474 research outputs found
Quaedam adnotationes de asphyxia : dissertatio inauguralis
http://tartu.ester.ee/record=b1867464~S1*es
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Supporting Performance in the Face of Age-Related Neural Changes: Testing Mechanistic Roles of Cognitive Reserve
Age impacts multiple neural measures and these changes do not always directly translate into alterations in clinical and cognitive measures. This partial protection from the deleterious effects of age in some individuals is referred to as cognitive reserve (CR) and although linked to variations in intelligence and life experiences, its mechanism is still unclear. Within the framework of a theoretical model we tested two potential mechanistic roles of CR to maintain task performance, neural reserve and neural compensation, in young and older adults using functional and structural MRI. Neural reserve refers to increased efficiency and/or capacity of existing functional neural resources. Neural compensation refers to the increased ability to recruit new, additional functional resources. Using structural and functional measures and task performance, the roles of CR were tested using path analysis. Results supported both mechanistic theories of CR and the use of our general theoretical model
A new physical interpretation of optical and infrared variability in quasars
Changing-look quasars are a recently identified class of active galaxies in
which the strong UV continuum and/or broad optical hydrogen emission lines
associated with unobscured quasars either appear or disappear on timescales of
months to years. The physical processes responsible for this behaviour are
still debated, but changes in the black hole accretion rate or accretion disk
structure appear more likely than changes in obscuration. Here we report on
four epochs of spectroscopy of SDSS J110057.70-005304.5, a quasar at a redshift
of whose UV continuum and broad hydrogen emission lines have faded,
and then returned over the past 20 years. The change in this quasar
was initially identified in the infrared, and an archival spectrum from 2010
shows an intermediate phase of the transition during which the flux below
rest-frame 3400\AA\ has decreased by close to an order of magnitude.
This combination is unique compared to previously published examples of
changing-look quasars, and is best explained by dramatic changes in the
innermost regions of the accretion disk. The optical continuum has been rising
since mid-2016, leading to a prediction of a rise in hydrogen emission line
flux in the next year. Increases in the infrared flux are beginning to follow,
delayed by a 3 year observed timescale. If our model is confirmed, the
physics of changing-look quasars are governed by processes at the innermost
stable circular orbit (ISCO) around the black hole, and the structure of the
innermost disk. The easily identifiable and monitored changing-look quasars
would then provide a new probe and laboratory of the nuclear central engine.Comment: 13 pages, 4 figures, 3 tables. Published in MNRAS. All code and data
links on GitHub, https://github.com/d80b2t/WISE_L
Meteoritic Input of Amino Acids and Nucleobases: Methodology and Implications for the Origins of Life
The discoveries of amino acids of extraterrestrial origin in many meteorites over the last 40 years have revolutionized the Astrobiology field. A variety of non-terrestrial amino acids similar to those found in life on Earth have been detected in meteorites. A few amino acids have even been found with chiral excesses, suggesting that meteorites could have contributed to the origin of homochirality in life on Earth. In addition to amino acids, which have been productively studied for years, sugar-like molecules, activated phosphates, and nucleobases have also been determined to be indigenous to numerous meteorites. Because these molecules are essential for life as we know it, and meteorites have been delivering them to the Earth since accretion, it is plausible that the origin(s) of life on Earth were aided by extraterrestrially-synthesized molecules. Understanding the origins of life on Earth guides our search for life elsewhere, helping to answer the question of whether biology is unique to Earth. This tutorial review focuses on meteoritic amino acids and nucleobases, exploring modern analytical methods and possible formation mechanisms. We will also discuss the unique window that meteorites provide into the chemistry that preceded life on Earth, a chemical record we do not have access to on Earth due to geologic recycling of rocks and the pervasiveness of biology across the planet. Finally, we will address the future of meteorite research, including asteroid sample return mIssIons
Extreme Variability in a Broad Absorption Line Quasar
CRTS J084133.15+200525.8 is an optically bright quasar at z=2.345 that has
shown extreme spectral variability over the past decade. Photometrically, the
source had a visual magnitude of V~17.3 between 2002 and 2008. Then, over the
following five years, the source slowly brightened by approximately one
magnitude, to V~16.2. Only ~1 in 10,000 quasars show such extreme variability,
as quantified by the extreme parameters derived for this quasar assuming a
damped random walk model. A combination of archival and newly acquired spectra
reveal the source to be an iron low-ionization broad absorption line (FeLoBAL)
quasar with extreme changes in its absorption spectrum. Some absorption
features completely disappear over the 9 years of optical spectra, while other
features remain essentially unchanged. We report the first definitive redshift
for this source, based on the detection of broad H-alpha in a Keck/MOSFIRE
spectrum. Absorption systems separated by several 1000 km/s in velocity show
coordinated weakening in the depths of their troughs as the continuum flux
increases. We interpret the broad absorption line variability to be due to
changes in photoionization, rather than due to motion of material along our
line of sight. This source highlights one sort of rare transition object that
astronomy will now be finding through dedicated time-domain surveys.Comment: 6 pages, 4 figures; accepted for publication in Ap
Ventricular Pacing in Children
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/75351/1/j.1540-8159.1982.tb06565.x.pd
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Construct Validity of Cognitive Reserve in a Multiethnic Cohort: The Northern Manhattan Study
Cognitive reserve is a hypothetical construct that has been used to inform models of cognitive aging and is presumed to be indicative of life experiences that may mitigate the effects of brain pathology. The purpose of this study was to evaluate the construct validity of cognitive reserve by examining both its convergent and its discriminant validity across three different samples of participants using structural equation modeling. The cognitive reserve variables were found to correlate highly with one another (thereby providing evidence of convergent validity), but demanding tests of discriminant validity indicated that, in two of the samples, the cognitive reserve construct was highly related to an executive functioning construct
Comparing Aberration Detection Methods with Simulated Data
We compared aberration detection methods requiring historical data to those that require little background by using simulated data. Methods that require less historical data are as sensitive and specific as those that require 3–5 years of data. These simulations can determine which method produces appropriate sensitivity and specificity
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