1,587 research outputs found
Presynaptic Inhibition in the Striatum of the Basal Ganglia Improves Pattern Classification and Thus Promotes Superior Goal Selection
This review article takes a multidisciplinary approach to understand how presynaptic inhibition in the striatum of the basal ganglia (BG) contributes to pattern classification and the selection of goals that control behavior. It is a difficult problem both because it is multidimensional and because it is has complex system dynamics. We focus on the striatum because, as the main site for input to the BG, it gets to decide what goals are important to consider
The search for habitable worlds: 1. The viability of a starshade mission
As part of NASA's mission to explore habitable planets orbiting nearby stars,
this paper explores the detection and characterization capabilities of a 4-m
space telescope plus 50-m starshade located at the Earth-Sun L2 point, a.k.a.
the New Worlds Observer (NWO). Our calculations include the true spectral types
and distribution of stars on the sky, an iterative target selection protocol
designed to maximize efficiency based on prior detections, and realistic
mission constraints. We carry out both analytical calculations and simulated
observing runs for a wide range in exozodiacal background levels ({\epsilon} =
1 - 100 times the local zodi brightness) and overall prevalence of Earth-like
terrestrial planets ({\eta}\oplus = 0.1 - 1). We find that even without any
return visits, the NWO baseline architecture (IWA = 65 mas, limiting FPB =
4\times10-11) can achieve a 95% probability of detecting and spectrally
characterizing at least one habitable Earth-like planet, and an expectation
value of ~3 planets found, within the mission lifetime and {\Delta}V budgets,
even in the worst-case scenario ({\eta}\oplus = 0.1 and {\epsilon} = 100 zodis
for every target). This achievement requires about one year of integration time
spread over the 5 year mission, leaving the remainder of the telescope time for
UV-NIR General Astrophysics. Cost and technical feasibility considerations
point to a "sweet spot" in starshade design near a 50-m starshade effective
diameter, with 12 or 16 petals, at a distance of 70,000-100,000 km from the
telescope.Comment: Refereed and accepted to PASP, scheduled for publication in the May
2012 issue (Vol. 124, No. 915
Target selection for the SUNS and DEBRIS surveys for debris discs in the solar neighbourhood
Debris discs - analogous to the Asteroid and Kuiper-Edgeworth belts in the
Solar system - have so far mostly been identified and studied in thermal
emission shortward of 100 um. The Herschel space observatory and the SCUBA-2
camera on the James Clerk Maxwell Telescope will allow efficient photometric
surveying at 70 to 850 um, which allow for the detection of cooler discs not
yet discovered, and the measurement of disc masses and temperatures when
combined with shorter wavelength photometry. The SCUBA-2 Unbiased Nearby Stars
(SUNS) survey and the DEBRIS Herschel Open Time Key Project are complimentary
legacy surveys observing samples of ~500 nearby stellar systems. To maximise
the legacy value of these surveys, great care has gone into the target
selection process. This paper describes the target selection process and
presents the target lists of these two surveys.Comment: 67 pages with full tables, 7 figures, accepted to MNRA
Radial Distribution of Dust Grains Around HR 4796A
We present high-dynamic-range images of circumstellar dust around HR 4796A
that were obtained with MIRLIN at the Keck II telescope at lambda = 7.9, 10.3,
12.5 and 24.5 um. We also present a new continuum measurement at 350 um
obtained at the Caltech Submillimeter Observatory. Emission is resolved in Keck
images at 12.5 and 24.5 um with PSF FWHM's of 0.37" and 0.55", respectively,
and confirms the presence of an outer ring centered at 70 AU. Unresolved excess
infrared emission is also detected at the stellar position and must originate
well within 13 AU of the star. A model of dust emission fit to flux densities
at 12.5, 20.8, and 24.5 um indicates dust grains are located 4(+3/-2) AU from
the star with effective size, 28+/-6 um, and an associated temperature of
260+/-40 K.
We simulate all extant data with a simple model of exozodiacal dust and an
outer exo-Kuiper ring. A two-component outer ring is necessary to fit both Keck
thermal infrared and HST scattered-light images. Bayesian parameter estimates
yield a total cross-sectional area of 0.055 AU^2 for grains roughly 4 AU from
the star and an outer-dust disk composed of a narrow large-grain ring embedded
within a wider ring of smaller grains. The narrow ring is 14+/-1 AU wide with
inner radius 66+/-1 AU and total cross-sectional area 245 AU^2. The outer ring
is 80+/-15 AU wide with inner radius 45+/-5 AU and total cross-sectional area
90 AU^2. Dust grains in the narrow ring are about 10 times larger and have
lower albedos than those in the wider ring. These properties are consistent
with a picture in which radiation pressure dominates the dispersal of an
exo-Kuiper belt.Comment: Accepted by Astrophysical Journal (Part1) on September 9, 2004. 13
pages, 10 figures, 2 table
Neurology
Contains research objectives and reports on three research projects.Office of Naval Research (Nonr-609(39))U. S. Air Force (AF33(616)-7282)Army Chemical Corps (DA-18-108-405-Cml-942)U.S. Public Health Service (B-3055)U. S. Air Force (AF49(638)-1130)U.S. Public Health Service (B-3090
Functional diversity of microbial ecologies estimated from ancient human coprolites and dental calculus
Human microbiome studies are increasingly incorporating macroecological approaches, such as community assembly, network analysis and functional redundancy to more fully characterize the microbiome. Such analyses have not been applied to ancient human microbiomes, preventing insights into human microbiome evolution. We address this issue by analysing published ancient microbiome datasets: coprolites from Rio Zape (n = 7; 700 CE Mexico) and historic dental calculus (n = 44; 1770-1855 CE, UK), as well as two novel dental calculus datasets: Maya (n = 7; 170 BCE-885 CE, Belize) and Nuragic Sardinians (n = 11; 1400-850 BCE, Italy). Periodontitis-associated bacteria (Treponema denticola, Fusobacterium nucleatum and Eubacterium saphenum) were identified as keystone taxa in the dental calculus datasets. Coprolite keystone taxa included known short-chain fatty acid producers (Eubacterium biforme, Phascolarctobacterium succinatutens) and potentially disease-associated bacteria (Escherichia, Brachyspira). Overlap in ecological profiles between ancient and modern microbiomes was indicated by similarity in functional response diversity profiles between contemporary hunter-gatherers and ancient coprolites, as well as parallels between ancient Maya, historic UK, and modern Spanish dental calculus; however, the ancient Nuragic dental calculus shows a distinct ecological structure. We detected key ecological signatures from ancient microbiome data, paving the way to expand understanding of human microbiome evolution. This article is part of the theme issue 'Insights into health and disease from ancient biomolecules'
Fundamental Properties of the Highly Ionized Plasmas in the Milky Way
The cooling transition temperature gas in the interstellar medium (ISM),
traced by the high ions, Si IV, C IV, N V, and O VI, helps to constrain the
flow of energy from the hot ISM with T >10^6 K to the warm ISM with T< 2x10^4
K. We investigate the properties of this gas along the lines of sight to 38
stars in the Milky Way disk using 1.5-2.7 km/s resolution spectra of Si IV, C
IV, and N V absorption from the Space Telescope Imaging Spectrograph (STIS),
and 15 km/s resolution spectra of O VI absorption from the Far Ultraviolet
Spectroscopic Explorer (FUSE). The absorption by Si IV and C IV exhibits broad
and narrow components while only broad components are seen in N V and O VI. The
narrow components imply gas with T<7x10^4 K and trace two distinct types of
gas. The strong, saturated, and narrow Si IV and C IV components trace the gas
associated with the vicinities of O-type stars and their supershells. The
weaker narrow Si IV and C IV components trace gas in the general ISM that is
photoionized by the EUV radiation from cooling hot gas or has radiatively
cooled in a non-equilibrium manner from the transition temperature phase, but
rarely the warm ionized medium (WIM) probed by Al III. The broad Si IV, C IV, N
V, and O VI components trace collisionally ionized gas that is very likely
undergoing a cooling transition from the hot ISM to the warm ISM. The cooling
process possibly provides the regulation mechanism that produces N(C IV)/N(Si
IV) = 3.9 +/- 1.9. The cooling process also produces absorption lines where the
median and mean values of the line widths increase with the energy required to
create the ion.Comment: Accepted for publication in the ApJ. Only this PDF file contains all
the figures and tables in a single fil
- …