1,896 research outputs found
Prior expectation mediates neural adaptation to repeated sounds in the auditory cortex: An MEG study
Contains fulltext :
99626.pdf (publisher's version ) (Open Access)Repetition suppression, the phenomenon that the second presentation of a stimulus attenuates neural activity, is typically viewed as an automatic consequence of repeated stimulus presentation. However, a recent neuroimaging study has suggested that repetition suppression may be driven by top-down expectations. Here we examined whether and when repetition suppression can be modulated by top-down expectation. Participants listened to auditory stimuli in blocks where tone repetitions were either expected or unexpected, while we recorded ongoing neural activity using magnetoencephalography. We found robust repetition suppression in the auditory cortex for repeated tones. Interestingly, this reduction was significantly larger for expected than unexpected repetitions, both in terms of evoked activity and gamma-band synchrony. These findings indicate a role of top-down expectation in generating repetition suppression and are in line with predictive coding models of perception, in which the difference between expected and actual input is propagated from lower to higher cortical areas.6 p
The form factor of the pion in "point-form" of relativistic dynamics revisited
The electromagnetic form factor of the pion is calculated in the "point-form"
of relativistic quantum mechanics using simple, phenomenological wave
functions. It is found that the squared charge radius of the pion is predicted
one order of magnitude larger than the experimental value and the asymptotic
behavior expected from QCD cannot be reproduced. The origin of these
discrepancies is analyzed. The present results confirm previous ones obtained
from a theoretical model and call for major improvements in the implementation
of the "point-form" approach.Comment: 8 pages, 3 eps figure
Hamiltonian light-front field theory within an AdS/QCD basis
Non-perturbative Hamiltonian light-front quantum field theory presents
opportunities and challenges that bridge particle physics and nuclear physics.
Fundamental theories, such as Quantum Chromodynmamics (QCD) and Quantum
Electrodynamics (QED) offer the promise of great predictive power spanning
phenomena on all scales from the microscopic to cosmic scales, but new tools
that do not rely exclusively on perturbation theory are required to make
connection from one scale to the next. We outline recent theoretical and
computational progress to build these bridges and provide illustrative results
for nuclear structure and quantum field theory. As our framework we choose
light-front gauge and a basis function representation with two-dimensional
harmonic oscillator basis for transverse modes that corresponds with
eigensolutions of the soft-wall AdS/QCD model obtained from light-front
holography.Comment: To appear in the proceedings of Light-Cone 2009: Relativistic
Hadronic and Particle Physics, July 8-13, 2009, Sao Jose dos Campos, Brazi
The sign of the day-night asymmetry for solar neutrinos
A qualitative understanding of the day-night asymmetry for solar neutrinos is
provided. The greater night flux in nu_e is seen to be a consequence of the
fact that the matter effect in the sun and that in the earth have the same
sign. It is shown in the adiabatic approximation for the sun that for all
values of the mixing angle theta_V between 0 and pi/2, the night flux of
neutrinos is greater than the day flux. Only for small values of theta_V where
the adiabatic approximation badly fails does the sign of the day-night
asymmetry reverse.Comment: 3 pages, 3 figures, typos corrected and references adde
Chiral Symmetry Breaking with Scalar Confinement
Spontaneous chiral symmetry breaking is accepted to occur in low energy
hadronic physics, resulting in the several successful theorems of PCAC. On the
other hand scalar confinement is suggested both by the spectroscopy of hadrons
and by the string picture of confinement. However these two evidences are
apparently conflicting, because chiral symmetry breaking requires a chiral
invariant coupling to the quarks, say a vector coupling like in QCD. Here we
reformulate the coupling of the quarks to the string, and we are able to comply
with chiral symmetry breaking, using scalar confinement. The results are quite
encouraging.Comment: 4 pages, 5 figures, contribution to the XXXVIIIth Rencontres de
Moriond QCD and High Energy Hadronic Interaction
Spontaneous chiral symmetry breaking in the linked cluster expansion
We investigate dynamical chiral symmetry breaking in the Coulomb gauge
Hamiltonian QCD. Within the framework of the linked cluster expansion we extend
the BCS ansatz for the vacuum and include correlation beyond the
quark-antiquark paring. In particular we study the effects of the three-body
correlations involving quark-antiquark and transverse gluons. The high momentum
behavior of the resulting gap equation is discussed and numerical computation
of the chiral symmetry breaking is presented.Comment: 13 pages, 9 figure
Travelling waves for the Gross-Pitaevskii equation II
The purpose of this paper is to provide a rigorous mathematical proof of the
existence of travelling wave solutions to the Gross-Pitaevskii equation in
dimensions two and three. Our arguments, based on minimization under
constraints, yield a full branch of solutions, and extend earlier results,
where only a part of the branch was built. In dimension three, we also show
that there are no travelling wave solutions of small energy.Comment: Final version accepted for publication in Communications in
Mathematical Physics with a few minor corrections and added remark
Electromagnetic form factors of light vector mesons
The electromagnetic form factors G_E(q^2), G_M(q^2), and G_Q(q^2), charge
radii, magnetic and quadrupole moments, and decay widths of the light vector
mesons rho^+, K^{*+} and K^{*0} are calculated in a Lorentz-covariant,
Dyson-Schwinger equation based model using algebraic quark propagators that
incorporate confinement, asymptotic freedom, and dynamical chiral symmetry
breaking, and vector meson Bethe-Salpeter amplitudes closely related to the
pseudoscalar amplitudes obtained from phenomenological studies of pi and K
mesons. Calculated static properties of vector mesons include the charge radii
and magnetic moments: r_{rho+} = 0.61 fm, r_{K*+} = 0.54 fm, and r^2_{K*0} =
-0.048 fm^2; mu_{rho+} = 2.69, mu_{K*+} = 2.37, and mu_{K*0} = -0.40. The
calculated static limits of the rho-meson form factors are similar to those
obtained from light-front quantum mechanical calculations, but begin to differ
above q^2 = 1 GeV^2 due to the dynamical evolution of the quark propagators in
our approach.Comment: 8 pages of RevTeX, 5 eps figure
Imaging the first light: experimental challenges and future perspectives in the observation of the Cosmic Microwave Background Anisotropy
Measurements of the cosmic microwave background (CMB) allow high precision
observation of the Last Scattering Surface at redshift 1100. After the
success of the NASA satellite COBE, that in 1992 provided the first detection
of the CMB anisotropy, results from many ground-based and balloon-borne
experiments have showed a remarkable consistency between different results and
provided quantitative estimates of fundamental cosmological properties. During
2003 the team of the NASA WMAP satellite has released the first improved
full-sky maps of the CMB since COBE, leading to a deeper insight into the
origin and evolution of the Universe. The ESA satellite Planck, scheduled for
launch in 2007, is designed to provide the ultimate measurement of the CMB
temperature anisotropy over the full sky, with an accuracy that will be limited
only by astrophysical foregrounds, and robust detection of polarisation
anisotropy. In this paper we review the experimental challenges in high
precision CMB experiments and discuss the future perspectives opened by second
and third generation space missions like WMAP and Planck.Comment: To be published in "Recent Research Developments in Astronomy &
Astrophysics Astrophysiscs" - Vol I
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