888 research outputs found
A computational framework for the morpho-elastic development of molluskan shells by surface and volume growth
Mollusk shells are an ideal model system for understanding the morpho-elastic
basis of morphological evolution of invertebrates' exoskeletons. During the
formation of the shell, the mantle tissue secretes proteins and minerals that
calcify to form a new incremental layer of the exoskeleton. Most of the
existing literature on the morphology of mollusks is descriptive. The
mathematical understanding of the underlying coupling between pre-existing
shell morphology, de novo surface deposition and morpho-elastic volume growth
is at a nascent stage, primarily limited to reduced geometric representations.
Here, we propose a general, three-dimensional computational framework coupling
pre-existing morphology, incremental surface growth by accretion, and
morpho-elastic volume growth. We exercise this framework by applying it to
explain the stepwise morphogenesis of seashells during growth: new material
surfaces are laid down by accretive growth on the mantle whose form is
determined by its morpho-elastic growth. Calcification of the newest surfaces
extends the shell as well as creates a new scaffold that constrains the next
growth step. We study the effects of surface and volumetric growth rates, and
of previously deposited shell geometries on the resulting modes of mantle
deformation, and therefore of the developing shell's morphology. Connections
are made to a range of complex shells ornamentations.Comment: Main article is 20 pages long with 15 figures. Supplementary material
is 4 pages long with 6 figures and 6 attached movies. To be published in PLOS
Computational Biolog
Thermocapillary valve for droplet production and sorting
Droplets are natural candidates for use as microfluidic reactors, if active
control of their formation and transport can be achieved. We show here that
localized heating from a laser can block the motion of a water-oil interface,
acting as a microfluidic valve for two-phase flows. A theoretical model is
developed to explain the forces acting on a drop due to thermocapillary flow,
predicting a scaling law which favors miniaturization. Finally, we show how the
laser forcing can be applied to sorting drops, thus demonstrating how it may be
integrated in complex droplet microfluidic systems.Comment: Five pages, four figure
The QUEST-La Silla AGN Variability Survey
We present the characterization and initial results from the QUEST-La Silla
AGN variability survey. This is an effort to obtain well sampled optical light
curves in extragalactic fields with unique multi-wavelength observations. We
present photometry obtained from 2010 to 2012 in the XMM-COSMOS field, which
was observed over 150 nights using the QUEST camera on the ESO-Schmidt
telescope. The survey uses a broadband filter, the -band, similar to the
union of the and the filters, achieving an intrinsic photometric
dispersion of mag, and a systematic error of mag in the
zero-point. Since some detectors of the camera show significant non-linearity,
we use a linear correlation to fit the zero-points as a function of the
instrumental magnitudes, thus obtaining a good correction to the non-linear
behavior of these detectors. We obtain good photometry to an equivalent
limiting magnitude of . Studying the optical variability of X-ray
detected sources in the XMM-COSMOS field, we find that the survey is
% complete to magnitudes , and % complete to a
magnitude . The determination and parameterization of the structure
function () of the variable sources shows
that most BL AGN are characterized by and . It is
further shown that variable NL AGN and GAL sources occupying the same parameter
space in and are very likely to correspond to obscured or low
luminosity AGN. Our samples are, however, small, and we expect to revisit these
results using larger samples with longer light curves obtained as part of our
ongoing survey.Comment: Accepted for publication in Ap
Étude des mécanismes de localisation auditive et de leur plasticité dans le cortex auditif humain
Pouvoir déterminer la provenance des sons est fondamental pour bien interagir avec notre environnement. La localisation auditive est une faculté importante et complexe du système auditif humain. Le cerveau doit décoder le signal acoustique pour en extraire les indices qui lui permettent de localiser une source sonore. Ces indices de localisation auditive dépendent en partie de propriétés morphologiques et environnementales qui ne peuvent être anticipées par l'encodage génétique. Le traitement de ces indices doit donc être ajusté par l'expérience durant la période de développement. À l’âge adulte, la plasticité en localisation auditive existe encore. Cette plasticité a été étudiée au niveau comportemental, mais on ne connaît que très peu ses corrélats et mécanismes neuronaux. La présente recherche avait pour objectif d'examiner cette plasticité, ainsi que les mécanismes d'encodage des indices de localisation auditive, tant sur le plan comportemental, qu'à travers les corrélats neuronaux du comportement observé. Dans les deux premières études, nous avons imposé un décalage perceptif de l’espace auditif horizontal à l’aide de bouchons d’oreille numériques. Nous avons montré que de jeunes adultes peuvent rapidement s’adapter à un décalage perceptif important. Au moyen de l’IRM fonctionnelle haute résolution, nous avons observé des changements de l’activité corticale auditive accompagnant cette adaptation, en termes de latéralisation hémisphérique. Nous avons également pu confirmer l’hypothèse de codage par hémichamp comme représentation de l'espace auditif horizontal. Dans une troisième étude, nous avons modifié l’indice auditif le plus important pour la perception de l’espace vertical à l’aide de moulages en silicone. Nous avons montré que l’adaptation à cette modification n’était suivie d’aucun effet consécutif au retrait des moulages, même lors de la toute première présentation d’un stimulus sonore. Ce résultat concorde avec l’hypothèse d’un mécanisme dit de many-to-one mapping, à travers lequel plusieurs profils spectraux peuvent être associés à une même position spatiale. Dans une quatrième étude, au moyen de l’IRM fonctionnelle et en tirant profit de l’adaptation aux moulages de silicone, nous avons révélé l’encodage de l’élévation sonore dans le cortex auditif humain.Spatial hearing is an important but complex capacity of the auditory system. The human auditory system infers the location of a sound source from a variety of acoustic cues, known as auditory localization cues. Because these cues depend to some degree on morphological and environmental factors that cannot be predicted by the genetic makeup, their processing has to fine-tuned during development. Even in adulthood, some plasticity in the processing of localization cues remains. This plasticity has been studied behaviorally, but very little is known about its neural correlates and mechanisms. The present research aimed to investigate this plasticity, as well as the encoding mechanisms of the auditory localization cues, using both behavioral and neuroimaging techniques. In the first two studies, we shifted the perception of horizontal auditory space using digital earplugs. We showed that young adults rapidly adapt to a large perceived shift and that adaptation is accompanied by changes in hemispheric lateralization of auditory cortex activity, as observed with high-resolution functional MRI. We also confirmed the hypothesis of a hemifield code for horizontal sound source location representation in the human auditory cortex. In a third study, we modified the major cue for vertical space perception using silicone earmolds and showed that the adaptation to this modification was not followed by any aftereffect upon earmolds removal, even at the very first sound presentation. This result is consistent with the hypothesis of a “many-to-one mapping” mechanism in which several spectral profiles can become associated with a given spatial direction. In a fourth study, using functional MRI and taking advantage of the adaptation to silicone earmolds, we revealed the encoding of sound source elevation in the human auditory cortex
High count rate {\gamma}-ray spectroscopy with LaBr3:Ce scintillation detectors
The applicability of LaBr3:Ce detectors for high count rate {\gamma}-ray
spectroscopy is investigated. A 3"x3" LaBr3:Ce detector is used in a test setup
with radioactive sources to study the dependence of energy resolution and photo
peak efficiency on the overall count rate in the detector. Digitized traces
were recorded using a 500 MHz FADC and analysed with digital signal processing
methods. In addition to standard techniques a pile-up correction method is
applied to the data in order to further improve the high-rate capabilities and
to reduce the losses in efficiency due to signal pile-up. It is shown, that
{\gamma}-ray spectroscopy can be performed with high resolution at count rates
even above 1 MHz and that the performance can be enhanced in the region between
500 kHz and 10 MHz by using pile-up correction techniques
A single mutation results in diploid gamete formation and parthenogenesis in a Drosophila yemanuclein-alpha meiosis I defective mutant
<p>Abstract</p> <p>Background</p> <p>Sexual reproduction relies on two key events: formation of cells with a haploid genome (the gametes) and restoration of diploidy after fertilization. Therefore the underlying mechanisms must have been evolutionary linked and there is a need for evidence that could support such a model.</p> <p>Results</p> <p>We describe the identification and the characterization of <it>yem<sup>1</sup></it>, the first <it>yem-alpha </it>mutant allele (V478E), which to some extent affects diploidy reduction and its restoration. Yem-alpha is a member of the Ubinuclein/HPC2 family of proteins that have recently been implicated in playing roles in chromatin remodeling in concert with HIRA histone chaperone. The <it>yem<sup>1 </sup></it>mutant females exhibited disrupted chromosome behavior in the first meiotic division and produced very low numbers of viable progeny. Unexpectedly these progeny did not display paternal chromosome markers, suggesting that they developed from diploid gametes that underwent gynogenesis, a form of parthenogenesis that requires fertilization.</p> <p>Conclusions</p> <p>We focus here on the analysis of the meiotic defects exhibited by <it>yem<sup>1 </sup></it>oocytes that could account for the formation of diploid gametes. Our results suggest that <it>yem<sup>1 </sup></it>affects chromosome segregation presumably by affecting kinetochores function in the first meiotic division.</p> <p>This work paves the way to further investigations on the evolution of the mechanisms that support sexual reproduction.</p
- …