851 research outputs found
Cosmic-ray exposure ages of fossil micrometeorites from mid-Ordovician sediments at Lynna River, Russia
We measured the He and Ne concentrations of 50 individual extraterrestrial
chromite grains recovered from mid-Ordovician (lower Darriwilian) sediments
from the Lynna River section near St. Petersburg, Russia. High concentrations
of solar wind-like He and Ne found in most grains indicate that they were
delivered to Earth as micrometeoritic dust, while their abundance,
stratigraphic position and major element composition indicate an origin related
to the L chondrite parent body (LCPB) break-up event, 470 Ma ago. Compared to
sediment-dispersed extraterrestrial chromite (SEC) grains extracted from coeval
sediments at other localities, the grains from Lynna River are both highly
concentrated and well preserved. As in previous work, in most grains from Lynna
River, high concentrations of solar wind-derived He and Ne impede a clear
quantification of cosmic-ray produced He and Ne. However, we have found several
SEC grains poor in solar wind Ne, showing a resolvable contribution of
cosmogenic 21Ne. This makes it possible, for the first time, to determine
robust cosmic-ray exposure (CRE) ages in these fossil micrometeorites, on the
order of a few hundred-thousand years. These ages are similar to the CRE ages
measured in chromite grains from cm-sized fossil meteorites recovered from
coeval sediments in Sweden. As the CRE ages are shorter than the orbital decay
time of grains of this size by Poynting-Robertson drag, this suggests that the
grains were delivered to Earth through direct injection into an orbital
resonance. We demonstrate how CRE ages of fossil micrometeorites can be used,
in principle, to determine sedimentation rates, and to correlate the sediments
at Lynna River with the fossil meteorite-bearing sediment layers in Sweden.Comment: 25 pages, 4 figures, 2 table
Two-step actions in infancy—the TWAIN model
In this paper, we propose a novel model—the TWAIN model—to describe the durations of two-step actions in a reach-to-place task in human infants. Previous research demonstrates that infants and adults plan their actions across multiple steps. They adjust, for instance, the velocity of a reaching action depending on what they intend to do with the object once it is grasped. Despite these findings and irrespective of the larger context in which the action occurs, current models (e.g., Fitts’ law) target single, isolated actions, as, for example, pointing to a goal. In the current paper, we develop and empirically test a more ecologically valid model of two-step action planning. More specifically, 61 18-month olds took part in a reach-to-place task and their reaching and placing durations were measured with a motion-capture system. Our model explained the highest amount of variance in placing duration and outperformed six previously suggested models, when using model comparison. We show that including parameters of the first action step, here the duration of the reaching action, can improve the description of the second action step, here the duration of the placing action. This move towards more ecologically valid models of action planning contributes knowledge as well as a framework for assessing human machine interactions. The TWAIN model provides an updated way to quantify motor learning by the time these abilities develop, which might help to assess performance in typically developing human children
Measuring degree-degree association in networks
The Pearson correlation coefficient is commonly used for quantifying the
global level of degree-degree association in complex networks. Here, we use a
probabilistic representation of the underlying network structure for assessing
the applicability of different association measures to heavy-tailed degree
distributions. Theoretical arguments together with our numerical study indicate
that Pearson's coefficient often depends on the size of networks with equal
association structure, impeding a systematic comparison of real-world networks.
In contrast, Kendall-Gibbons' is a considerably more robust measure
of the degree-degree association
Transcriptomics resources of human tissues and organs
Quantifying the differential expression of genes in various human organs, tissues, and cell types is vital to understand human physiology and disease. Recently, several large-scale transcriptomics studies have analyzed the expression of protein-coding genes across tissues. These datasets provide a framework for defining the molecular constituents of the human body as well as for generating comprehensive lists of proteins expressed across tissues or in a tissue-restricted manner. Here, we review publicly available human transcriptome resources and discuss body-wide data from independent genome-wide transcriptome analyses of different tissues. Gene expression measurements from these independent datasets, generated using samples from fresh frozen surgical specimens and postmortem tissues, are consistent. Overall, the different genome-wide analyses support a distribution in which many proteins are found in all tissues and relatively few in a tissue-restricted manner. Moreover, we discuss the applications of publicly available omics data for building genome-scale metabolic models, used for analyzing cell and tissue functions both in physiological and in disease contexts
Middle Ordovician carbonate facies development, conodont biostratigraphy and faunal diversity patterns at the Lynna River, northwestern Russia
The Ordovician Period has emerged as a highly dynamic time in Earth history. Comprehensive work on chrono, chemo-and biostratigraphy has resulted in an overall wellconstrained systemic framework, but several local successions around the globe still await detailed analysis in many respects. Herein we perform a highresolution analysis of abiotic and biotic signals in the Lynna River section, a key locality in northwestern Russia. As this section has been pivotal in documenting the temporal evolution of the Great Ordovician Biodiversification Event on Baltica, the macroscopic and microscopic characteristics of the local succession reveal important paleoenvironmental information that ties into the global development during the Middle Ordovician. The results add particularly to the understanding of the characteristics and largescale sedimentary ‘behavior’ of the Baltoscandian paleobasin. Microfacies vary consistently with the macroscopic appearance of the rocks, with intervals characterized by competent limestone being associated with coarser carbonate textures and intervals dominated by marly beds associated with finer textures. Along with carbonate textures, fossil grain assemblages vary in a rhythmic (~cyclic) manner. The local rocks are commonly partly dolomitized, with the proportion of dolomitization increasing upsection. Regional comparisons suggest that the changes in overall macro and microfacies were strongly related to variations in sea level. New highresolution conodont biostratigraphic data largely confirm previous regional correlations based on lithostratigraphy and trilobite faunas, and enable more robust correlations worldwide
The Bivariate Normal Copula
We collect well known and less known facts about the bivariate normal
distribution and translate them into copula language. In addition, we prove a
very general formula for the bivariate normal copula, we compute Gini's gamma,
and we provide improved bounds and approximations on the diagonal.Comment: 24 page
Transfer Functions for Protein Signal Transduction: Application to a Model of Striatal Neural Plasticity
We present a novel formulation for biochemical reaction networks in the
context of signal transduction. The model consists of input-output transfer
functions, which are derived from differential equations, using stable
equilibria. We select a set of 'source' species, which receive input signals.
Signals are transmitted to all other species in the system (the 'target'
species) with a specific delay and transmission strength. The delay is computed
as the maximal reaction time until a stable equilibrium for the target species
is reached, in the context of all other reactions in the system. The
transmission strength is the concentration change of the target species. The
computed input-output transfer functions can be stored in a matrix, fitted with
parameters, and recalled to build discrete dynamical models. By separating
reaction time and concentration we can greatly simplify the model,
circumventing typical problems of complex dynamical systems. The transfer
function transformation can be applied to mass-action kinetic models of signal
transduction. The paper shows that this approach yields significant insight,
while remaining an executable dynamical model for signal transduction. In
particular we can deconstruct the complex system into local transfer functions
between individual species. As an example, we examine modularity and signal
integration using a published model of striatal neural plasticity. The modules
that emerge correspond to a known biological distinction between
calcium-dependent and cAMP-dependent pathways. We also found that overall
interconnectedness depends on the magnitude of input, with high connectivity at
low input and less connectivity at moderate to high input. This general result,
which directly follows from the properties of individual transfer functions,
contradicts notions of ubiquitous complexity by showing input-dependent signal
transmission inactivation.Comment: 13 pages, 5 tables, 15 figure
A phonon scattering assisted injection and extraction based terahertz quantum cascade laser
A novel lasing scheme for terahertz quantum cascade lasers, based on
consecutive phonon-photon-phonon emissions per module, is proposed and
experimentally demonstrated. The charge transport of the proposed structure is
modeled using a rate equation formalism. An optimization code based on a
genetic algorithm was developed to find a four-well design in the
material system that maximizes the product
of population inversion and oscillator strength at 150 K. The fabricated
devices using Au double-metal waveguides show lasing at 3.2 THz up to 138 K.
The electrical characteristics display no sign of differential resistance drop
at lasing threshold, which suggests - thanks to the rate equation model - a
slow depopulation rate of the lower lasing state, a hypothesis confirmed by
non-equilibrium Green's function calculations.Comment: 11 pages, 10 figure
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