262 research outputs found
Impaired visual short-term memory capacity is distinctively associated with structural connectivity of the posterior thalamic radiation and the splenium of the corpus callosum in preterm-born adults
Preterm birth is associated with an increased risk for lasting changes in both the cortico-thalamic system and attention; however, the link between cortico-thalamic and attention changes is as yet little understood. In preterm newborns, cortico-cortical and cortico-thalamic structural connectivity are distinctively altered, with increased local clustering for cortico-cortical and decreased integrity for cortico-thalamic connectivity. In preterm-born adults, among the various attention functions, visual short-term memory (vSTM) capacity is selectively impaired. We hypothesized distinct associations between vSTM capacity and the structural integrity of cortico-thalamic and cortico-cortical connections, respectively, in preterm-born adults. A whole-report paradigm of briefly presented letter arrays based on the computationally formalized Theory of Visual Attention (TVA) was used to quantify parameter vSTM capacity in 26 preterm- and 21 full-term-born adults. Fractional anisotropy (FA) of posterior thalamic radiations and the splenium of the corpus callosum obtained by diffusion tensor imaging were analyzed by tract-based spatial statistics and used as proxies for cortico-thalamic and cortico-cortical structural connectivity. The relationship between vSTM capacity and cortico-thalamic and cortico-cortical connectivity, respectively, was significantly modified by prematurity. In full-term-born adults, the higher FA in the right posterior thalamic radiation the higher vSTM capacity; in preterm-born adults this FA-vSTM-relationship was inversed. In the splenium, higher FA was correlated with higher vSTM capacity in preterm-born adults, whereas no significant relationship was evident in full-term-born adults. These results indicate distinct associations between cortico-thalamic and cortico-cortical integrity and vSTM capacity in preterm-and full-term-born adults. Data suggest compensatory cortico-cortical fiber re-organization for attention deficits after preterm delivery
Relational color constancy in achromatic and isoluminant images
Relational color constancy, which refers to the constancy of perceived relations between surface colors under
changes in illuminant, may be based on the computation of spatial ratios of cone excitations. As this activity
need occur only within rather than between cone pathways, relational color constancy might be assumed to be
based on relative luminance processing. This hypothesis was tested in a psychophysical experiment in which
observers viewed simulated images of Mondrian patterns undergoing colorimetric changes that could be attributed
either to an illuminant change or to a nonilluminant change; the images were isoluminant, achromatic,
or unmodified. Observers reliably discriminated the two types of changes in all three conditions, implying
that relational color constancy is not based on luminance cues alone. A computer simulation showed
that in these isoluminant and achromatic images spatial ratios of cone excitations and of combinations of cone
excitations were almost invariant under illuminant changes and that discrimination performance could be predicted
from deviations in these ratios.Biotechnology and Biological Sciences Research Council (BBSRC
Density-Matrix Algorithm for Phonon Hilbert Space Reduction in the Numerical Diagonalization of Quantum Many-Body Systems
Combining density-matrix and Lanczos algorithms we propose a new optimized
phonon approach for finite-cluster diagonalizations of interacting
electron-phonon systems. To illustrate the efficiency and reliability of our
method, we investigate the problem of bipolaron band formation in the extended
Holstein Hubbard model.Comment: 14 pages, 6 figures, Workshop on High Performance Computing in
Science and Engineering, Stuttgart 200
Considerations on the quantum double-exchange Hamiltonian
Schwinger bosons allow for an advantageous representation of quantum
double-exchange. We review this subject, comment on previous results, and
address the transition to the semiclassical limit. We derive an effective
fermionic Hamiltonian for the spin-dependent hopping of holes interacting with
a background of local spins, which is used in a related publication within a
two-phase description of colossal magnetoresistant manganites.Comment: 7 pages, 3 figure
Peierls Dimerization with Non-Adiabatic Spin-Phonon Coupling
We study the magnetic properties of a frustrated Heisenberg spin chain with a
dynamic spin-phonon interaction. By Lanczos diagonalization, preserving the
full lattice dynamics, we explore the non-adiabatic regime with phonon
frequencies comparable to the exchange coupling energy which is e.g. the
relevant limit for the spin-Peierls compound . When compared to the
static limit of an alternating spin chain the magnetic properties are strongly
renormalized due to the coupled dynamics of spin and lattice degrees of
freedom. The magnitude of the spin triplet excitation gap changes from a strong
to a weak dimerization dependence with increasing phonon frequencies implying
the necessity to include dynamic effects in an attempt for a quantitative
description of the spin-Peierls state.Comment: 4 pages, 5 figure
Reduced apparent fiber density in the white matter of premature-born adults
Premature-born adults exhibit lasting white matter alterations as demonstrated by widespread reduction in fractional anisotropy (FA) based on diffusion-weighted imaging (DWI). FA reduction, however, is non-specific for microscopic underpinnings such as aberrant myelination or fiber density (FD). Using recent advances in DWI, we tested the hypothesis of reduced FD in premature-born adults and investigated its link with the degree of prematurity and cognition. 73 premature- and 89 mature-born adults aged 25â27 years underwent single-shell DWI, from which a FD measure was derived using convex optimization modeling for microstructure informed tractography (COMMIT). Premature-born adults exhibited lower FD in numerous tracts including the corpus callosum and corona radiata compared to mature-born adults. These FD alterations were associated with both the degree of prematurity, as assessed via gestational age and birth weight, as well as with reduced cognition as measured by full-scale IQ. Finally, lower FD overlapped with lower FA, suggesting lower FD underlie unspecific FA reductions. Results provide evidence that premature birth leads to lower FD in adulthood which links with lower full-scale IQ. Data suggest that lower FD partly underpins FA reductions of premature birth but that other processes such as hypomyelination might also take place
Mid-Infrared Conductivity from Mid-Gap States Associated with Charge Stripes
The optical conductivity of La(2-x)Sr(x)NiO(4) has been interpreted in
various ways, but so far the proposed interpretations have neglected the fact
that the holes doped into the NiO(2) planes order in diagonal stripes, as
established by neutron and X-ray scattering. Here we present a study of optical
conductivity in La(2)NiO(4+d) with d=2/15, a material in which the charge
stripes order three-dimensionally. We show that the conductivity can be
decomposed into two components, a mid-infrared peak that we attribute to
transitions from the filled valence band into empty mid-gap states associated
with the stripes, and a Drude peak that appears at higher temperatures as
carriers are thermally excited into the mid-gap states. The shift of the mid-IR
peak to lower energy with increasing temperature is explained in terms of the
Franck-Condon effect. The relevance of these results to understanding the
optical conductivity in the cuprates is discussed.Comment: final version of paper (minor changes from previous version
Automated quantitative evaluation of brain MRI may be more accurate for discriminating preterm born adults
Objective
To investigate the structural brain abnormalities and their diagnostic accuracy through qualitative and quantitative analysis in term born and very preterm birth or with very low birth weight (VP/VLBW) adults.
Methods
We analyzed 3-T MRIs acquired in 2011â2013 from 67 adults (27 term born controls, mean age 26.4 years, 8 females; 40 VP/VLBWs, mean age 26.6 years, 16 females). We compared automatic segmentations of the white matter, deep gray matter and cortical gray matter, manual corpus callosum measurements and visual ratings of the ventricles and white matter with t tests, logistic regression, and receiver operator characteristic (ROC) curves.
Results
Automatic segmentation correctly classified 84% of cases; visual ratings correctly classified 63%. Quantitative volumetry based on automatic segmentation revealed higher ventricular volume, lower posterior corpus callosum, and deep gray matter volumes in VP/VLBW subjects compared to controls (pâ<â0.01). Visual rating and manual measurement revealed a thinner corpus callosum in VP/VLBW adults (pâ=â0.04) and deformed lateral ventricles (pâ=â0.03) and tendency towards more âdirtyâ white matter (pâ=â0.06). Automatic/manual measures combined with visual ratings correctly classified 87% of cases. Stepwise logistic regression identified three independent features that correctly classify 81% of cases: ventricular volume, deep gray matter volume, and white matter aspect.
Conclusion
Enlarged and deformed lateral ventricles, thinner corpus callosum, and âdirtyâ white matter are prevalent in preterm born adults. Their visual evaluation has low diagnostic accuracy. Automatic volume quantification is more accurate but time consuming. It may be useful to ask for prematurity before initiating further diagnostics in subjects with these alterations
Hippocampal subfield volumes are nonspecifically reduced in prematureâborn adults
Reduced global hippocampus volumes have been demonstrated in prematureâborn individuals, from newborns to adults; however, it is unknown whether hippocampus subfield (HCSF) volumes are differentially affected by premature birth and how relevant they are for cognitive performance. To address these questions, we investigated magnetic resonance imaging (MRI)âderived HCSF volumes in very prematureâborn adults, and related them with general cognitive performance in adulthood. We assessed 103 very prematureâborn (gestational age [GA] <32âweeks and/or birth weight <1,500âg) and 109 termâborn individuals with cognitive testing and structural MRI at 26âyears of age. HCSFs were automatically segmented based on threeâdimensional T1â and T2âweighted sequences and studied both individually and grouped into three functional units, namely hippocampus proper (HP), subicular complex (SC), and dentate gyrus (DG). Cognitive performance was measured using the WechslerâAdultâIntelligenceâScale (fullâscale intelligence quotient [FSâIQ]) at 26âyears. We observed bilateral volume reductions for almost all HCSF volumes in prematureâborn adults and associations with GA and neonatal treatment intensity but not birth weight. Leftâsided HP, SC, and DG volumes were associated with adult FSâIQ. Furthermore, left DG volume was a mediator of the association between GA and adult FSâIQ in prematureâborn individuals. Results demonstrate nonspecifically reduced HCSF volumes in prematureâborn adults; but specific associations with cognitive outcome highlight the importance of the left DG. Data suggest that specific interventions toward hippocampus function might be promising to lower adverse cognitive effects of prematurity
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