281 research outputs found
Comment on "New Methods for Old Coulomb Few-Body Problems"
In this Comment on the above mentioned paper by F. E. Harris, A. M. Frolov,
and V. H. Smith, we briefly review our contributions to development of new
methods for solution of the Coulomb four-body problem. We show that our
research group, headed by Prof. T. K. Rebane, had a priority in using the fully
correlated exponential basis for variational calculations of four-body systems.
We also draw attention to the fact that our group subsequently implemented a
more advanced method, which uses highly efficient exponential-trigonometric
basis functions for solution of the same problem.Comment: Accepted by the International Journal of Quantum Chemistr
Analytic Evaluation of Four-Particle Integrals with Complex Parameters
The method for analytic evaluation of four-particle integrals, proposed by
Fromm and Hill, is generalized to include complex exponential parameters. An
original procedure of numerical branch tracking for multiple valued functions
is developed. It allows high precision variational solution of the Coulomb
four-body problem in a basis of exponential-trigonometric functions of
interparticle separations. Numerical results demonstrate high efficiency and
versatility of the new method.Comment: 13 pages, 4 figure
Explicit integration of one problem of motion of the generalized Kowalevski top
In the problem of motion of the Kowalevski top in a double force field the
4-dimensional invariant submanifold of the phase space was pointed out by
M.P.Kharlamov (Mekh. Tverd. Tela, 32, 2002). We show that the equations of
motion on this manifold can be separated by the appropriate change of
variables, the new variables s1, s2 being elliptic functions of time. The
natural phase variables (components of the angular velocity and the direction
vectors of the forces with respect to the movable basis) are expressed via s1,
s2 explicitly in elementary algebraic functions.Comment: 6 page
Microtesla MRI of the human brain combined with MEG
One of the challenges in functional brain imaging is integration of
complementary imaging modalities, such as magnetoencephalography (MEG) and
functional magnetic resonance imaging (fMRI). MEG, which uses highly sensitive
superconducting quantum interference devices (SQUIDs) to directly measure
magnetic fields of neuronal currents, cannot be combined with conventional
high-field MRI in a single instrument. Indirect matching of MEG and MRI data
leads to significant co-registration errors. A recently proposed imaging method
- SQUID-based microtesla MRI - can be naturally combined with MEG in the same
system to directly provide structural maps for MEG-localized sources. It
enables easy and accurate integration of MEG and MRI/fMRI, because microtesla
MR images can be precisely matched to structural images provided by high-field
MRI and other techniques. Here we report the first images of the human brain by
microtesla MRI, together with auditory MEG (functional) data, recorded using
the same seven-channel SQUID system during the same imaging session. The images
were acquired at 46 microtesla measurement field with pre-polarization at 30
mT. We also estimated transverse relaxation times for different tissues at
microtesla fields. Our results demonstrate feasibility and potential of human
brain imaging by microtesla MRI. They also show that two new types of imaging
equipment - low-cost systems for anatomical MRI of the human brain at
microtesla fields, and more advanced instruments for combined functional (MEG)
and structural (microtesla MRI) brain imaging - are practical.Comment: 8 pages, 5 figures - accepted by JM
Aging, rejuvenation, and memory effects in short-range Ising spin glass: CuCoCl-FeCl graphite bi-intercalation compound
Non-equilibrium aging dynamics in 3D Ising spin glass
CuCoCl-FeCl GBIC has been studied by zero-field
cooled (ZFC) magnetization and low frequency AC magnetic susceptibility ( Hz), where K. The time dependence of the
relaxation rate dd for the ZFC magnetization
after the ZFC aging protocol, shows a peak at a characteristic time
near a wait time (aging behavior), corresponding to a crossover from
quasi equilibrium dynamics to non-equilibrium. The time strongly
depends on , temperature (), magnetic field (), and the
temperature shift (). The rejuvenation effect is observed in both
and under the -shift and -shift
procedures. The memory of the specific spin configurations imprinted during the
ZFC aging protocol can be recalled when the system is re-heated at a constant
heating rate. The aging, rejuvenation, and memory effects observed in the
present system are discussed in terms of the scaling concepts derived from
numerical studies on 3D Edwards-Anderson spin glass model.Comment: 14 pages, 14 figures; Eur. Phys. J. B accepted for publicatio
Correlation between amygdala BOLD activity and frontal EEG asymmetry during real-time fMRI neurofeedback training in patients with depression
Real-time fMRI neurofeedback (rtfMRI-nf) is an emerging approach for studies
and novel treatments of major depressive disorder (MDD). EEG performed
simultaneously with an rtfMRI-nf procedure allows an independent evaluation of
rtfMRI-nf brain modulation effects. Frontal EEG asymmetry in the alpha band is
a widely used measure of emotion and motivation that shows profound changes in
depression. However, it has never been directly related to simultaneously
acquired fMRI data. We report the first study investigating
electrophysiological correlates of the rtfMRI-nf procedure, by combining
rtfMRI-nf with simultaneous and passive EEG recordings. In this pilot study,
MDD patients in the experimental group (n=13) learned to upregulate BOLD
activity of the left amygdala using an rtfMRI-nf during a happy emotion
induction task. MDD patients in the control group (n=11) were provided with a
sham rtfMRI-nf. Correlations between frontal EEG asymmetry in the upper alpha
band and BOLD activity across the brain were examined. Average individual
changes in frontal EEG asymmetry during the rtfMRI-nf task for the experimental
group showed a significant positive correlation with the MDD patients'
depression severity ratings, consistent with an inverse correlation between the
depression severity and frontal EEG asymmetry at rest. Temporal correlations
between frontal EEG asymmetry and BOLD activity were significantly enhanced,
during the rtfMRI-nf task, for the amygdala and many regions associated with
emotion regulation. Our findings demonstrate an important link between amygdala
BOLD activity and frontal EEG asymmetry. Our EEG asymmetry results suggest that
the rtfMRI-nf training targeting the amygdala is beneficial to MDD patients,
and that alpha-asymmetry EEG-nf would be compatible with the amygdala
rtfMRI-nf. Combination of the two could enhance emotion regulation training and
benefit MDD patients.Comment: 28 pages, 16 figures, to appear in NeuroImage: Clinica
- …