2,237 research outputs found
Realisations of Symmetry
We perform a systematic investigation of free-scalar realisations of the
Za\-mo\-lod\-chi\-kov algebra in which the operator product of two
spin-three generators contains a non-zero operator of spin four which has
vanishing norm. This generalises earlier work where such an operator was
required to be absent. By allowing this spin-four null operator we obtain
several realisations of the algebra both in terms of two scalars as well
as in terms of an arbitrary number of free scalars. Our analysis is
complete for the case of two-scalar realisations.Comment: 14 pages, LATEX, UG-6/9
A dendritic mechanism for decoding traveling waves: Principles and applications to motor cortex
Traveling waves of neuronal oscillations have been observed in many cortical regions, including the motor and sensory cortex. Such waves are often modulated in a task-dependent fashion although their precise functional role remains a matter of debate. Here we conjecture that the cortex can utilize the direction and wavelength of traveling waves to encode information. We present a novel neural mechanism by which such information may be decoded by the spatial arrangement of receptors within the dendritic receptor field. In particular, we show how the density distributions of excitatory and inhibitory receptors can combine to act as a spatial filter of wave patterns. The proposed dendritic mechanism ensures that the neuron selectively responds to specific wave patterns, thus constituting a neural basis of pattern decoding. We validate this proposal in the descending motor system, where we model the large receptor fields of the pyramidal tract neurons — the principle outputs of the motor cortex — decoding motor commands encoded in the direction of traveling wave patterns in motor cortex. We use an existing model of field oscillations in motor cortex to investigate how the topology of the pyramidal cell receptor field acts to tune the cells responses to specific oscillatory wave patterns, even when those patterns are highly degraded. The model replicates key findings of the descending motor system during simple motor tasks, including variable interspike intervals and weak corticospinal coherence. By additionally showing how the nature of the wave patterns can be controlled by modulating the topology of local intra-cortical connections, we hence propose a novel integrated neuronal model of encoding and decoding motor commands
DARIS : a low-frequency distributed aperture array for radio astronomy in space
The frequency band below 30 MHz is one of the last unexplored bands in radio astronomy. This band is well suited for studying the early cosmos at high hydrogen redshifts, the so-called dark ages, extragalactic surveys, (extra) solar planetary bursts, and high energy particle physics. In addition, space research such as space weather tomography, are also areas of scientific interest. \ud
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Due to ionospheric scintillation (below 30MHz) and its opaqueness (below 15MHz), earth-bound radio astronomy observations in these bands are either severely limited in sensitivity and spatial resolution or entirely impossible. A radio telescope in space obviously would not be hampered by the Earth's ionosphere. In the past, several (limited) studies have been conducted to explore possibilities for such an array in space. These studies considered aperture synthesis arrays in space, at the back-side of the Moon, or a satellite constellation operating in a coherent mode. \u
Systemic sclerosis: are anti-nuclear antibodies our guiding stars?
In this thesis the role of anti-nuclear antibodies to function as biomarkers in systemic sclerosis has been evaluated. Respectively, the heterogeneity of the disease, the need for biomarkers and the role of auto-antibodies to function as such, with specific attention for anti-topoisomerase I, have been outlined in this thesis. Boehringer Ingelheim; Chipsoft; Stichting NVLE fondsLUMC / Geneeskund
Mirror effect induced by the dilaton field on the Hawking radiation
We discuss the string creation in the near-extremal NS1 black string
solution. The string creation is described by an effective field equation
derived from a fundamental string action coupled to the dilaton field in a
conformally invariant manner. In the non-critical string model the dilaton
field causes a timelike mirror surface outside the horizon when the size of the
black string is comparable to the Planck scale. Since the fundamental strings
are reflected by the mirror surface, the negative energy flux does not
propagate across the surface. This means that the evaporation stops just before
the naked singularity of the extremal black string appears even though the
surface gravity is non-zero in the extremal limit.Comment: 15 page
A two-stage thermal modification of wood
Thermal modification or heat treatment is an effective method to improve the dimensional stability and biological durability of wood. An undesired side effect is the reduction of mechanical properties of wood, mainly due to the high temperatures involved (150-280°C). In this thesis the effects of heat treatment on wood properties are discussed in relation to anatomical and molecular changes found in wood after heat treatment. The results are based on investigations of a full-scale industrial heat treatment method under relative mild conditions (< 200°C) including a moist and dry treatment stage. Several changes and/or modifications of the main components of wood (cellulose, hemicelluloses and lignin) appear to be involved in the effects of heat treatment on important wood properties like dimensional stability, strength/stiffness and biological durability. Although the effect of heat treatment on the anatomical structure of wood is limited, changes have been found affecting the properties of wood. This thesis provides a better understanding of thermal modification, which probably contributes to a more controlled use of heat-treated wood in service conditions
Generalized Conformal Quantum Mechanics of D0-brane
We study the generalized conformal quantum mechanics of the probe D0-brane in
the near horizon background of the bound state of source D0-branes. We
elaborate on the relationship of such model to the M theory in the light cone
frame.Comment: 14 pages, RevTeX, revised version with added references to appear in
Phys. Rev.
Consistent Sphere Reductions and Universality of the Coulomb Branch in the Domain-Wall/QFT Correspondence
We prove that any D-dimensional theory comprising gravity, an antisymmetric
n-index field strength and a dilaton can be consistently reduced on S^n in a
truncation in which just scalar fields and the metric are retained in
(D-n)-dimensions, provided only that the strength of the couping of the dilaton
to the field strength is appropriately chosen. A consistent reduction can then
be performed for n\le 5; with D being arbitrary when n\le 3, whilst D\le 11 for
n=4 and D\le 10 for n=5. (Or, by Hodge dualisation, can be replaced by
(D-n) in these conditions.) We obtain the lower dimensional scalar potentials
and construct associated domain wall solutions. We use the consistent reduction
Ansatz to lift domain-wall solutions in the (D-n)-dimensional theory back to D
dimensions, where we show that they become certain continuous distributions of
(D-n-2)-branes. We also examine the spectrum for a minimally-coupled scalar
field in the domain-wall background, showing that it has a universal structure
characterised completely by the dimension n of the compactifying sphere.Comment: latex file, 21 pages, 1 figure, minor typo correction
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