1,021 research outputs found
Neuron dynamics in the presence of 1/f noise
Interest in understanding the interplay between noise and the response of a
non-linear device cuts across disciplinary boundaries. It is as relevant for
unmasking the dynamics of neurons in noisy environments as it is for designing
reliable nanoscale logic circuit elements and sensors. Most studies of noise in
non-linear devices are limited to either time-correlated noise with a
Lorentzian spectrum (of which the white noise is a limiting case) or just white
noise. We use analytical theory and numerical simulations to study the impact
of the more ubiquitous "natural" noise with a 1/f frequency spectrum.
Specifically, we study the impact of the 1/f noise on a leaky integrate and
fire model of a neuron. The impact of noise is considered on two quantities of
interest to neuron function: The spike count Fano factor and the speed of
neuron response to a small step-like stimulus. For the perfect (non-leaky)
integrate and fire model, we show that the Fano factor can be expressed as an
integral over noise spectrum weighted by a (low pass) filter function. This
result elucidates the connection between low frequency noise and disorder in
neuron dynamics. We compare our results to experimental data of single neurons
in vivo, and show how the 1/f noise model provides much better agreement than
the usual approximations based on Lorentzian noise. The low frequency noise,
however, complicates the case for information coding scheme based on interspike
intervals by introducing variability in the neuron response time. On a positive
note, the neuron response time to a step stimulus is, remarkably, nearly
optimal in the presence of 1/f noise. An explanation of this effect elucidates
how the brain can take advantage of noise to prime a subset of the neurons to
respond almost instantly to sudden stimuli.Comment: Phys. Rev. E in pres
Strings from Feynman Graph counting : without large N
A well-known connection between n strings winding around a circle and
permutations of n objects plays a fundamental role in the string theory of
large N two dimensional Yang Mills theory and elsewhere in topological and
physical string theories. Basic questions in the enumeration of Feynman graphs
can be expressed elegantly in terms of permutation groups. We show that these
permutation techniques for Feynman graph enumeration, along with the Burnside
counting lemma, lead to equalities between counting problems of Feynman graphs
in scalar field theories and Quantum Electrodynamics with the counting of
amplitudes in a string theory with torus or cylinder target space. This string
theory arises in the large N expansion of two dimensional Yang Mills and is
closely related to lattice gauge theory with S_n gauge group. We collect and
extend results on generating functions for Feynman graph counting, which
connect directly with the string picture. We propose that the connection
between string combinatorics and permutations has implications for QFT-string
dualities, beyond the framework of large N gauge theory.Comment: 55 pages + 10 pages Appendices, 23 figures ; version 2 - typos
correcte
Recommended from our members
Tumor Response to Radiotherapy is Dependent on Genotype-Associated Mechanisms in vitro and in vivo
Background: We have previously shown that in vitro radiosensitivity of human tumor cells segregate non-randomly into a limited number of groups. Each group associates with a specific genotype. However we have also shown that abrogation of a single gene (p21) in a human tumor cell unexpectedly sensitized xenograft tumors comprised of these cells to radiotherapy while not affecting in vitro cellular radiosensitivity. Therefore in vitro assays alone cannot predict tumor response to radiotherapy. In the current work, we measure in vitro radiosensitivity and in vivo response of their xenograft tumors in a series of human tumor lines that represent the range of radiosensitivity observed in human tumor cells. We also measure response of their xenograft tumors to different radiotherapy protocols. We reduce these data into a simple analytical structure that defines the relationship between tumor response and total dose based on two coefficients that are specific to tumor cell genotype, fraction size and total dose. Methods: We assayed in vitro survival patterns in eight tumor cell lines that vary in cellular radiosensitivity and genotype. We also measured response of their xenograft tumors to four radiotherapy protocols: 8 × 2 Gy; 2 × 5Gy, 1 × 7.5 Gy and 1 × 15 Gy. We analyze these data to derive coefficients that describe both in vitro and in vivo responses. Results: Response of xenografts comprised of human tumor cells to different radiotherapy protocols can be reduced to only two coefficients that represent 1) total cells killed as measured in vitro 2) additional response in vivo not predicted by cell killing. These coefficients segregate with specific genotypes including those most frequently observed in human tumors in the clinic. Coefficients that describe in vitro and in vivo mechanisms can predict tumor response to any radiation protocol based on tumor cell genotype, fraction-size and total dose. Conclusions: We establish an analytical structure that predicts tumor response to radiotherapy based on coefficients that represent in vitro and in vivo responses. Both coefficients are dependent on tumor cell genotype and fraction-size. We identify a novel previously unreported mechanism that sensitizes tumors in vivo; this sensitization varies with tumor cell genotype and fraction size
Searching for New Physics Through AMO Precision Measurements
We briefly review recent experiments in atomic, molecular, and optical
physics using precision measurements to search for physics beyond the Standard
Model. We consider three main categories of experiments: searches for changes
in fundamental constants, measurements of the anomalous magnetic moment of the
electron, and searches for an electric dipole moment of the electron.Comment: Prepared for Comments on AMO Physics at Physica Script
シェアビジネスにおけるSNSの信頼性に関する研究-シェアビジネスの先端事例ケーススタディとSNSの信頼性の影響に関する実証分析
Otaru University of Commerce(小樽商科大学)博士(商学)doctoralcours
Counting and construction of holomorphic primary fields in free CFT4 from rings of functions on Calabi-Yau orbifolds
SCOAP
Epidermal Growth Factor Receptor Inhibition Modulates the Microenvironment by Vascular Normalization to Improve Chemotherapy and Radiotherapy Efficacy
Background: Epidermal growth factor receptor (EGFR) inhibitors have shown only modest clinical activity when used as single agents to treat cancers. They decrease tumor cell expression of hypoxia-inducible factor 1-a (HIF-1a) and vascular endothelial growth factor (VEGF). Hypothesizing that this might normalize tumor vasculature, we examined the effects of the EGFR inhibitor erlotinib on tumor vascular function, tumor microenvironment (TME) and chemotherapy and radiotherapy sensitivity. Methodology/Principal Findings: Erlotinib treatment of human tumor cells in vitro and mice bearing xenografts in vivo led to decreased HIF-1a and VEGF expression. Treatment altered xenograft vessel morphology assessed by confocal microscopy (following tomato lectin injection) and decreased vessel permeability (measured by Evan’s blue extravasation), suggesting vascular normalization. Erlotinib increased tumor blood flow measured by Power Doppler ultrasound and decreased hypoxia measured by EF5 immunohistochemistry and tumor O2 saturation measured by optical spectroscopy. Predictin
Transit Photometry as an Exoplanet Discovery Method
Photometry with the transit method has arguably been the most successful
exoplanet discovery method to date. A short overview about the rise of that
method to its present status is given. The method's strength is the rich set of
parameters that can be obtained from transiting planets, in particular in
combination with radial velocity observations; the basic principles of these
parameters are given. The method has however also drawbacks, which are the low
probability that transits appear in randomly oriented planet systems, and the
presence of astrophysical phenomena that may mimic transits and give rise to
false detection positives. In the second part we outline the main factors that
determine the design of transit surveys, such as the size of the survey sample,
the temporal coverage, the detection precision, the sample brightness and the
methods to extract transit events from observed light curves. Lastly, an
overview over past, current and future transit surveys is given. For these
surveys we indicate their basic instrument configuration and their planet
catch, including the ranges of planet sizes and stellar magnitudes that were
encountered. Current and future transit detection experiments concentrate
primarily on bright or special targets, and we expect that the transit method
remains a principal driver of exoplanet science, through new discoveries to be
made and through the development of new generations of instruments.Comment: Review chapte
Mass-loss rates for transiting exoplanets
Exoplanets at small orbital distances from their host stars are submitted to
intense levels of energetic radiations, X-rays and extreme ultraviolet (EUV).
Depending on the masses and densities of the planets and on the atmospheric
heating efficiencies, the stellar energetic inputs can lead to atmospheric mass
loss. These evaporation processes are observable in the ultraviolet during
planetary transits. The aim of the present work is to quantify the mass-loss
rates (dm/dt), heating efficiencies (eta), and lifetimes for the whole sample
of transiting exoplanets, now including hot jupiters, hot neptunes, and hot
super-earths. The mass-loss rates and lifetimes are estimated from an "energy
diagram" for exoplanets, which compares the planet gravitational potential
energy to the stellar X/EUV energy deposited in the atmosphere. We estimate the
mass-loss rates of all detected transiting planets to be within 10^6 to 10^13
g/s for various conservative assumptions. High heating efficiencies would imply
that hot exoplanets such the gas giants WASP-12b and WASP-17b could be
completely evaporated within 1 Gyr. We further show that the heating efficiency
can be constrained when dm/dt is inferred from observations and the stellar
X/EUV luminosity is known. This leads us to suggest that eta ~ 100% in the
atmosphere of the hot jupiter HD209458b, while it could be lower for HD189733b.
Simultaneous observations of transits in the ultraviolet and X-rays are
necessary to further constrain the exospheric properties of exoplanets.Comment: 10 pages, 5 figures, 2 tables. Accepted for publication in Astronomy
& Astrophysic
Kepler observations of variability in B-type stars
The analysis of the light curves of 48 B-type stars observed by Kepler is
presented. Among these are 15 pulsating stars, all of which show low
frequencies characteristic of SPB stars. Seven of these stars also show a few
weak, isolated high frequencies and they could be considered as SPB/beta Cep
hybrids. In all cases the frequency spectra are quite different from what is
seen from ground-based observations. We suggest that this is because most of
the low frequencies are modes of high degree which are predicted to be unstable
in models of mid-B stars. We find that there are non-pulsating stars within the
beta Cep and SPB instability strips. Apart from the pulsating stars, we can
identify stars with frequency groupings similar to what is seen in Be stars but
which are not Be stars. The origin of the groupings is not clear, but may be
related to rotation. We find periodic variations in other stars which we
attribute to proximity effects in binary systems or possibly rotational
modulation. We find no evidence for pulsating stars between the cool edge of
the SPB and the hot edge of the delta Sct instability strips. None of the stars
show the broad features which can be attributed to stochastically-excited modes
as recently proposed. Among our sample of B stars are two chemically peculiar
stars, one of which is a HgMn star showing rotational modulation in the light
curve.Comment: 19 pages, 11 figures, 4 table
- …