5,053 research outputs found
Estimating the distribution of dynamic invariants: illustrated with an application to human photo-plethysmographic time series
Dynamic invariants are often estimated from experimental time series with the aim of differentiating between different physical states in the underlying system. The most popular schemes for estimating dynamic invariants are capable of estimating confidence intervals, however, such confidence intervals do not reflect variability in the underlying dynamics. We propose a surrogate based method to estimate the expected distribution of values under the null hypothesis that the underlying deterministic dynamics are stationary. We demonstrate the application of this method by considering four recordings of human pulse waveforms in differing physiological states and show that correlation dimension and entropy are insufficient to differentiate between these states. In contrast, algorithmic complexity can clearly differentiate between all four rhythms
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Subregional Hippocampal Thickness Abnormalities in Older Adults with a History of Heavy Cannabis Use.
Background and Aims: Legalization of cannabis (CB) for both medicinal and, in some states, recreational use, has given rise to increasing usage rates across the country. Of particular concern are indications that frequent CB use may be selectively harmful to the developing adolescent brain compared with adult-onset usage. However, the long-term effects of heavy, adolescent CB use on brain structure and cognitive performance in late-life remain unknown. A critical brain region is the hippocampus (HC), where there is a striking intersection between high concentrations of cannabinoid 1 (CB1) receptors and age-related pathology. Design: We investigated whether older adults (average age=66.6+7.2 years old) with a history of early life CB use show morphological differences in hippocampal subregions compared with older, nonusers. Methods: We performed high-resolution magnetic resonance imaging combined with computational techniques to assess cortical thickness of the medial temporal lobe, neuropsychological testing, and extensive drug use histories on 50 subjects (24 formerly heavy cannabis users [CB+ group] abstinent for an average of 28.7 years, 26 nonusers [CB- group]). We investigated group differences in hippocampal subregions, controlling for age, sex, and intelligence (as measured by the Wechsler Test of Adult Reading), years of education, and cigarette use. Results: The CB+ subjects exhibited thinner cortices in subfields cornu ammonis 1 [CA1; F(1,42)=9.96, p=0.0003], and CA2, 3, and the dentate gyrus [CA23DG; F(1,42)=23.17, p<0.0001], and in the entire HC averaged over all subregions [F(1,42)=8.49, p=0.006]. Conclusions: Negative effects of chronic adolescent CB use on hippocampal structure are maintained well into late life. Because hippocampal cortical loss underlies and exacerbates age-related cognitive decline, these findings have profound implications for aging adults with a history of early life usage. Clinical Trial Registration: ClinicalTrials.gov # NCT01874886
On the Cosmological Evolution of the Luminosity Function and the Accretion Rate of Quasars
We consider a class of models for the redshift evolution (between 0\lsim z
\lsim 4) of the observed optical and X-ray quasar luminosity functions (LFs),
with the following assumptions: (i) the mass-function of dark matter halos
follows the Press-Schechter theory, (ii) the black hole (BH) mass scales
linearly with the halo mass, (iii) quasars have a constant universal lifetime,
and (iv) a thin accretion disk provides the optical luminosity of quasars,
while the X-ray/optical flux ratio is calibrated from a sample of observed
quasars. The mass accretion rate onto quasar BHs is a free parameter
of the models, that we constrain using the observed LFs. The accretion rate
inferred from either the optical or X-ray data under these assumptions
generally decreases as a function of cosmic time from to . We find that a comparable accretion rate is inferred from the X-ray and
optical LF only if the X-ray/optical flux ratio decreases with BH mass. Near
, drops to substantially sub-Eddington values at which
advection-dominated accretion flows (ADAFs) exist. Such a decline of ,
possibly followed by a transition to radiatively inefficient ADAFs, could
explain both the absence of bright quasars in the local universe and the
faintness of accreting BHs at the centers of nearby galaxies. We argue that a
decline of the accretion rate of the quasar population is indeed expected in
cosmological structure formation models.Comment: Latex, 23 pages, 9 figures, accepted for publication in Ap
Telemetry system driven by radiation power for use in gravitational wave detectors
2005-2006 > Academic research: refereed > Publication in refereed journalVersion of RecordPublishe
Rhythmic dynamics and synchronization via dimensionality reduction : application to human gait
Reliable characterization of locomotor dynamics of human walking is vital to understanding the neuromuscular control of human locomotion and disease diagnosis. However, the inherent oscillation and ubiquity of noise in such non-strictly periodic signals pose great challenges to current methodologies. To this end, we exploit the state-of-the-art technology in pattern recognition and, specifically, dimensionality reduction techniques, and propose to reconstruct and characterize the dynamics accurately on the cycle scale of the signal. This is achieved by deriving a low-dimensional representation of the cycles through global optimization, which effectively preserves the topology of the cycles that are embedded in a high-dimensional Euclidian space. Our approach demonstrates a clear advantage in capturing the intrinsic dynamics and probing the subtle synchronization patterns from uni/bivariate oscillatory signals over traditional methods. Application to human gait data for healthy subjects and diabetics reveals a significant difference in the dynamics of ankle movements and ankle-knee coordination, but not in knee movements. These results indicate that the impaired sensory feedback from the feet due to diabetes does not influence the knee movement in general, and that normal human walking is not critically dependent on the feedback from the peripheral nervous system
Large-Amplitude Ultraviolet Variations in the RR Lyrae Star ROTSE-I J143753.84+345924.8
The NASA Galaxy Evolution Explorer (GALEX) satellite has obtained
simultaneous near and far ultraviolet light curves of the ROTSE-I Catalog RR
Lyrae ab-type variable star J143753.84+345924.8. A series of 38 GALEX Deep
Imaging Survey observations well distributed in phase within the star's
0.56432d period shows an AB=4.9mag variation in the far UV (1350-1750A) band
and an AB=1.8mag variation in the near UV (1750-2750A) band, compared with only
a 0.8mag variation in the broad, unfiltered ROTSE-I (4500-10000A) band. These
GALEX UV observations are the first to reveal a large RR Lyrae amplitude
variation at wavelengths below 1800A. We compare the GALEX and ROTSE-I
observations to predictions made by recent Kurucz stellar atmosphere models. We
use published physical parameters for the comparable period (0.57433d),
well-observed RR Lyrae star WY Antliae to compute predicted FUV, NUV, and
ROTSE-I light curves for J143753.84+345924.8. The observed light curves agree
with the Kurucz predictions for [Fe/H]=-1.25 to within AB=0.2mag in the GALEX
NUV and ROTSE-I bands, and within 0.5mag in the FUV. At all metallicities
between solar and one hundredth solar, the Kurucz models predict 6-8mag of
variation at wavelengths between 1000-1700A. Other variable stars with similar
temperature variations, such as Cepheids, should also have large-amplitude FUV
light curves, observable during the ongoing GALEX imaging surveys.Comment: This paper will be published as part of the Galaxy Evolution Explorer
(GALEX) Astrophysical Journal Letters Special Issue. Links to the full set of
papers will be available at http:/www.galex.caltech.edu/PUBLICATIONS after
November 22, 200
GALEX UV Spectroscopy and Deep Imaging of LIRGs in the ELAIS S1 field
The ELAIS S1 field was observed by GALEX in both its Wide Spectroscopic and
Deep Imaging Survey modes. This field was previously observed by the Infrared
Space Observatory and we made use of the catalogue of multi-wavelength data
published by the ELAIS consortium to select galaxies common to the two samples.
Among the 959 objects with GALEX spectroscopy, 88 are present in the ELAIS
catalog and 19 are galaxies with an optical spectroscopic redshift. The
distribution of redshifts covers the range . The selected galaxies
have bolometric IR luminosities (deduced from the flux using ISOCAM) which means that we cover a wide range of galaxies from
normal to Ultra Luminous IR Galaxies. The mean () UV luminosity (not
corrected for extinction) amounts to
L_\sun for the low-z () sample. The UV slope (assuming
) correlates with the GALEX FUV-NUV color if
the sample is restricted to galaxies below . Taking advantage of the
UV and IR data, we estimate the dust attenuation from the IR/UV ratio and
compare it to the UV slope . We find that it is not possible to uniquely
estimate the dust attenuation from for our sample of galaxies. These
galaxies are highly extinguished with a median value .
Once the dust correction applied, the UV- and IR-based SFRs correlate. For the
closest galaxy with the best quality spectrum, we see a feature consistent with
being produced by a bump near 220nm in the attenuation curve.Comment: This paper has been published as part of the GALEX ApJL Special Issue
(ApJ 619, L63
Dynamic Evolution of a Quasi-Spherical General Polytropic Magnetofluid with Self-Gravity
In various astrophysical contexts, we analyze self-similar behaviours of
magnetohydrodynamic (MHD) evolution of a quasi-spherical polytropic magnetized
gas under self-gravity with the specific entropy conserved along streamlines.
In particular, this MHD model analysis frees the scaling parameter in the
conventional polytropic self-similar transformation from the constraint of
with being the polytropic index and therefore
substantially generalizes earlier analysis results on polytropic gas dynamics
that has a constant specific entropy everywhere in space at all time. On the
basis of the self-similar nonlinear MHD ordinary differential equations, we
examine behaviours of the magnetosonic critical curves, the MHD shock
conditions, and various asymptotic solutions. We then construct global
semi-complete self-similar MHD solutions using a combination of analytical and
numerical means and indicate plausible astrophysical applications of these
magnetized flow solutions with or without MHD shocks.Comment: 21 pages, 7 figures, accepted for publication in APS
UV to IR SEDs of UV selected galaxies in the ELAIS fields: evolution of dust attenuation and star formation activity from z=0.7 to z=0.2
We study the ultraviolet to far-infrared (hereafter UV-to-IR) SEDs of a
sample of intermediate redshift (0.2 < z < 0.7) UV-selected galaxies from the
ELAIS-N1 and ELAIS-N2 fields by fitting a multi-wavelength dataset to a library
of GRASIL templates. Star formation related properties of the galaxies are
derived from the library of models by using the Bayesian statistics. We find a
decreasing presence of galaxies with low attenuation and low total luminosity
as redshift decreases, which does not hold for high total luminosity galaxies.
In addition the dust attenuation of low mass galaxies increases as redshift
decreases, and this trend seems to disappear for galaxies with M* > 10^11
M_sun. This result is consistent with a mass dependent evolution of the dust to
gas ratio, which could be driven by a mass dependent efficiency of star
formation in star forming galaxies. The specific star formation rates (SSFR)
decrease with increasing stellar mass at all redshifts, and for a given stellar
mass the SSFR decreases with decreasing redshift. The differences in the slope
of the M*--SSFR relation found between this work and others at similar redshift
could be explained by the adopted selection criteria of the samples which, for
a UV selected sample, favours blue, star forming galaxies.Comment: 21 figures, accepted for publication in Ap
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