any
real solution of the Wheeler-De Witt equation yields a generate four-geometry
compatible with the strong gravity limit of General Relativity and the Carroll
group. Due to the more detailed description of quantum geometrodynamics given
by the Bohm-de Broglie interpretation, some new boundary conditions on
solutions of the Wheeler-DeWitt equation must be imposed in order to preserve
consistency of this finer view.Comment: 42 pages LaTeX, last version with minor corrections, being the most
importants on pages 0, 6, 11, 21, 23, and 30 . The new title does not change
our conclusion
Quantum Scalar Field on the Massless (2+1)-Dimensional Black Hole Background
The behavior of a quantum scalar field is studied in the metric ground state
of the (2+1)-dimensional black hole of Ba\~nados, Teitelboim and Zanelli which
contains a naked singularity. The one-loop BTZ partition function and the
associate black hole effective entropy, the expectation value of the quantum
fluctuation as well as the renormalized expectation value of the stress tensor
are explicitly computed in the framework of the -function procedure.
This is done for all values of the coupling with the curvature, the mass of the
field and the temperature of the quantum state. In the massless conformally
coupled case, the found stress tensor is used for determining the quantum back
reaction on the metric due to the scalar field in the quantum vacuum state, by
solving the semiclassical Einstein equations. It is finally argued that, within
the framework of the 1/N expansion, the Cosmic Censorship Hypothesis is
implemented since the naked singularity of the ground state metric is shielded
by an event horizon created by the back reaction.Comment: 18 pages, RevTeX, no figures, minor changes, final version accepted
for publication in Phys. Rev.
The Bohm Interpretation of Quantum Cosmology
I make a review on the aplications of the Bohm-De Broglie interpretation of
quantum mechanics to quantum cosmology. In the framework of minisuperspaces
models, I show how quantum cosmological effects in Bohm's view can avoid the
initial singularity, isotropize the Universe, and even be a cause for the
present observed acceleration of the Universe. In the general case, we
enumerate the possible structures of quantum space and time.Comment: 28 pages, 1 figure, contribution to the James Cushing festschrift to
appear in Foundations of Physic
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