Post-training load-related changes of auditory working memory: An EEG study

Working memory (WM) refers to the temporary retention and manipulation of information, and its capacity is highly susceptible to training. Yet, the neural mechanisms that allow for increased performance under demanding conditions are not fully understood. We expected that post-training efficiency in WM performance modulates neural processing during high load tasks. We tested this hypothesis, using electroencephalography (EEG) (N = 39), by comparing source space spectral power of healthy adults performing low and high load auditory WM tasks. Prior to the assessment, participants either underwent a modality-specific auditory WM training, or a modality-irrelevant tactile WM training, or were not trained (active control). After a modality-specific training participants showed higher behavioral performance, compared to the control. EEG data analysis revealed general effects of WM load, across all training groups, in the theta-, alpha-, and beta-frequency bands. With increased load theta-band power increased over frontal, and decreased over parietal areas. Centro-parietal alpha-band power and central beta-band power decreased with load. Interestingly, in the high load condition a tendency toward reduced beta-band power in the right medial temporal lobe was observed in the modality-specific WM training group compared to the modality-irrelevant and active control groups. Our finding that WM processing during the high load condition changed after modality-specific WM training, showing reduced beta-band activity in voice-selective regions, possibly indicates a more efficient maintenance of task-relevant stimuli. The general load effects suggest that WM performance at high load demands involves complementary mechanisms, combining a strengthening of task-relevant and a suppression of task-irrelevant processing

Evaluation of hyperelastic models for unidirectional short fibre reinforced materials using a representative volume element with refined boundary conditions

The simulation of a short fibre reinforced structure by means of the FEM requires the knowledge of the material behaviour at every Gauss point. In order to obtain such information, a representative volume element (RVE) containing unidirectional short fibres is analysed in the presented work. In order to cover the complete anisotropic effect of the fibres, deformations with different angles to the fibre direction have to be conducted. In contrast to other works, this task is tackled using the application of periodic boundary conditions to the RVE in tensorial form, which enables a simple access to consider varying fibre angles with one and the same RVE. As the RVE’s average response represents the homogenised behaviour at a macroscopic material point, the material models’ parameters can be identified by fitting them to stress-strain curves obtained from simulations with the RVE. The findings of these analyses are used to assess the applicability of several hyperelastic models describing transversal isotropic materials under consideration of large deformations. For example it is shown, that the formulation of mixed invariants with the isochoric right Cauchy-Green tensor is insufficient to reproduce the RVE’s behaviour at purely volumetric deformations. Both the modelling and the calculations are carried out with the commercial FEMsoftware ABAQUS. Insight is given to the implementation of the boundary conditions as well as the underlying constitutive equations

Power-laws in recurrence networks from dynamical systems

Recurrence networks are a novel tool of nonlinear time series analysis allowing the characterisation of higher-order geometric properties of complex dynamical systems based on recurrences in phase space, which are a fundamental concept in classical mechanics. In this Letter, we demonstrate that recurrence networks obtained from various deterministic model systems as well as experimental data naturally display power-law degree distributions with scaling exponents $\gamma$ that can be derived exclusively from the systems' invariant densities. For one-dimensional maps, we show analytically that $\gamma$ is not related to the fractal dimension. For continuous systems, we find two distinct types of behaviour: power-laws with an exponent $\gamma$ depending on a suitable notion of local dimension, and such with fixed $\gamma=1$.Comment: 6 pages, 7 figure

Long-term changes in the north-south asymmetry of solar activity: A nonlinear dynamics characterization using visibility graphs

Solar activity is characterized by complex dynamics superimposed onto an almost periodic, approximately 11-year cycle. One of its main features is the presence of a marked, time-varying hemispheric asymmetry, the deeper reasons for which have not yet been completely uncovered. Traditionally, this asymmetry has been studied by considering amplitude and phase differences. Here, we use visibility graphs, a novel tool of nonlinear time series analysis, to obtain complementary information on hemispheric asymmetries in dynamical properties. Our analysis provides deep insights into the potential and limitations of this method, revealing a complex interplay between factors relating to statistical and dynamical properties, i.e., effects due to the probability distribution and the regularity of observed fluctuations. We demonstrate that temporal changes in the hemispheric predominance of the graph properties lag those directly associated with the total hemispheric sunspot areas. Our findings open a new dynamical perspective on studying the north-south sunspot asymmetry, which is to be further explored in future work

Long-term changes in the north–south asymmetry of solar activity : a nonlinear dynamics characterization using visibility graphs

Peer reviewedPublisher PD

On the Origin of Nuclear Star Clusters in Late Type Spiral Galaxies

A large fraction of bulgeless disk galaxies contain young compact stellar systems at their centers, in spite of the local gravitational stability of these disks. We evaluate two contrasting hypotheses for the origin of the nuclear star clusters in late-type disk galaxies. The clusters could not have migrated from distant eccentric locations in the disk. Instead they must have formed in situ, requiring radial transport of gas toward the center of the disk. This transport could be a consequence of the development of the magnetorotational instability in the differentially rotating warm neutral medium. We evaluate the rate of gas transport into the disk center and find that it is sufficient to support continuous star formation in that location. Enhanced stellar surface brightness in the inner few hundred parsecs and the formation of a compact stellar system in the central few parsecs are unavoidable in dark matter halos with divergent density profiles. We illustrate our conclusions on a model of the nearest late-type disk galaxy M33.Comment: 4 pages, ApJ Letters, in pres