34,931 research outputs found
Eye and hand movements during reconstruction of spatial memory
© 2012 a Pion publicationRecent behavioural and biological evidence indicates common mechanisms serving working memory and attention (e.g., Awh et al, 2006 Neuroscience 139 201-208). This study explored the role of spatial attention and visual search in an adapted Corsi spatial memory task. Eye movements and touch responses were recorded from participants who recalled locations (signalled by colour or shape change) from an array presented either simultaneously or sequentially. The time delay between target presentation and recall (0, 5, or 10 s) and the number of locations to be remembered (2-5) were also manipulated. Analysis of the response phase revealed subjects were less accurate (touch data) and fixated longer (eye data) when responding to sequentially presented targets suggesting higher cognitive effort. Fixation duration on target at recall was also influenced by whether spatial location was initially signalled by colour or shape change. Finally, we found that the sequence tasks encouraged longer fixations on the signalled targets than simultaneous viewing during encoding, but no difference was observed during recall. We conclude that the attentional manipulations (colour/shape) mainly affected the eye movement parameters, whereas the memory manipulation (sequential versus simultaneous, number of items) mainly affected the performance of the hand during recall, and thus the latter is more important for ascertaining if an item is remembered or forgotten. In summary, the nature of the stimuli that is used and how it is presented play key roles in determining subject performance and behaviour during spatial memory tasks
RECENT STATE DEVELOPMENTS IN AGRICULTURAL LAW
This article surveys recent legislation and bills in process that deal with agricultural and food law in selected agricultural states in the United States. In addition to categorizing statutory and legislative developments, emerging issues are analyzed in terms of their background and potential for new legislation in the years ahead. Some of the law surveyed is being addressed at both federal and state levels; other legislation reflects local problem situations.Agricultural and Food Policy,
Microwave Nanotube Transistor Operation at High Bias
We measure the small signal, 1 GHz source-drain dynamical conductance of a
back-gated single-walled carbon nanotube field effect transistor at both low
and high dc bias voltages. At all bias voltages, the intrinsic device dynamical
conductance at 1 GHz is identical to the low frequency dynamical conductance,
consistent with the prediction of a cutoff frequency much higher than 1 GHz.
This work represents a significant step towards a full characterization of a
nanotube transistor for RF and microwave amplifiers.Comment: 3 pages, 4 figure
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A combined R-matrix eigenstate basis set and finite-differences propagation method for the time-dependent Schr\"{od}dinger equation: the one-electron case
In this work we present the theoretical framework for the solution of the
time-dependent Schr\"{o}dinger equation (TDSE) of atomic and molecular systems
under strong electromagnetic fields with the configuration space of the
electron's coordinates separated over two regions, that is regions and
. In region the solution of the TDSE is obtained by an R-matrix basis
set representation of the time-dependent wavefunction. In region a grid
representation of the wavefunction is considered and propagation in space and
time is obtained through the finite-differences method. It appears this is the
first time a combination of basis set and grid methods has been put forward for
tackling multi-region time-dependent problems. In both regions, a high-order
explicit scheme is employed for the time propagation. While, in a purely
hydrogenic system no approximation is involved due to this separation, in
multi-electron systems the validity and the usefulness of the present method
relies on the basic assumption of R-matrix theory, namely that beyond a certain
distance (encompassing region ) a single ejected electron is distinguishable
from the other electrons of the multi-electron system and evolves there (region
II) effectively as a one-electron system. The method is developed in detail for
single active electron systems and applied to the exemplar case of the hydrogen
atom in an intense laser field.Comment: 13 pages, 6 figures, submitte
The Hubbard Dimer: A density functional case study of a many-body problem
This review explains the relationship between density functional theory and
strongly correlated models using the simplest possible example, the two-site
Hubbard model. The relationship to traditional quantum chemistry is included.
Even in this elementary example, where the exact ground-state energy and site
occupations can be found analytically, there is much to be explained in terms
of the underlying logic and aims of Density Functional Theory. Although the
usual solution is analytic, the density functional is given only implicitly. We
overcome this difficulty using the Levy-Lieb construction to create a
parametrization of the exact function with negligible errors. The symmetric
case is most commonly studied, but we find a rich variation in behavior by
including asymmetry, as strong correlation physics vies with charge-transfer
effects. We explore the behavior of the gap and the many-body Green's function,
demonstrating the `failure' of the Kohn-Sham method to reproduce the
fundamental gap. We perform benchmark calculations of the occupation and
components of the KS potentials, the correlation kinetic energies, and the
adiabatic connection. We test several approximate functionals (restricted and
unrestricted Hartree-Fock and Bethe Ansatz Local Density Approximation) to show
their successes and limitations. We also discuss and illustrate the concept of
the derivative discontinuity. Useful appendices include analytic expressions
for Density Functional energy components, several limits of the exact
functional (weak- and strong-coupling, symmetric and asymmetric), the Kohn-Sham
hopping energy functional for 3 sites, various adiabatic connection results,
proofs of exact conditions for this model, and the origin of the Hubbard model
from a minimal basis model for stretched H
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