316 research outputs found
Specific heat and thermal conductivity in the vortex state of the two-gap superconductor MgB_2
The specific heat coefficient gamma_s(H) and the electronic thermal
conductivity kappa_{es}(H) are calculated for Abrikosov's vortex lattice by
taking into account the effects of supercurrent flow and Andreev scattering.
First we solve the gap equation for the entire range of magnetic fields. We
take into account vertex corrections due to impurity scattering calculated in
the Born approximation. The function gamma_s(H)/gamma_n increases from zero and
becomes approximately linear above H/H_{c2} \sim 0.1. The dependence on
impurity scattering is substantially reduced by the vertex corrections. The
upward curvature of kappa_{es}(H)/kappa_{en}, which is caused by decreasing
Andreev scattering for increasing field, is reduced for increasing impurity
scattering. We also calculate the temperature dependence of the scattering
rates 1/tau_{ps}(H) of a phonon and 1/tau_{es}(H) of a quasiparticle due to
quasiparticle and phonon scattering, respectively. At low temperatures the
ratio tau_{pn}/tau_{ps}(H) increases rapidly to one as H tends to H_{c2} which
yields a rapid drop in the phononic thermal conductivity kappa_{ph}. Our
results are in qualitative agreement with the experiments on the two-gap
superconductor MgB_2.Comment: 12 pages, 5 figures, additions to figures 1, 2, and 3. Accepted by
Phys. Rev.
Ultrasonic attenuation in magnetic fields for superconducting states with line nodes in Sr2RuO4
We calculate the ultrasonic attenuation in magnetic fields for
superconducting states with line nodes vertical or horizontal relative to the
RuO_2 planes. This theory, which is valid for fields near Hc2 and not too low
temperatures, takes into account the effects of supercurrent flow and Andreev
scattering by the Abrikosov vortex lattice. For rotating in-plane field
H(theta) the attenuation alpha(theta)exhibits variations of fourfold symmetry
in the rotation angle theta. In the case of vertical nodes, the transverse T100
sound mode yields the weakest(linear)H and T dependence of alpha, while the
longitudinal L100 mode yields a stronger (quadratic) H and T dependence. This
is in strong contrast to the case of horizontal line nodes where alpha is the
same for the T100 and L100 modes (apart from a shift of pi/4 in field
direction) and is roughly a quadratic function of H and T. Thus we conclude
that measurements of alpha in in-plane magnetic fields for different in-plane
sound modes may be an important tool for probing the nodal structure of the gap
in Sr_2RuO_4.Comment: 5 pages, 6 figures, replaced in non-preprint form, to appear in Phys.
Rev.
Spherical harmonic decomposition applied to spatial-temporal analysis of human high-density EEG
We demonstrate an application of spherical harmonic decomposition to analysis
of the human electroencephalogram (EEG). We implement two methods and discuss
issues specific to analysis of hemispherical, irregularly sampled data.
Performance of the methods and spatial sampling requirements are quantified
using simulated data. The analysis is applied to experimental EEG data,
confirming earlier reports of an approximate frequency-wavenumber relationship
in some bands.Comment: 12 pages, 8 figures, submitted to Phys. Rev. E, uses APS RevTeX
style
Alpha-band rhythms in visual task performance: phase-locking by rhythmic sensory stimulation
Oscillations are an important aspect of neuronal activity. Interestingly, oscillatory patterns are also observed in behaviour, such as in visual performance measures after the presentation of a brief sensory event in the visual or another modality. These oscillations in visual performance cycle at the typical frequencies of brain rhythms, suggesting that perception may be closely linked to brain oscillations. We here investigated this link for a prominent rhythm of the visual system (the alpha-rhythm, 8-12 Hz) by applying rhythmic visual stimulation at alpha-frequency (10.6 Hz), known to lead to a resonance response in visual areas, and testing its effects on subsequent visual target discrimination. Our data show that rhythmic visual stimulation at 10.6 Hz: 1) has specific behavioral consequences, relative to stimulation at control frequencies (3.9 Hz, 7.1 Hz, 14.2 Hz), and 2) leads to alpha-band oscillations in visual performance measures, that 3) correlate in precise frequency across individuals with resting alpha-rhythms recorded over parieto-occipital areas. The most parsimonious explanation for these three findings is entrainment (phase-locking) of ongoing perceptually relevant alpha-band brain oscillations by rhythmic sensory events. These findings are in line with occipital alpha-oscillations underlying periodicity in visual performance, and suggest that rhythmic stimulation at frequencies of intrinsic brain-rhythms can be used to reveal influences of these rhythms on task performance to study their functional roles
Long-term effects of cranial irradiation and intrathecal chemotherapy in treatment of childhood leukemia: a MEG study of power spectrum and correlated cognitive dysfunction
<p>Abstract</p> <p>Background</p> <p>Prophylaxis to prevent relapses in the central nervous system after childhood acute lymphoblastic leukemia (ALL) used to consist of both intrathecal chemotherapy (CT) and cranial irradiation (CRT). CRT was mostly abolished in the eighties because of its neurotoxicity, and replaced with more intensive intrathecal CT. In this study, a group of survivors treated with CRT before 1983 and another group treated without CRT thereafter are investigated 20–25 years later, giving a much stronger perspective on long-term quality of life than previous studies. The outcomes will help to better understand these groups’ current needs and will aid in anticipating late effects of prophylactic CRT that is currently applied for other diseases. This study evaluates oscillatory neuronal activity in these long-term survivors. Power spectrum deviations are hypothesized to correlate with cognitive dysfunction.</p> <p>Methods</p> <p>Resting state eyes-closed magnetoencephalography (MEG) recordings were obtained from 14 ALL survivors treated with CT + CRT, 18 treated with CT alone and 35 controls. Relative spectral power was calculated in the δ, θ, α1, α2, β and γ frequency bands. The Amsterdam Neuropsychological Tasks (ANT) program was used to assess cognition in the executive functions domain. MEG data and ANT scores were correlated.</p> <p>Results</p> <p>In the CT + CRT group, relative θ power was slightly increased (p = 0.069) and α2 power was significantly decreased (p = 0.006). The CT + CRT group performed worse on various cognitive tests. A deficiency in visuomotor accuracy, especially of the right hand, could be clearly associated with the deviating regional θ and α2 powers (0.471 < r < 0.697). A significant association between decreased regional α2 power and less attentional fluctuations was found for CT + CRT patients as well as controls (0.078 < r < 0.666). Patients treated with CT alone displayed a power spectrum similar to controls, except for a significantly increased level of left frontal α2 power (p = 0.030).</p> <p>Conclusions</p> <p>The tendency towards global slowing of brain oscillatory activity, together with the fact that dementia has been reported as a late effect of CRT and the neuropsychological deficiencies currently present, suggest that the irradiated brain might be aging faster and could be at risk for early‐onset dementia. The CT group showed no signs of early aging.</p
Deconstructing Insight: EEG Correlates of Insightful Problem Solving
Background:
Cognitive insight phenomenon lies at the core of numerous discoveries. Behavioral research indicates four salient features of insightful problem solving: (i) mental impasse, followed by (ii) restructuring of the problem representation, which leads to (iii) a deeper understanding of the problem, and finally culminates in (iv) an “Aha!” feeling of suddenness and obviousness of the solution. However, until now no efforts have been made to investigate the neural mechanisms of these constituent features of insight in a unified framework.
Methodology/Principal Findings:
In an electroencephalographic study using verbal remote associate problems, we identified neural correlates of these four features of insightful problem solving. Hints were provided for unsolved problems or after mental impasse. Subjective ratings of the restructuring process and the feeling of suddenness were obtained on trial-by-trial basis. A negative correlation was found between these two ratings indicating that sudden insightful solutions, where restructuring is a key feature, involve automatic, subconscious recombination of information. Electroencephalogram signals were analyzed in the space×time×frequency domain with a nonparametric cluster randomization test. First, we found strong gamma band responses at parieto-occipital regions which we interpreted as (i) an adjustment of selective attention (leading to a mental impasse or to a correct solution depending on the gamma band power level) and (ii) encoding and retrieval processes for the emergence of spontaneous new solutions. Secondly, we observed an increased upper alpha band response in right temporal regions (suggesting active suppression of weakly activated solution relevant information) for initially unsuccessful trials that after hint presentation led to a correct solution. Finally, for trials with high restructuring, decreased alpha power (suggesting greater cortical excitation) was observed in right prefrontal area.
Conclusions/Significance:
Our results provide a first account of cognitive insight by dissociating its constituent components and potential neural correlates
THETA-rhythm makes the world go round:dissociative effects of TMS theta versus alpha entrainment of right pTPJ on embodied perspective transformations
Being able to imagine another person's experience and perspective of the world is a crucial human ability and recent reports suggest that humans "embody" another's viewpoint by mentally rotating their own body representation into the other's orientation. Our recent Magnetoencephalography (MEG) data further confirmed this notion of embodied perspective transformations and pinpointed the right posterior temporo-parietal junction (pTPJ) as the crucial hub in a distributed network oscillating at theta frequency (3-7 Hz). In a subsequent transcranial magnetic stimulation (TMS) experiment we interfered with right pTPJ processing and observed a modulation of the embodied aspects of perspective transformations. While these results corroborated the role of right pTPJ, the notion of theta oscillations being the crucial neural code remained a correlational observation based on our MEG data. In the current study we therefore set out to confirm the importance of theta oscillations directly by means of TMS entrainment. We compared entrainment of right pTPJ at 6 Hz vs. 10 Hz and confirmed that only 6 Hz entrainment facilitated embodied perspective transformations (at 160° angular disparity) while 10 Hz slowed it down. The reverse was true at low angular disparity (60° between egocentric and target perspective) where a perspective transformation was not strictly necessary. Our results further corroborate right pTPJ involvement in embodied perspective transformations and highlight theta oscillations as a crucial neural code
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