9,078 research outputs found
Development of modularity in the neural activity of children's brains
We study how modularity of the human brain changes as children develop into
adults. Theory suggests that modularity can enhance the response function of a
networked system subject to changing external stimuli. Thus, greater cognitive
performance might be achieved for more modular neural activity, and modularity
might likely increase as children develop. The value of modularity calculated
from fMRI data is observed to increase during childhood development and peak in
young adulthood. Head motion is deconvolved from the fMRI data, and it is shown
that the dependence of modularity on age is independent of the magnitude of
head motion. A model is presented to illustrate how modularity can provide
greater cognitive performance at short times, i.e.\ task switching. A fitness
function is extracted from the model. Quasispecies theory is used to predict
how the average modularity evolves with age, illustrating the increase of
modularity during development from children to adults that arises from
selection for rapid cognitive function in young adults. Experiments exploring
the effect of modularity on cognitive performance are suggested. Modularity may
be a potential biomarker for injury, rehabilitation, or disease.Comment: 29 pages, 11 figure
Modularity Enhances the Rate of Evolution in a Rugged Fitness Landscape
Biological systems are modular, and this modularity affects the evolution of
biological systems over time and in different environments. We here develop a
theory for the dynamics of evolution in a rugged, modular fitness landscape. We
show analytically how horizontal gene transfer couples to the modularity in the
system and leads to more rapid rates of evolution at short times. The model, in
general, analytically demonstrates a selective pressure for the prevalence of
modularity in biology. We use this model to show how the evolution of the
influenza virus is affected by the modularity of the proteins that are
recognized by the human immune system. Approximately 25\% of the observed rate
of fitness increase of the virus could be ascribed to a modular viral
landscape.Comment: 45 pages; 7 figure
Hierarchy of Gene Expression Data is Predictive of Future Breast Cancer Outcome
We calculate measures of hierarchy in gene and tissue networks of breast
cancer patients. We find that the likelihood of metastasis in the future is
correlated with increased values of network hierarchy for expression networks
of cancer-associated genes, due to correlated expression of cancer-specific
pathways. Conversely, future metastasis and quick relapse times are negatively
correlated with values of network hierarchy in the expression network of all
genes, due to dedifferentiation of gene pathways and circuits. These results
suggest that hierarchy of gene expression may be useful as an additional
biomarker for breast cancer prognosis.Comment: 14 pages, 5 figure
Field Theoretic Approach to Long Range Reactions
We analyze bimolecular reactions that proceed by a long-ranged reactive
interaction, using a field theoretic approach that takes into account
fluctuations.
We consider both the one-species, reaction and the
two-species, reaction. We consider both mobile and immobile
reactants, both in the presence and in the absence of adsorption.Comment: 9 pages. 4 figures. Uses svjour macros. To appear in Europ. Phys. J.
Higher-Rank Numerical Ranges and Compression Problems
We consider higher-rank versions of the standard numerical range for
matrices. A central motivation for this investigation comes from quantum error
correction. We develop the basic structure theory for the higher-rank numerical
ranges, and give a complete description in the Hermitian case. We also consider
associated projection compression problems.Comment: 14 pages, 3 figures, to appear in Linear Algebra and its Application
An assessment of multibody simulation tools for articulated spacecraft
A survey of multibody simulation codes was conducted in the spring of 1988, to obtain an assessment of the state of the art in multibody simulation codes from the users of the codes. This survey covers the most often used articulated multibody simulation codes in the spacecraft and robotics community. There was no attempt to perform a complete survey of all available multibody codes in all disciplines. Furthermore, this is not an exhaustive evaluation of even robotics and spacecraft multibody simulation codes, as the survey was designed to capture feedback on issues most important to the users of simulation codes. We must keep in mind that the information received was limited and the technical background of the respondents varied greatly. Therefore, only the most often cited observations from the questionnaire are reported here. In this survey, it was found that no one code had both many users (reports) and no limitations. The first section is a report on multibody code applications. Following applications is a discussion of execution time, which is the most troublesome issue for flexible multibody codes. The representation of component flexible bodies, which affects both simulation setup time as well as execution time, is presented next. Following component data preparation, two sections address the accessibility or usability of a code, evaluated by considering its user interface design and examining the overall simulation integrated environment. A summary of user efforts at code verification is reported, before a tabular summary of the questionnaire responses. Finally, some conclusions are drawn
An exact effective two-qubit gate in a chain of three spins
We show that an effective two-qubit gate can be obtained from the free
evolution of three spins in a chain with nearest neighbor XY coupling, without
local manipulations. This gate acts on the two remote spins and leaves the
mediating spin unchanged. It can be used to perfectly transfer an arbitrary
quantum state from the first spin to the last spin or to simultaneously
communicate one classical bit in each direction. One ebit can be generated in
half of the time for state transfer.
For longer spin chains, we present methods to create or transfer entanglement
between the two end spins in half of the time required for quantum state
transfer, given tunable coupling strength and local magnetic field. We also
examine imperfect state transfer through a homogeneous XY chain.Comment: RevTeX4, 7 pages, 4 figue
Galaxy pairs as a probe for mergers at z ~ 2
In this work I investigate the redshift evolution of pair fraction of a
sample of 196 massive galaxies from z = 0 to 3, selected from the COSMOS field.
We find that on average a massive galaxy undergoes ~ 1.1 \pm 0.5 major merger
since z = 3. I will review the current limitations of using the pair fraction
as a probe for quantifying the impact of mergers on galaxy evolution. This work
is based on the paper Man et al. (2011).Comment: 4 pages; to appear on the Conference Proceedings for "Galaxy Mergers
in an Evolving Universe", held in Hualien, Taiwan (October 2011
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