233,932 research outputs found
Probing the dynamics of identified neurons with a data-driven modeling approach
In controlling animal behavior the nervous system has to perform within the operational limits set by the requirements of each specific behavior. The implications for the corresponding range of suitable network, single neuron, and ion channel properties have remained elusive. In this article we approach the question of how well-constrained properties of neuronal systems may be on the neuronal level. We used large data sets of the activity of isolated invertebrate identified cells and built an accurate conductance-based model for this cell type using customized automated parameter estimation techniques. By direct inspection of the data we found that the variability of the neurons is larger when they are isolated from the circuit than when in the intact system. Furthermore, the responses of the neurons to perturbations appear to be more consistent than their autonomous behavior under stationary conditions. In the developed model, the constraints on different parameters that enforce appropriate model dynamics vary widely from some very tightly controlled parameters to others that are almost arbitrary. The model also allows predictions for the effect of blocking selected ionic currents and to prove that the origin of irregular dynamics in the neuron model is proper chaoticity and that this chaoticity is typical in an appropriate sense. Our results indicate that data driven models are useful tools for the in-depth analysis of neuronal dynamics. The better consistency of responses to perturbations, in the real neurons as well as in the model, suggests a paradigm shift away from measuring autonomous dynamics alone towards protocols of controlled perturbations. Our predictions for the impact of channel blockers on the neuronal dynamics and the proof of chaoticity underscore the wide scope of our approach
The precision of line position measurements of unresolved quasar absorption lines and its influence on the search for variations of fundamental constants
Optical quasar spectra can be used to trace variations of the fine-structure
constant alpha. Controversial results that have been published in last years
suggest that in addition to to wavelength calibration problems systematic
errors might arise because of insufficient spectral resolution. The aim of this
work is to estimate the impact of incorrect line decompositions in fitting
procedures due to asymmetric line profiles. Methods are developed to
distinguish between different sources of line position shifts and thus to
minimize error sources in future work. To simulate asymmetric line profiles,
two different methods were used. At first the profile was created as an
unresolved blend of narrow lines and then, the profile was created using a
macroscopic velocity field of the absorbing medium. The simulated spectra were
analysed with standard methods to search for apparent shifts of line positions
that would mimic a variation of fundamental constants. Differences between
position shifts due to an incorrect line decomposition and a real variation of
constants were probed using methods that have been newly developed or adapted
for this kind of analysis. The results were then applied to real data. Apparent
relative velocity shifts of several hundred meters per second are found in the
analysis of simulated spectra with asymmetric line profiles. It was found that
each system has to be analysed in detail to distinguish between different
sources of line position shifts. A set of 16 FeII systems in seven quasar
spectra was analysed. With the methods developed, the mean alpha variation that
appeared in these systems was reduced from the original
Dalpha/alpha=(2.1+/-2.0)x10^-5 to Dalpha/alpha=(0.1+/-0.8)x10^-5. We thus
conclude that incorrect line decompositions can be partly responsible for the
conflicting results published so far
Laser frequency comb techniques for precise astronomical spectroscopy
Precise astronomical spectroscopic analyses routinely assume that individual
pixels in charge-coupled devices (CCDs) have uniform sensitivity to photons.
Intra-pixel sensitivity (IPS) variations may already cause small systematic
errors in, for example, studies of extra-solar planets via stellar radial
velocities and cosmological variability in fundamental constants via quasar
spectroscopy, but future experiments requiring velocity precisions approaching
~1 cm/s will be more strongly affected. Laser frequency combs have been shown
to provide highly precise wavelength calibration for astronomical
spectrographs, but here we show that they can also be used to measure IPS
variations in astronomical CCDs in situ. We successfully tested a laser
frequency comb system on the Ultra-High Resolution Facility spectrograph at the
Anglo-Australian Telescope. By modelling the 2-dimensional comb signal recorded
in a single CCD exposure, we find that the average IPS deviates by <8 per cent
if it is assumed to vary symmetrically about the pixel centre. We also
demonstrate that series of comb exposures with absolutely known offsets between
them can yield tighter constraints on symmetric IPS variations from ~100
pixels. We discuss measurement of asymmetric IPS variations and absolute
wavelength calibration of astronomical spectrographs and CCDs using frequency
combs.Comment: 11 pages, 7 figures. Accepted for publication in MNRA
Analysis of Scarp Profiles: Evaluation of Errors in Morphologic Dating
Morphologic analysis of scarp degradation can be used quantitatively to determine relative ages of different scarps formed in cohesionless materials, under the same climatic conditions. Scarps of tectonic origin as well as wavecut or rivercut terraces can be treated as topographic impulses that are attenuated by surface erosional processes. This morphological evolution can be modelled as the convolution of the initial shape with erosion (or degradation) function whose width increases with time. Such modeling applies well to scarps less than 10m high, formed in unconsolidated fanglomerates. To a good approximation, the degradation function is Gaussian with a variance measuring the degree of rounding of the initial shape. This geometric parameter can be called the degradation coefficient. A synthetic experiment shows that the degradation coefficient can be obtained by least squares fitting of profiles levelled perpendicular to the scarp. Gravitational collapse of the free face is accounted for by assuming initial scarp slopes at the angle of repose of the cohesionless materials (30°–35°). Uncertainties in the measured profiles result in an uncertainty in degradation coefficient that can be evaluated graphically. Because the degradation coefficient is sensitive to the regional slope and to three-dimensional processes (gullying, loess accumulation, stream incision, etc.), a reliable and accurate determination of degradation coefficient requires several long profiles across the same scarp. The linear diffusion model of scarp degradation is a Gaussian model in which the degradation coefficient is proportional to numerical age. In that case, absolute dating requires only determination of the propotionality constant, called the mass diffusivity constant. For Holocene scarps a few meters high, in loose alluvium under arid climatic conditions, mass diffusivity constants generally range between 1 and 6 m^2/kyr. Morphologic analysis is a reliable method to compare ages of different scarps in a given area, and it can provide approximate absolute ages of Holocene scarplike landforms
Smear fitting: a new deconvolution method for interferometric data
A new technique is presented for producing images from interferometric data.
The method, ``smear fitting'', makes the constraints necessary for
interferometric imaging double as a model, with uncertainties, of the sky
brightness distribution. It does this by modelling the sky with a set of
functions and then convolving each component with its own elliptical gaussian
to account for the uncertainty in its shape and location that arises from
noise. This yields much sharper resolution than CLEAN for significantly
detected features, without sacrificing any sensitivity. Using appropriate
functional forms for the components provides both a scientifically interesting
model and imaging constraints that tend to be better than those used by
traditional deconvolution methods. This allows it to avoid the most serious
problems that limit the imaging quality of those methods. Comparisons of smear
fitting to CLEAN and maximum entropy are given, using both real and simulated
observations. It is also shown that the famous Rayleigh criterion (resolution =
wavelength / baseline) is inappropriate for interferometers as it does not
consider the reliability of the measurements.Comment: 16 pages, 38 figures (some have been lossily compressed for
astro-ph). Uses the hyperref LaTeX package. Accepted for publication by the
Monthly Notices of the Royal Astronomical Societ
Air mass factor formulation for spectroscopic measurements from satellites: Application to formaldehyde retrievals from the Global Ozone Monitoring Experiment
Abstract. We present a new formulation for the air mass factor (AMF) to convert slant column measurements of optically thin atmospheric species from space into total vertical columns. Because of atmospheric scattering, the AMF depends on the vertical distribution of the species. We formulate the AMF as the integral of the relative vertical distribution (shape factor) of the species over the depth of the atmosphere, weighted by altitudedependent coefficients (scattering weights) computed independently from a radiative transfer model. The scattering weights are readily tabulated, and one can then obtain the AMF for any observation scene by using shape factors from a three dimensional (3-D) atmospheric chemistry model for the period of observation. This approach subsequently allows objective evaluation of the 3-D model with the observed vertical columns, since the shape factor and the vertical column in the model represent two independent pieces of information. We demonstrate the AMF method by using slant column measurements of formaldehyde at 346 nm from the Global Ozone Monitoring Experiment satellite instrument over North America during July 1996. Shape factors are computed with the Global Earth Observing System CHEMistry (GEOS-CHEM) global 3-D model and are checked for consistency with the few available aircraft measurements. Scattering weights increase by an order of magnitude from the surface to the upper troposphere. The AMFs are typically 20-40 % less over continents than over the oceans and are approximately half the values calculated in the absence of scattering. Model-induced errors in the AMF are estimated to be • 10%. The GEOS-CHEM model captures 50 % and 60 % of the variances in the observed slant and vertical columns, respectively. Comparison of the simulated and observed vertical columns allows assessment of model bias. 1
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