868 research outputs found
Roughening and preroughening in the six vertex model with an extended range of interaction
We study the phase diagram of the BCSOS model with an extended interaction
range using transfer matrix techniques, pertaining to the (100) surface of
single component fcc and bcc crystals. The model shows a 2x2 reconstructed
phase and a disordered flat phase. The deconstruction transition between these
phases merges with a Kosterlitz-Thouless line, showing an interplay of Ising
and Gaussian degrees of freedom. As in studies of the fully frustrated XY
model, exponents deviating from Ising are found. We conjecture that
tri-critical Ising behavior may be a possible explanation for the non-Ising
exponents found in those models.Comment: 25 pages in RevTeX 3.0, seven uuencoded postscript figures, REPLACED
because of submission error (figures were not included
Is surface melting a surface phase transition?
Monte Carlo or Molecular Dynamics calculations of surfaces of Lennard-Jones
systems often indicate, apart from a gradual disordering of the surface called
surface melting, the presence of a phase transition at the surface, but cannot
determine the nature of the transition. In the present paper, we provide for a
link between the continuous Lennard-Jones system and a lattice model. We apply
the method for the (001) surface of a Lennard-Jones fcc structure pertaining to
Argon. The corresponding lattice model is a Body Centered Solid on Solid model
with an extended range of interaction, showing in principle rough, flat and
disordered flat phases. We observe that entropy effects considerably lower the
strength of the effective couplings between the atoms. The Argon (001) face is
shown to exhibit a phase transition at T=70.5 +- 0.5 K, and we identify this
transition as roughening. The roughening temperature is in good correspondence
with experimental results for Argon.Comment: 17 pages REVTeX, 14 uuencoded postscript figures appende
Anticipatory attention: An event-related desynchronization approach
This paper addresses the question of whether anticipatory attention - i.e. attention directed towards an upcoming stimulus in order to facilitate its processing - is realized at the neurophysiological level by a pre-stimulus desynchronization of the sensory cortex corresponding to the modality of the anticipated stimulus, reflecting then opening of a thalamocortical gate in the relevant sensory modality. It is argued that a technique called Event-Related Desynchronization (ERD) of rhythmic 10-Hz activity is well suited to study the thalamocortical processes that are thought to mediate anticipatory attention. In a series of experiments, ERD was computed on EEG and MEG data, recorded while subjects performed a time estimation task and were informed about the quality of their time estimation by stimuli providing Knowledge of Results (KR). The modality of the KR stimuli (auditory, visual, or somatosensory) was manipulated both within and between experiments. The results indicate to varying degrees that preceding the presentation of the KR stimuli, ERD is present over the sensory cortex, which corresponds to the modality of the KR stimulus. The general pattern of results supports the notion that a thalamocortical gating mechanism forms the neurophysiological basis of anticipatory attention. Furthermore, the results support the notion that Event-Related Potential(ERP) and ERD measures reflect fundamentally different neurophysiological processes
Understanding counterfactuals in discourse modulates ERP and oscillatory gamma rhythms in the EEG
This study provides ERP and oscillatory dynamics data associated with the comprehension of narratives involving counterfactual events. Participants were given short stories describing an initial situation (“Marta wanted to plant flowers in her garden…”), followed by a critical sentence describing a new situation in either a factual (“Since she found a spade, she started to dig a hole”) or counterfactual format (“If she had found a spade, she would have started to dig a hole”), and then a continuation sentence that was either related to the initial situation (“she bought a spade”) or to the new one (“she planted roses”). The ERPs recorded for the continuation sentences related to the initial situation showed larger negativity after factuals than after counterfactuals, suggesting that the counterfactual's presupposition – the events did not occur – prevents updating the here-and-now of discourse. By contrast, continuation sentences related to the new situation elicited similar ERPs under both factual and counterfactual contexts, suggesting that counterfactuals also activate momentarily an alternative “as if” meaning. However, the reduction of gamma power following counterfactuals, suggests that the “as if” meaning is not integrated into the discourse, nor does it contribute to semantic unification processes
Correlated percolation and the correlated resistor network
We present some exact results on percolation properties of the Ising model,
when the range of the percolating bonds is larger than nearest-neighbors. We
show that for a percolation range to next-nearest neighbors the percolation
threshold Tp is still equal to the Ising critical temperature Tc, and present
the phase diagram for this type of percolation. In addition, we present Monte
Carlo calculations of the finite size behavior of the correlated resistor
network defined on the Ising model. The thermal exponent t of the conductivity
that follows from it is found to be t = 0.2000 +- 0.0007. We observe no
corrections to scaling in its finite size behavior.Comment: 16 pages, REVTeX, 6 figures include
Event-related desynchronization related to the anticipation of a stimulus providing knowledge of results
In the present paper, event-related desynchronization (ERD) in the alpha and beta frequency bands is quantified in order to investigate the processes related to the anticipation of a knowledge of results (KR) stimulus. In a time estimation task, 10 subjects were instructed to press a button 4 s after the presentation of an auditory stimulus. Two seconds after the response they received auditory or visual feedback on the timing of their response. Preceding the button press, a centrally maximal ERD is found. Preceding the visual KR stimulus, an ERD is present that has an occipital maximum. Contrary to expectation, preceding the auditory KR stimulus there are no signs of a modalityspecific ERD. Results are related to a thalamo-cortical gating model which predicts a correspondence between negative slow potentials and ERD during motor preparation and stimulus anticipation
Is beta in agreement with the relatives? Using relative clause sentences to investigate MEG beta power dynamics during sentence comprehension.
There remains some debate about whether beta power effects observed during sentence comprehension reflect ongoing syntactic unification operations (beta-syntax hypothesis), or instead reflect maintenance or updating of the sentence-level representation (beta-maintenance hypothesis). In this study, we used magnetoencephalography to investigate beta power neural dynamics while participants read relative clause sentences that were initially ambiguous between a subject- or an object-relative reading. An additional condition included a grammatical violation at the disambiguation point in the relative clause sentences. The beta-maintenance hypothesis predicts a decrease in beta power at the disambiguation point for unexpected (and less preferred) object-relative clause sentences and grammatical violations, as both signal a need to update the sentence-level representation. While the beta-syntax hypothesis also predicts a beta power decrease for grammatical violations due to a disruption of syntactic unification operations, it instead predicts an increase in beta power for the object-relative clause condition because syntactic unification at the point of disambiguation becomes more demanding. We observed decreased beta power for both the agreement violation and object-relative clause conditions in typical left hemisphere language regions, which provides compelling support for the beta-maintenance hypothesis. Mid-frontal theta power effects were also present for grammatical violations and object-relative clause sentences, suggesting that violations and unexpected sentence interpretations are registered as conflicts by the brain's domain-general error detection system.</p
Fat-free noncontrast whole-heart CMR with fast and power-optimized off-resonant water excitation pulses
Background: Cardiovascular MRI (CMR) faces challenges due to the interference
of bright fat signals in visualizing anatomical structures. Effective fat
suppression is crucial when using whole-heart CMR. Conventional methods often
fall short due to rapid fat signal recovery and water-selective off-resonant
pulses come with tradeoffs between scan time and RF energy deposit. A
lipid-insensitive binomial off-resonant (LIBOR) RF pulse is introduced,
addressing concerns about RF energy and scan time for CMR at 3T. Methods: A
short LIBOR pulse was developed and implemented in a free-breathing respiratory
self-navigated whole-heart sequence at 3T. A BORR pulse with matched duration,
as well as previously used LIBRE pulses, were implemented and optimized for fat
suppression in numerical simulations and validated in healthy subjects (n=3).
Whole-heart CMR was performed in healthy subjects (n=5) with all four pulses.
The SNR of ventricular blood, skeletal muscle, myocardium, and subcutaneous
fat, and the coronary vessel sharpness and length were compared. Results:
Experiments validated numerical findings and near homogeneous fat suppression
was achieved with all pulses. Comparing the short pulses (1ms), LIBOR reduced
the RF power two-fold compared with LIBRE, and three-fold compared with BORR,
and LIBOR significantly decreased overall fat SNR. The reduction in RF duration
shortened the whole-heart acquisition from 8.5min to 7min. No significant
differences in coronary arteries detection and sharpness were found when
comparing all four pulses. Conclusion: LIBOR enabled whole-heart CMR under 7
minutes at 3T, with large volume fat signal suppression, while reducing RF
power compared with LIBRE and BORR. LIBOR is an excellent candidate to address
SAR problems encountered in CMR where fat suppression remains challenging and
short RF pulses are required.Comment: 25 pages, 7 figures, 2 table
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