972 research outputs found
Neural Prediction Errors Reveal a Risk-Sensitive Reinforcement-Learning Process in the Human Brain
Humans and animals are exquisitely, though idiosyncratically, sensitive to risk or variance in the outcomes of their actions. Economic, psychological, and neural aspects of this are well studied when information about risk is provided explicitly. However, we must normally learn about outcomes from experience, through trial and error. Traditional models of such reinforcement learning focus on learning about the mean reward value of cues and ignore higher order moments such as variance. We used fMRI to test whether the neural correlates of human reinforcement learning are sensitive to experienced risk. Our analysis focused on anatomically delineated regions of a priori interest in the nucleus accumbens, where blood oxygenation level-dependent (BOLD) signals have been suggested as correlating with quantities derived from reinforcement learning. We first provide unbiased evidence that the raw BOLD signal in these regions corresponds closely to a reward prediction error. We then derive from this signal the learned values of cues that predict rewards of equal mean but different variance and show that these values are indeed modulated by experienced risk. Moreover, a close neurometric–psychometric coupling exists between the fluctuations of the experience-based evaluations of risky options that we measured neurally and the fluctuations in behavioral risk aversion. This suggests that risk sensitivity is integral to human learning, illuminating economic models of choice, neuroscientific models of affective learning, and the workings of the underlying neural mechanisms
SIMPle Dark Matter: Self-Interactions and keV Lines
We consider a simple supersymmetric hidden sector: pure SU(N) gauge theory.
Dark matter is made up of hidden glueballinos with mass and hidden
glueballs with mass near the confinement scale . For and , the glueballinos freeze out
with the correct relic density and self-interact through glueball exchange to
resolve small-scale structure puzzles. An immediate consequence is that the
glueballino spectrum has a hyperfine splitting of order . We show that the radiative decays of the excited state can
explain the observed 3.5 keV X-ray line signal from clusters of galaxies,
Andromeda, and the Milky Way.Comment: v1: 6 pages, 2 figures; v2: added references, published version; v3:
note adde
A new type of X-ray pulsar
X-ray emission from stars much more massive than the Sun was discovered only
35 years ago. Such stars drive fast stellar winds where shocks can develop, and
it is commonly assumed that the X-rays emerge from the shock-heated plasma.
Many massive stars additionally pulsate. However, hitherto it was neither
theoretically predicted nor observed that these pulsations would affect their
X-ray emission. Here we report the discovery of pulsating X-rays from the
massive B-type star Xi1 Canis Majoris. This star is a variable of beta Cephei
type and has a strong magnetic field. Our observations with the XMM-Newton
telescope reveal X-ray pulsations with the same period as the fundamental
stellar pulsation. This discovery challenges our understanding of stellar winds
from massive stars, their X-ray emission, and their magnetism.Comment: manuscript draft. The revised paper is published in Nature
Communication
An X-ray investigation of the NGC 346 field in the SMC (3): XMM-Newton data
We present new XMM-Newton results on the field around the NGC346 star cluster
in the SMC. This continues and extends previously published work on Chandra
observations of the same field. The two XMM-Newton observations were obtained,
respectively, six months before and six months after the previously published
Chandra data. Of the 51 X-ray sources detected with XMM-Newton, 29 were already
detected with Chandra. Comparing the properties of these X-ray sources in each
of our three datasets has enabled us to investigate their variability on times
scales of a year. Changes in the flux levels and/or spectral properties were
observed for 21 of these sources. In addition, we discovered long-term
variations in the X-ray properties of the peculiar system HD5980, a luminous
blue variable star, that is likely to be a colliding wind binary system, which
displayed the largest luminosity during the first XMM-Newton observation.Comment: 19 pages, 5 figures (in gif), accepted by ApJ, also available from
http://vela.astro.ulg.ac.be/Preprints/P89/index.htm
Biofluid Markers for Prodromal Parkinson's Disease:Evidence From a Catecholaminergic Perspective
Parkinson's disease (PD) is the most frequent of all Lewy body diseases, a family of progressive neurodegenerative disorders characterized by intra-neuronal cytoplasmic inclusions of α-synuclein. Its most defining features are bradykinesia, tremor, rigidity and postural instability. By the time PD manifests with motor signs, 70% of dopaminergic midbrain neurons are lost, and the disease is already in the middle or late stage. However, there are various non-motor symptoms occurring up to 20 years before the actual parkinsonism that are closely associated with profound deficiency of myocardial noradrenaline content and peripheral sympathetic denervation, as evidenced by neuroimaging experiments in recent years. Additionally, there is an inherent autotoxicity of catecholamines in the neuronal cells in which they are produced, forming toxic catecholaldehyde intermediates that make α-synuclein prone to aggregation, initiating a cascade of events that ultimately leads to neuronal death. The etiopathogenesis of PD and related synucleinopathies thus may well be a prototypical example of a catecholamine-regulated neurodegeneration, given that the synucleinopathy in PD spreads in synergy with central and peripheral catecholaminergic dysfunction from the earliest phases onward. That is why catecholamines and their metabolites, precursors, or derivatives in cerebrospinal fluid or plasma could be of particular interest as biomarkers for prodromal and de novo PD. Because there is great demand for such markers, this mini-review summarizes all catecholamine-related studies to date, in addition to providing profound neurochemical evidence on a systemic and cellular level to further emphasize this hypothesis and with emphasis on extracellular vesicles as a novel diagnostic and therapeutic incentive
Supersymmetry Breaking and Duality in SU(N)xSU(N-M) Theories
We consider a class of N=1 supersymmetric Yang-Mills theories, with gauge
group SU(N)xSU(N - M) and fundamental matter content. Duality plays an
essential role in analyzing the nonperturbative infrared dynamics of these
models. We find that Yukawa couplings drive these theories into the confining
phase, and show how the nonperturbative superpotentials arise in the dual
picture. We show that the odd-N, M = 2 models with an appropriate tree-level
superpotential break supersymmetry.Comment: 12p., LaTeX; some typos corrected, version to appear in Phys. Lett.
Multi-wavelength INTEGRAL NEtwork (MINE) observations of the microquasar GRS 1915+105
We present the international collaboration MINE (Multi-lambda Integral
NEtwork) aimed at conducting multi-wavelength observations of X-ray binaries
and microquasars simultaneously with the INTEGRAL gamma-ray satellite. We will
focus on the 2003 March-April campaign of observations of the peculiar
microquasar GRS 1915+105 gathering radio, IR and X-ray data. The source was
observed 3 times in the plateau state, before and after a major radio and X-ray
flare. It showed strong steady optically thick radio emission corresponding to
powerful compact jets resolved in the radio images, bright near-infrared
emission, a strong QPO at 2.5 Hz in the X-rays and a power law dominated
spectrum without cutoff in the 3-300 keV range. We compare the different
observations, their multi-wavelength light curves, including JEM-X, ISGRI and
SPI, and the parameters deduced from fitting the spectra obtained with these
instruments on board INTEGRAL.Comment: 4 pages, 9 fig., Proc. of the 5th INTEGRAL Workshop (Feb. 16-20
2004), to be published by ES
Simultaneous multi-wavelength observations of microquasars (the MINE collaboration)
We present the international collaboration MINE (Multi-lambda INTEGRAL
NEtwork) aimed at conducting multi-wavelength observations of microquasars
simultaneously with the INTEGRAL satellite. The first results on GRS 1915+105
are encouraging and those to come should help us to understand the physics of
the accretion and ejection phenomena around a compact object.Comment: 2 p, 3 fig., proc. of the IAU Coll. 194, ``Compact Binaries in the
Galaxy and Beyond'', Nov. 2003, La Paz, Mexico, to be published in the Conf.
Series of Revista Mexicana de Astronomia y Astrofisica, Eds. G. Tovmassian &
E. Sio
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