972 research outputs found
The state of MIIND
MIIND (Multiple Interacting Instantiations of Neural Dynamics) is a highly modular multi-level C++ framework, that aims to shorten the development time for models in Cognitive Neuroscience (CNS). It offers reusable code modules (libraries of classes and functions) aimed at solving problems that occur repeatedly in modelling, but tries not to impose a specific modelling philosophy or methodology. At the lowest level, it offers support for the implementation of sparse networks. For example, the library SparseImplementationLib supports sparse random networks and the library LayerMappingLib can be used for sparse regular networks of filter-like operators. The library DynamicLib, which builds on top of the library SparseImplementationLib, offers a generic framework for simulating network processes. Presently, several specific network process implementations are provided in MIIND: the WilsonâCowan and OrnsteinâUhlenbeck type, and population density techniques for leaky-integrate-and-fire neurons driven by Poisson input. A design principle of MIIND is to support detailing: the refinement of an originally simple model into a form where more biological detail is included. Another design principle is extensibility: the reuse of an existing model in a larger, more extended one. One of the main uses of MIIND so far has been the instantiation of neural models of visual attention. Recently, we have added a library for implementing biologically-inspired models of artificial vision, such as HMAX and recent successors. In the long run we hope to be able to apply suitably adapted neuronal mechanisms of attention to these artificial models
Origin of Graphite in the Southwestern Grenville Province
Two graphite deposits in the southwestern Grenville Province are investigated to evaluate the origin of graphitic carbon and to test if the graphite mineralization is syngenetic or epigenetic. Graphite mineralization in the Bissett Creek deposit is characterized by homogeneously distributed and disseminated graphite flakes (approximately 1 to 5 mm in size and 2 to 10 vol.%) within graphitic gneisses. The graphite flakes are intergrown with metamorphic minerals, most notably biotite. The Montpellier graphite showing in QuÂŽebec contains graphite concentrations of up to 20 vol.%. In contrast to the disseminated and homogenously distributed graphite in the Bissett Creek deposit, graphite mineralization at Montpellier forms lenses of variable sizes that occur at the top of a calc-silicate unit and as graphite-rich lenses in biotite-sillimanite-rich paragneiss. The ÎŽ13C of graphite ranges from â29 to â17 â° at Bissett Creek and from â18 to â14 â° at Montpellier. Carbon isotope compositions of graphite from both deposits support a biogenic source for the carbon and the spread in ÎŽ13C can be generated through Rayleigh fractionation. A minor contribution of inorganic carbon from the devolatilization of carbonate minerals is possible at Montpellier. Mineralization at Bissett Creek and Montpellier is interpreted to represent syngenetic graphite mineralization from organic-rich material during high-temperature metamorphism
Hydrologic Transport of Dissolved Inorganic Carbon and Its Control on Chemical Weathering
Chemical weathering is one of the major processes interacting with climate and tectonics to form clays, supply nutrients to soil microorganisms and plants, and sequester atmospheric CO2. Hydrology and dissolution kinetics have been emphasized as factors controlling chemical weathering rates. However, the interaction between hydrology and transport of dissolved inorganic carbon (DIC) in controlling weathering has received less attention. In this paper, we present an analytical model that couples subsurface water and chemical molar balance equations to analyze the roles of hydrology and DIC transport on chemical weathering. The balance equations form a dynamical system that fully determines the dynamics of the weathering zone chemistry as forced by the transport of DIC. The model is formulated specifically for the silicate mineral albite, but it can be extended to other minerals, and is studied as a function of percolation rate and water transit time. Three weathering regimes are elucidated. For very small or large values of transit time, the weathering is limited by reaction kinetics or transport, respectively. For intermediate values, the system is transport controlled and is sensitive to transit time. We apply the model to a series of watersheds for which we estimate transit times and identify the type of weathering regime. The results suggest that hydrologic transport of DIC may be as important as reaction kinetics and dilution in determining chemical weathering rates
High-sensitivity optical measurement of mechanical Brownian motion
We describe an experiment in which a laser beam is sent into a high-finesse
optical cavity with a mirror coated on a mechanical resonator. We show that the
reflected light is very sensitive to small mirror displacements. We have
observed the Brownian motion of the resonator with a very high sensitivity.Comment: 4 pages, 4 figures, RevTe
Optical decay from a Fabry-Perot cavity faster than the decay time
The dynamical response of an optical Fabry-Perot cavity is investigated
experimentally. We observe oscillations in the transmitted and reflected light
intensity if the frequency of the incoupled light field is rapidly changed. In
addition, the decay of a cavity-stored light field is accelerated if the phase
and intensity of the incoupled light are switched in an appropriate way. The
theoretical model by M. J. Lawrence em et al, JOSA B 16, 523 (1999) agrees with
our observations.Comment: submitted to Josa
Optical vernier technique for in-situ measurement of the length of long Fabry-Perot cavities
We propose a method for in-situ measurement of the length of kilometer size
Fabry-Perot cavities in laser gravitational wave detectors. The method is based
on the vernier, which occurs naturally when the laser incident on the cavity
has a sideband. By changing the length of the cavity over several wavelengths
we obtain a set of carrier resonances alternating with sideband resonances.
From the measurement of the separation between the carrier and a sideband
resonance we determine the length of the cavity. We apply the technique to the
measurement of the length of a Fabry-Perot cavity in the Caltech 40m
Interferometer and discuss the accuracy of the technique.Comment: LaTeX 2e, 12 pages, 4 figure
Interferometry
The following recommended programs are reviewed: (1) infrared and optical interferometry (a ground-based and space programs); (2) compensation for the atmosphere with adaptive optics (a program for development and implementation of adaptive optics); and (3) gravitational waves (high frequency gravitational wave sources (LIGO), low frequency gravitational wave sources (LAGOS), a gravitational wave observatory program, laser gravitational wave observatory in space, and technology development during the 1990's). Prospects for international collaboration and related issues are also discussed
Laser Interferometric Detectors of Gravitational Waves
A laser interferometric detector of gravitational waves is studied and a
complete solution (to first order in the metric perturbation) of the coupled
Einstein-Maxwell equations with appropriate boundary conditions for the light
beams is determined. The phase shift, the light deflection and the rotation of
the polarization axis induced by gravitational waves are computed. The results
are compared with previous literature, and are shown to hold also for detectors
which are large in comparison with the gravitational wavelength.Comment: 13 pages, LaTe
Geophysical studies with laser-beam detectors of gravitational waves
The existing high technology laser-beam detectors of gravitational waves may
find very useful applications in an unexpected area - geophysics. To make
possible the detection of weak gravitational waves in the region of high
frequencies of astrophysical interest, ~ 30 - 10^3 Hz, control systems of laser
interferometers must permanently monitor, record and compensate much larger
external interventions that take place in the region of low frequencies of
geophysical interest, ~ 10^{-5} - 3 X 10^{-3} Hz. Such phenomena as tidal
perturbations of land and gravity, normal mode oscillations of Earth,
oscillations of the inner core of Earth, etc. will inevitably affect the
performance of the interferometers and, therefore, the information about them
will be stored in the data of control systems. We specifically identify the
low-frequency information contained in distances between the interferometer
mirrors (deformation of Earth) and angles between the mirrors' suspensions
(deviations of local gravity vectors and plumb lines). We show that the access
to the angular information may require some modest amendments to the optical
scheme of the interferometers, and we suggest the ways of doing that. The
detailed evaluation of environmental and instrumental noises indicates that
they will not prevent, even if only marginally, the detection of interesting
geophysical phenomena. Gravitational-wave instruments seem to be capable of
reaching, as a by-product of their continuous operation, very ambitious
geophysical goals, such as observation of the Earth's inner core oscillations.Comment: 29 pages including 8 figures, modifications and clarifications in
response to referees' comments, to be published in Class. Quant. Gra
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