799 research outputs found
Competence, specification and induction of Pax-3 in the trigeminal placode
Placodes are discrete regions of thickened ectoderm that contribute extensively to the peripheral nervous system in the vertebrate head. The paired-domain transcription factor Pax-3 is an early molecular marker for the avian ophthalmic trigeminal (opV) placode, which forms sensory neurons in the ophthalmic lobe of the trigeminal ganglion. Here, we use collagen gel cultures and heterotopic quail-chick grafts to examine the competence, specification and induction of Pax-3 in the opV placode. At the 3-somite stage, the whole head ectoderm rostral to the first somite is competent to express Pax-3 when grafted to the opV placode region, though competence is rapidly lost thereafter in otic-level ectoderm. Pax-3 specification in presumptive opV placode ectoderm occurs by the 8-somite stage, concomitant with robust Pax-3 expression. From the 8-somite stage onwards, significant numbers of cells are committed to express Pax-3. The entire length of the neural tube has the ability to induce Pax-3 expression in competent head ectoderm and the inductive interaction is direct. We propose a detailed model for Pax-3 induction in the opV placode
High frequency GaAs nano-optomechanical disk resonator
Optomechanical coupling between a mechanical oscillator and light trapped in
a cavity increases when the coupling takes place in a reduced volume. Here we
demonstrate a GaAs semiconductor optomechanical disk system where both optical
and mechanical energy can be confined in a sub-micron scale interaction volume.
We observe giant optomechanical coupling rate up to 100 GHz/nm involving
picogram mass mechanical modes with frequency between 100 MHz and 1 GHz. The
mechanical modes are singled-out measuring their dispersion as a function of
disk geometry. Their Brownian motion is optically resolved with a sensitivity
of 10^(-17)m/sqrt(Hz) at room temperature and pressure, approaching the quantum
limit imprecision.Comment: 7 pages, 3 figure
Modelling of spin decoherence in a Si hole qubit perturbed by a single charge fluctuator
Spin qubits in semiconductor quantum dots are one of the promizing devices to
realize a quantum processor. A better knowledge of the noise sources affecting
the coherence of such a qubit is therefore of prime importance. In this work,
we study the effect of telegraphic noise induced by the fluctuation of a single
electric charge. We simulate as realistically as possible a hole spin qubit in
a quantum dot defined electrostatically by a set of gates along a silicon
nanowire channel. Calculations combining Poisson and time-dependent
Schr\"odinger equations allow to simulate the relaxation and the dephasing of
the hole spin as a function of time for a classical random telegraph signal. We
show that dephasing time is well given by a two-level model in a wide
range of frequency. Remarkably, in the most realistic configuration of a low
frequency fluctuator, the system has a non-Gaussian behavior in which the phase
coherence is lost as soon as the fluctuator has changed state. The Gaussian
description becomes valid only beyond a threshold frequency , when
the two-level system reacts to the statistical distribution of the fluctuator
states. We show that the dephasing time at this threshold
frequency can be considerably increased by playing on the orientation of the
magnetic field and the gate potentials, by running the qubit along "sweet"
lines. However, remains bounded due to dephasing induced
by the non-diagonal terms of the stochastic perturbation Hamiltonian. Our
simulations reveal that the spin relaxation cannot be described cleanly in the
two-level model because the coupling to higher energy hole levels impacts very
strongly the spin decoherence. This result suggests that multi-level
simulations including the coupling to phonons should be necessary to describe
the relaxation phenomenon in this type of qubit
Wavelength-sized GaAs optomechanical resonators with GHz frequency
We report on wavelength-sized GaAs optomechanical disk resonators showing
ultra-strong optomechanical interaction. We observe optical transduction of a
disk mechanical breathing mode with 1.4 GHz frequency and effective mass of ~ 2
pg. The measured vacuum optomechanical coupling rate reaches 0.8 MHz, with a
related differential optomechanical coupling factor of 485 GHz/nm. The disk
Brownian motion is optically resolved with a sensitivity of 10-17 m/{\sqrt}Hz
at room temperature and pressure.Comment: 8 pages, 3 figure
Origin of optical losses in gallium arsenide disk whispering gallery resonators
Whispering gallery modes in GaAs disk resonators reach half a million of
optical quality factor. These high Qs remain still well below the ultimate
design limit set by bending losses. Here we investigate the origin of residual
optical dissipation in these devices. A Transmission Electron Microscope
analysis is combined with an improved Volume Current Method to precisely
quantify optical scattering losses by roughness and waviness of the structures,
and gauge their importance relative to intrinsic material and radiation losses.
The analysis also provides a qualitative description of the surface
reconstruction layer, whose optical absorption is then revealed by comparing
spectroscopy experiments in air and in different liquids. Other linear and
nonlinear optical loss channels in the disks are evaluated likewise. Routes are
given to further improve the performances of these miniature GaAs cavities.Comment: 18 page
Biomonitoring of 2,4-Dichlorophenoxyacetic Acid Exposure and Dose in Farm Families
OBJECTIVE: We estimated 2,4-dichlorophenoxyacetic acid (2,4-D) exposure and systemic dose in farm family members following an application of 2,4-D on their farm. METHODS: Farm families were recruited from licensed applicators in Minnesota and South Carolina. Eligible family members collected all urine during five 24-hr intervals, 1 day before through 3 days after an application of 2,4-D. Exposure profiles were characterized with 24-hr urine 2,4-D concentrations, which then were related to potential predictors of exposure. Systemic dose was estimated using the urine collections from the application day through the third day after application. RESULTS: Median urine 2,4-D concentrations at baseline and day after application were 2.1 and 73.1 μ g/L for applicators, below the limit of detection, and 1.2 μ g/L for spouses, and 1.5 and 2.9 μ g/L for children. The younger children (4–11 years of age) had higher median post-application concentrations than the older children (≥ 12 years of age) (6.5 vs. 1.9 μ g/L). The geometric mean systemic doses (micrograms per kilogram body weight) were 2.46 (applicators), 0.8 (spouses), 0.22 (all children), 0.32 (children 4–11 years of age), and 0.12 (children ≥ 12 years of age). Exposure to the spouses and children was primarily determined by direct contact with the application process and the number of acres treated. Multivariate models identified glove use, repairing equipment, and number of acres treated as predictors of exposure in the applicators. CONCLUSIONS: We observed considerable heterogeneity of 2,4-D exposure among farm family members, primarily attributable to level of contact with the application process. Awareness of this variability and the actual magnitude of exposures are important for developing exposure and risk characterizations in 2,4-D–exposed agricultural populations
Recognition of leitmotives in Richard Wagner's music: chroma distance and listener expertise
The leitmotives in Richard Wagner’s Der Ring des Nibelungen serve a range of compositional and psychological functions, including the introduction of musical structure and mnemonic devices for the listener. Leitmotives in the Ring differ greatly in their construction, salient aspects (e.g. rhythmic, melodic, harmonic), and their usage in particular scenes and contexts.
We aim to understand listeners’ real-time processing of leitmotives, and have gathered data from a memory test, probing participants’ memory for different leit- motives contained in a 10-minute excerpt from the opera Siegfried. An item response theory (IRT) approach was used to estimate item difficulty parameters as well as parameters characterizing participants’ individual recognition ability. We fit a series of IRT models to the data obtained from 68 participants, finding that a Rasch Model with an unconstrained but fixed discrimination parameter fit the data best accord- ing to the Bayesian Information Criterion. We further investigated the relationship between model parameters and factors such as: number of leitmotive occurrences in the excerpt; acoustical distance using chroma features (Mauch & Dixon, 2010) and distance thresholding (Casey, Rhodes & Slaney, 2008); extent of musical training; and objective and self-reported Wagner expertise, finding that performance in the objective Wagner test and chroma distance were statistically significant predictors, while number of occurrences, self-reported Wagner expertise and extent of musical training did not reach significance
Holocene variations in Lake Titicaca water level and their implications for sociopolitical developments in the central Andes
Holocene climate in the high tropical Andes was characterized by both gradual and abrupt changes, which disrupted the hydrological cycle and impacted landscapes and societies. High-resolution paleoenvironmental records are essential to contextualize archaeological data and to evaluate the sociopolitical response of ancient societies to environmental variability. Middle-to-Late Holocene water levels in Lake Titicaca were reevaluated through a transfer function model based on measurements of organic carbon stable isotopes, combined with high-resolution profiles of other geochemical variables and paleoshoreline indicators. Our reconstruction indicates that following a prolonged low stand during the Middle Holocene (4000 to 2400 BCE), lake level rose rapidly ~15 m by 1800 BCE, and then increased another 3 to 6 m in a series of steps, attaining the highest values after ~1600 CE. The largest lake-level increases coincided with major sociopolitical changes reported by archaeologists. In particular, at the end of the Formative Period (500 CE), a major lake-level rise inundated large shoreline areas and forced populations to migrate to higher elevation, likely contributing to the emergence of the Tiwanaku culture
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