356 research outputs found
The Direct Effect of Toroidal Magnetic Fields on Stellar Oscillations: An Analytical Expression for the General Matrix Element
Where is the solar dynamo located and what is its modus operandi? These are
still open questions in solar physics. Helio- and asteroseismology can help
answer them by enabling us to study solar and stellar internal structures
through global oscillations. The properties of solar and stellar acoustic modes
are changing with the level of magnetic activity. However, until now, the
inference on subsurface magnetic fields with seismic measures has been very
limited. The aim of this paper is to develop a formalism to calculate the
effect of large-scale toroidal magnetic fields on solar and stellar global
oscillation eigenfunctions and eigenfrequencies. If the Lorentz force is added
to the equilibrium equation of motion, stellar eigenmodes can couple. In
quasi-degenerate perturbation theory, this coupling, also known as the direct
effect, can be quantified by the general matrix element. We present the
analytical expression of the matrix element for a superposition of subsurface
zonal toroidal magnetic field configurations. The matrix element is important
for forward calculations of perturbed solar and stellar eigenfunctions and
frequency perturbations. The results presented here will help to ascertain
solar and stellar large-scale subsurface magnetic fields, and their geometric
configuration, strength, and their change over the course of activity cycles.Comment: 20 pages, accepted for publication in The Astrophysical Journa
Executive control over unconscious cognition: attentional sensitization of unconscious information processing
Unconscious priming is a prototypical example of an automatic process, which is initiated without deliberate intention. Classical theories of automaticity assume that such unconscious automatic processes occur in a purely bottom-up driven fashion independent of executive control mechanisms. In contrast to these classical theories, our attentional sensitization model of unconscious information processing proposes that unconscious processing is susceptible to executive control and is only elicited if the cognitive system is configured accordingly. It is assumed that unconscious processing depends on attentional amplification of task-congruent processing pathways as a function of task sets. This article provides an overview of the latest research on executive control influences on unconscious information processing. I introduce refined theories of automaticity with a particular focus on the attentional sensitization model of unconscious cognition which is specifically developed to account for various attentional influences on different types of unconscious information processing. In support of the attentional sensitization model, empirical evidence is reviewed demonstrating executive control influences on unconscious cognition in the domains of visuo-motor and semantic processing: subliminal priming depends on attentional resources, is susceptible to stimulus expectations and is influenced by action intentions and task sets. This suggests that even unconscious processing is flexible and context-dependent as a function of higher-level executive control settings. I discuss that the assumption of attentional sensitization of unconscious information processing can accommodate conflicting findings regarding the automaticity of processes in many areas of cognition and emotion. This theoretical view has the potential to stimulate future research on executive control of unconscious processing in healthy and clinical populations
Determination of fundamental asteroseismic parameters using the Hilbert transform
Context. Solar-like oscillations exhibit a regular pattern of frequencies.
This pattern is dominated by the small and large frequency separations between
modes. The accurate determination of these parameters is of great interest,
because they give information about e.g. the evolutionary state and the mass of
a star.
Aims. We want to develop a robust method to determine the large and small
frequency separations for time series with low signal-tonoise ratio. For this
purpose, we analyse a time series of the Sun from the GOLF instrument aboard
SOHO and a time series of the star KIC 5184732 from the NASA Kepler satellite
by employing a combination of Fourier and Hilbert transform.
Methods. We use the analytic signal of filtered stellar oscillation time
series to compute the signal envelope. Spectral analysis of the signal envelope
then reveals frequency differences of dominant modes in the periodogram of the
stellar time series.
Results. With the described method the large frequency separation
can be extracted from the envelope spectrum even for data of poor
signal-to-noise ratio. A modification of the method allows for an overview of
the regularities in the periodogram of the time series.Comment: 7 pages, 7 figures, 2 tables, submitted to A&
Stellar magnetic activity and variability of oscillation parameters - An investigation of 24 solar-like stars observed by Kepler
Context. The Sun and solar-like stars undergo activity cycles for which the
underlying mechanisms are not well understood. The oscillations of the Sun are
known to vary with its activity cycle and these changes provide diagnostics on
the conditions below the photosphere. Kepler has detected oscillations in
hundreds of solar-like stars but as of yet, no widespread detection of
signatures of magnetic activity cycles in the oscillation parameters of these
stars have been reported.
Aims. We analyse the photometric short cadence Kepler time series of a set of
24 solar-like stars, which were observed for at least 960 days each, with the
aim to find signatures of stellar magnetic activity in the oscillation
parameters.
Methods. We analyse the temporal evolution of oscillation parameters by
measuring mode frequency shifts, changes in the height of the p-mode envelope,
as well as granulation time scales.
Results. For 23 of the 24 investigated stars, we find significant frequency
shifts in time. We present evidence for magnetic activity in six of them. We
find that the amplitude of the frequency shifts decreases with stellar age and
rotation period. For the most prominent example, KIC 8006161, we find that,
similar to the solar case, frequency shifts are smallest for the lowest and
largest for the highest p-mode frequencies.
Conclusions. These findings show that magnetic activity can be routinely
observed in the oscillation parameters for solar-like stars, which opens up the
possibility to place the solar activity cycle in the context of other stars by
asteroseismology.Comment: 25 pages, 34 figures. Accepted for publication in Astronomy &
Astrophysic
Disturbing visual working memory:electrophysiological evidence for a role of the prefrontal cortex in recovery from interference
Single cell recordings in monkeys support the notion that the lateral prefrontal cortex (PFC) controls reactivation of visual working memory representations when rehearsal is disrupted. In contrast, recent fMRI findings yielded a double dissociation for PFC and the medial temporal lobe (MTL) in a letter working memory task. PFC was engaged in interference protection during reactivation while MTL was prominently involved in the retrieval of the letter representations. We present event-related potential data (ERP) that support PFC involvement in the top-down control of reactivation during a visual working memory task with endogenously triggered recovery after visual interference. A differentiating view is proposed for the role of PFC in working memory with respect to endogenous/exogenous control and to stimulus type. General implications for binding and retention mechanisms are discussed
The Effect of Toroidal Magnetic Fields on Solar Oscillation Frequencies
Solar oscillation frequencies change with the level of magnetic activity. Localizing subsurface magnetic field concentrations in the Sun with helioseismology will help us to understand the solar dynamo. Because the magnetic fields are not considered in standard solar models, adding them to the basic equations of stellar structure changes the eigenfunctions and eigenfrequencies. We use quasi-degenerate perturbation theory to calculate the effect of toroidal magnetic fields on solar oscillation mean multiplet frequencies for six field configurations. In our calculations, we consider both the direct effect of the magnetic field, which describes the coupling of modes, and the indirect effect, which accounts for changes in stellar structure due to the magnetic field. We limit our calculations to self-coupling of modes. We find that the magnetic field affects the multiplet frequencies in a way that depends on the location and the geometry of the field inside the Sun. Comparing our theoretical results with observed shifts, we find that strong tachocline fields cannot be responsible for the observed frequency shifts of p modes over the solar cycle. We also find that part of the surface effect in helioseismic oscillation frequencies might be attributed to magnetic fields in the outer layers of the Sun. The theory presented here is also applicable to models of solar-like stars and their oscillation frequencies
GONG p-mode parameters through two solar cycles
We investigate the parameters of global solar p-mode oscillations, namely
damping width , amplitude , mean squared velocity , energy , and energy supply rate ,
derived from two solar cycles' worth (1996 - 2018) of Global Oscillation
Network Group (GONG) time series for harmonic degrees . We correct
for the effect of fill factor, apparent solar radius, and spurious jumps in the
mode amplitudes. We find that the amplitude of the activity related changes of
and depends on both frequency and harmonic degree of the modes,
with the largest variations of for modes with and with a min-to-max variation of
and of for modes with and with a min-to-max variation of
. The level of correlation between the solar radio flux
and mode parameters also depends on mode frequency and harmonic
degree. As a function of mode frequency, the mode amplitudes are found to
follow an asymmetric Voigt profile with
. From the mode parameters, we
calculate physical mode quantities and average them over specific mode
frequency ranges. This way, we find that the mean squared velocities and energies of p modes are anti-correlated with the level of
activity, varying by and , respectively, and that
the mode energy supply rates show no significant correlation with activity.
With this study we expand previously published results on the temporal
variation of solar p-mode parameters. Our results will be helpful to future
studies of the excitation and damping of p modes, i.e., the interplay between
convection, magnetic field, and resonant acoustic oscillations.Comment: Accepted for publication in Solar Physics. 33 pages, 16 figures, 5
table
The sound of concepts: The link between auditory and conceptual brain systems
Concepts in long-term memory are important building blocks of human cognition and are the basis for object recognition, language and thought. While it is well accepted that concepts are comprised of features related to sensory object attributes, it is still unclear how these features are represented in the brain. Of central interest is whether concepts are essentially grounded in perception. This would imply a common neuroanatomical substrate for perceptual and conceptual processing. Here we show using functional magnetic resonance imaging and recordings of event-related potentials that acoustic conceptual features rapidly recruit auditory areas even when implicitly presented through visual words. Recognizing words denoting objects for which acoustic features are highly relevant (e.g. "telephone") suffices to ignite cell assemblies in the posterior superior and middle temporal gyrus (pSTG/MTG) that were also activated by listening to real sounds. Activity in pSTG/MTG had an onset of 150 ms and increased parametrically as a function of acoustic feature relevance. Both findings suggest a conceptual origin of this effect rather than post-conceptual strategies such as imagery. The presently demonstrated link between auditory and conceptual brain systems parallels observations in other memory systems suggesting that modality-specificity represents a general organizational principle in cortical memory representation. The understanding of concepts as a partial reinstatement of brain activity during perception stresses the necessity of rich sensory experiences for concept acquisition. The modality-specific nature of concepts could also explain the difficulties in achieving a consensus about overall definitions of abstract concepts such as freedom or justice unless embedded in a concrete, experienced situation
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