14,610 research outputs found
High accuracy decoding of dynamical motion from a large retinal population
Motion tracking is a challenge the visual system has to solve by reading out
the retinal population. Here we recorded a large population of ganglion cells
in a dense patch of salamander and guinea pig retinas while displaying a bar
moving diffusively. We show that the bar position can be reconstructed from
retinal activity with a precision in the hyperacuity regime using a linear
decoder acting on 100+ cells. The classical view would have suggested that the
firing rates of the cells form a moving hill of activity tracking the bar's
position. Instead, we found that ganglion cells fired sparsely over an area
much larger than predicted by their receptive fields, so that the neural image
did not track the bar. This highly redundant organization allows for diverse
collections of ganglion cells to represent high-accuracy motion information in
a form easily read out by downstream neural circuits.Comment: 23 pages, 7 figure
Accretion variability of Herbig Ae/Be stars observed by X-Shooter. HD 31648 and HD 163296
This work presents X-Shooter/VLT spectra of the prototypical, isolated Herbig
Ae stars HD 31648 (MWC 480) and HD 163296 over five epochs separated by
timescales ranging from days to months. Each spectrum spans over a wide
wavelength range covering from 310 to 2475 nm. We have monitored the continuum
excess in the Balmer region of the spectra and the luminosity of twelve
ultraviolet, optical and near infrared spectral lines that are commonly used as
accretion tracers for T Tauri stars. The observed strengths of the Balmer
excesses have been reproduced from a magnetospheric accretion shock model,
providing a mean mass accretion rate of 1.11 x 10^-7 and 4.50 x 10^-7 Msun
yr^-1 for HD 31648 and HD 163296, respectively. Accretion rate variations are
observed, being more pronounced for HD 31648 (up to 0.5 dex). However, from the
comparison with previous results it is found that the accretion rate of HD
163296 has increased by more than 1 dex, on a timescale of ~ 15 years. Averaged
accretion luminosities derived from the Balmer excess are consistent with the
ones inferred from the empirical calibrations with the emission line
luminosities, indicating that those can be extrapolated to HAe stars. In spite
of that, the accretion rate variations do not generally coincide with those
estimated from the line luminosities, suggesting that the empirical
calibrations are not useful to accurately quantify accretion rate variability.Comment: 14 pages, 7 Figures, Accepted in Ap
Self-pulsing dynamics of ultrasound in a magnetoacoustic resonator
A theoretical model of parametric magnetostrictive generator of ultrasound is
considered, taking into account magnetic and magnetoacoustic nonlinearities.
The stability and temporal dynamics of the system is analized with standard
techniques revealing that, for a given set of parameters, the model presents a
homoclinic or saddle--loop bifurcation, which predicts that the ultrasound is
emitted in the form of pulses or spikes with arbitrarily low frequency.Comment: 5 pages, 5 figure
Excitability in a nonlinear magnetoacoustic resonator
We report a nonlinear acoustic system displaying excitability. The considered
system is a magnetostrictive material where acoustic waves are parametrically
generated. For a set of parameters, the system presents homoclinic and
heteroclinic dynamics, whose boundaries define a excitability domain. The
excitable behaviour is characterized by analyzing the response of the system to
different external stimuli. Single spiking and bursting regimes have been
identified.Comment: 4 pages, 5 figure
A second order minimality condition for the Mumford-Shah functional
A new necessary minimality condition for the Mumford-Shah functional is
derived by means of second order variations. It is expressed in terms of a sign
condition for a nonlocal quadratic form on , being a
submanifold of the regular part of the discontinuity set of the critical point.
Two equivalent formulations are provided: one in terms of the first eigenvalue
of a suitable compact operator, the other involving a sort of nonlocal capacity
of . A sufficient condition for minimality is also deduced. Finally, an
explicit example is discussed, where a complete characterization of the domains
where the second variation is nonnegative can be given.Comment: 30 page
Electronic structure of the ferromagnetic superconductor UCoGe from first principles
The superconductor UCoGe is analyzed with electronic structure calculations
using Linearized Augmented Plane Wave method based on Density Functional
Theory. Ferromagnetic and antiferromagnetic calculations with and without
correlations (via LDA+U) were done. In this compound the Fermi level is
situated in a region where the main contribution to DOS comes from the U-5f
orbital. The magnetic moment is mainly due to the Co-3d orbital with a small
contribution from the U-5f orbital. The possibility of fully non-collinear
magnetism in this compound seems to be ruled out. These results are compared
with the isostructural compound URhGe, in this case the magnetism comes mostly
from the U-5f orbital
The androgen receptor and signal-transduction pathways in hormone-refractory prostate cancer. Part 2: androgen-receptor cofactors and bypass pathways
Prostate cancer is the second leading cause of cancer related deaths in men from the western world. Treatment of prostate cancer has relied on androgen deprivation therapy for the past 50 years. Response rates are initially high (70-80%), however almost all patients develop androgen escape and subsequently die within 1-2 years. Unlike breast cancer, alternative approaches (chemotherapy and radiotherapy) do not increase survival time. The high rate of prostate cancer mortality is therefore strongly linked to both development of androgen escape and the lack of alternate therapies. AR mutations and amplifications can not explain all cases of androgen escape and post-translational modification of the AR has become an alternative theory. However recently it has been suggested that AR co-activators e.g. SRC-1 or pathways the bypass the AR (Ras/MAP kinase or PI3K/Akt) may stimulated prostate cancer progression independent of the AR. This review will focus on how AR coactivators may act to increase AR transactivation during sub-optimal DHT concentrations and
also how signal transduction pathways may promote androgen escape via activation of transcription factors, e.g. AP-1, c-Myc and Myb, that induce cell proliferation or inhibit apoptosis
The Energetic Costs of Cellular Computation
Cells often perform computations in response to environmental cues. A simple
example is the classic problem, first considered by Berg and Purcell, of
determining the concentration of a chemical ligand in the surrounding media. On
general theoretical grounds (Landuer's Principle), it is expected that such
computations require cells to consume energy. Here, we explicitly calculate the
energetic costs of computing ligand concentration for a simple two-component
cellular network that implements a noisy version of the Berg-Purcell strategy.
We show that learning about external concentrations necessitates the breaking
of detailed balance and consumption of energy, with greater learning requiring
more energy. Our calculations suggest that the energetic costs of cellular
computation may be an important constraint on networks designed to function in
resource poor environments such as the spore germination networks of bacteria.Comment: 9 Pages (including Appendix); 4 Figures; v3 corrects even more typo
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