2,397 research outputs found
Analysis of the Movement of Chlamydomonas Flagella: The Function of the Radial-spoke System Is Revealed by Comparison of Wild-type and Mutant Flagella
The mutation uni-1 gives rise to uniflagellate Chlamydomonas cells which rotate around a fixed point in the microscope field, so that the flagellar bending pattern can be photographed easily . This has allowed us to make a detailed analysis of the wild-type flagellar bending pattern and the bending patterns of flagella on several mutant strains. Cells containing uni-1, and recombinants of uni-1 with the suppressor mutations, sup(_pf)-1 and sup(_pf)-3, show the typical asymmetric bending pattern associated with forward swimming in Chlamydomonas,
although sup(_pf)-1 flagella have about one-half the normal beat frequency, apparently as the result of defective function of the outer dynein arms. The pf-17 mutation has been shown to produce nonmotile flagella in which radial spoke heads and five characteristic axonemal
polypeptides are missing. Recombinants containing pf-17 and either sup(_pf)-1 or sup(_pf)-3 have
motile flagella, but still lack radial-spoke heads and the associated polypeptides . The flagellar
bending pattern of these recombinants lacking radial-spoke heads is a nearly symmetric, large
amplitude pattern which is quite unlike the wild-type pattern . However, the presence of an
intact radial-spoke system is not required to convert active sliding into bending and is not
required for bend initiation and bend propagation, since all of these processes are active in the
sup(_pf) pf-17 recombinants. The function of the radial-spoke system appears to be to convert the
symmetric bending pattern displayed by these recombinants into the asymmetric bending
pattern required for efficient swimming, by inhibiting the development of reverse bends during
the recovery phase of the bending cycle
Structure of the stationary state of the asymmetric target process
We introduce a novel migration process, the target process. This process is
dual to the zero-range process (ZRP) in the sense that, while for the ZRP the
rate of transfer of a particle only depends on the occupation of the departure
site, it only depends on the occupation of the arrival site for the target
process. More precisely, duality associates to a given ZRP a unique target
process, and vice-versa. If the dynamics is symmetric, i.e., in the absence of
a bias, both processes have the same stationary-state product measure. In this
work we focus our interest on the situation where the latter measure exhibits a
continuous condensation transition at some finite critical density ,
irrespective of the dimensionality. The novelty comes from the case of
asymmetric dynamics, where the target process has a nontrivial fluctuating
stationary state, whose characteristics depend on the dimensionality. In one
dimension, the system remains homogeneous at any finite density. An alternating
scenario however prevails in the high-density regime: typical configurations
consist of long alternating sequences of highly occupied and less occupied
sites. The local density of the latter is equal to and their
occupation distribution is critical. In dimension two and above, the asymmetric
target process exhibits a phase transition at a threshold density much
larger than . The system is homogeneous at any density below ,
whereas for higher densities it exhibits an extended condensate elongated along
the direction of the mean current, on top of a critical background with density
.Comment: 30 pages, 16 figure
Object Segmentation in Images using EEG Signals
This paper explores the potential of brain-computer interfaces in segmenting
objects from images. Our approach is centered around designing an effective
method for displaying the image parts to the users such that they generate
measurable brain reactions. When an image region, specifically a block of
pixels, is displayed we estimate the probability of the block containing the
object of interest using a score based on EEG activity. After several such
blocks are displayed, the resulting probability map is binarized and combined
with the GrabCut algorithm to segment the image into object and background
regions. This study shows that BCI and simple EEG analysis are useful in
locating object boundaries in images.Comment: This is a preprint version prior to submission for peer-review of the
paper accepted to the 22nd ACM International Conference on Multimedia
(November 3-7, 2014, Orlando, Florida, USA) for the High Risk High Reward
session. 10 page
Dynamics at the angle of repose: jamming, bistability, and collapse
When a sandpile relaxes under vibration, it is known that its measured angle
of repose is bistable in a range of values bounded by a material-dependent
maximal angle of stability; thus, at the same angle of repose, a sandpile can
be stationary or avalanching, depending on its history. In the nearly jammed
slow dynamical regime, sandpile collapse to a zero angle of repose can also
occur, as a rare event. We claim here that fluctuations of {\it dilatancy} (or
local density) are the key ingredient that can explain such varied phenomena.
In this work, we model the dynamics of the angle of repose and of the density
fluctuations, in the presence of external noise, by means of coupled stochastic
equations. Among other things, we are able to describe sandpile collapse in
terms of an activated process, where an effective temperature (related to the
density as well as to the external vibration intensity) competes against the
configurational barriers created by the density fluctuations.Comment: 15 pages, 1 figure. Minor changes and update
Puromycin Sensitivity of Ribosomal Label after Incorporation of 14C-Labelled Amino Acids into Isolated Mitochondria from Neurospora crassa
Radioactive amino acids were incorporated into isolated mitochondria from Neurospora crassa. Then the mitochondrial ribosomes were isolated and submitted to density gradient centrifugation. A preferential labelling of polysomes was observed. However, when the mitochondrial suspension was treated with puromycin after amino acid incorporation, no radioactivity could be detected in either the monosomes or the polysomes. The conclusion is drawn that isolated mitochondria under these conditions do not incorporate significant amounts of amino acids into proteins of their ribosomes
Reddenings of FGK supergiants and classical Cepheids from spectroscopic data
Accurate and homogeneous atmospheric parameters (Teff, log (g), Vt, [Fe/H])
are derived for 74 FGK non-variable supergiants from high-resolution, high
signal-to-noise ratio, echelle spectra. Extremely high precision for the
inferred effective temperatures (10-40 K) is achieved by using the line-depth
ratio method. The new data are combined with atmospheric values for 164
classical Cepheids, observed at 675 different pulsation phases, taken from our
previously published studies. The derived values are correlated with unreddened
B-V colours compiled from the literature for the investigated stars in order to
obtain an empirical relationship of the form: (B-V)o = 57.984 - 10.3587(log
Teff)^2 + 1.67572(log Teff)^3 - 3.356(log (g)) + 0.0321(Vt) + 0.2615[Fe/H] +
0.8833((log (g))(log Teff)). The expression is used to estimate colour excesses
E(B-V) for individual supergiants and classical Cepheids, with a precision of
+-0.05 mag. for supergiants and Cepheids with n=1-2 spectra, reaching +-0.025
mag. for Cepheids with n>2 spectra, matching uncertainties for the most
sophisticated photometric techniques. The reddening scale is also a close match
to the system of space reddenings for Cepheids. The application range is for
spectral types F0--K0 and luminosity classes I and II.Comment: accepted for publication (MNRAS
Spectral properties of zero temperature dynamics in a model of a compacting granular column
The compacting of a column of grains has been studied using a one-dimensional
Ising model with long range directed interactions in which down and up spins
represent orientations of the grain having or not having an associated void.
When the column is not shaken (zero 'temperature') the motion becomes highly
constrained and under most circumstances we find that the generator of the
stochastic dynamics assumes an unusual form: many eigenvalues become
degenerate, but the associated multi-dimensional invariant spaces have but a
single eigenvector. There is no spectral expansion and a Jordan form must be
used. Many properties of the dynamics are established here analytically; some
are not. General issues associated with the Jordan form are also taken up.Comment: 34 pages, 4 figures, 3 table
A record-driven growth process
We introduce a novel stochastic growth process, the record-driven growth
process, which originates from the analysis of a class of growing networks in a
universal limiting regime. Nodes are added one by one to a network, each node
possessing a quality. The new incoming node connects to the preexisting node
with best quality, that is, with record value for the quality. The emergent
structure is that of a growing network, where groups are formed around record
nodes (nodes endowed with the best intrinsic qualities). Special emphasis is
put on the statistics of leaders (nodes whose degrees are the largest). The
asymptotic probability for a node to be a leader is equal to the Golomb-Dickman
constant omega=0.624329... which arises in problems of combinatorical nature.
This outcome solves the problem of the determination of the record breaking
rate for the sequence of correlated inter-record intervals. The process
exhibits temporal self-similarity in the late-time regime. Connections with the
statistics of the cycles of random permutations, the statistical properties of
randomly broken intervals, and the Kesten variable are given.Comment: 30 pages,5 figures. Minor update
Competition and cooperation:aspects of dynamics in sandpiles
In this article, we review some of our approaches to granular dynamics, now
well known to consist of both fast and slow relaxational processes. In the
first case, grains typically compete with each other, while in the second, they
cooperate. A typical result of {\it cooperation} is the formation of stable
bridges, signatures of spatiotemporal inhomogeneities; we review their
geometrical characteristics and compare theoretical results with those of
independent simulations. {\it Cooperative} excitations due to local density
fluctuations are also responsible for relaxation at the angle of repose; the
{\it competition} between these fluctuations and external driving forces, can,
on the other hand, result in a (rare) collapse of the sandpile to the
horizontal. Both these features are present in a theory reviewed here. An arena
where the effects of cooperation versus competition are felt most keenly is
granular compaction; we review here a random graph model, where three-spin
interactions are used to model compaction under tapping. The compaction curve
shows distinct regions where 'fast' and 'slow' dynamics apply, separated by
what we have called the {\it single-particle relaxation threshold}. In the
final section of this paper, we explore the effect of shape -- jagged vs.
regular -- on the compaction of packings near their jamming limit. One of our
major results is an entropic landscape that, while microscopically rough,
manifests {\it Edwards' flatness} at a macroscopic level. Another major result
is that of surface intermittency under low-intensity shaking.Comment: 36 pages, 23 figures, minor correction
Theoretical Models for Classical Cepheids: IV. Mean Magnitudes and Colors and the Evaluation of Distance, Reddening and Metallicity
We discuss the metallicity effect on the theoretical visual and near-infrared
PL and PLC relations of classical Cepheids, as based on nonlinear, nonlocal and
time--dependent convective pulsating models at varying chemical composition. In
view of the two usual methods of averaging (magnitude-weighted and
intensity-weighted) observed magnitudes and colors over the full pulsation
cycle, we briefly discuss the differences between static and mean quantities.
We show that the behavior of the synthetic mean magnitudes and colors fully
reproduces the observed trend of Galactic Cepheids, supporting the validity of
the model predictions. In the second part of the paper we show how the estimate
of the mean reddening and true distance modulus of a galaxy from Cepheid VK
photometry depend on the adopted metal content, in the sense that larger
metallicities drive the host galaxy to lower extinctions and distances.
Conversely, self-consistent estimates of the Cepheid mean reddening, distance
and metallicity may be derived if three-filter data are taken into account. By
applying the theoretical PL and PLC relations to available BVK data of Cepheids
in the Magellanic Clouds we eventually obtain Z \sim 0.008, E(B-V) \sim 0.02
mag, DM \sim 18.63 mag for LMC and Z \sim 0.004, E(B-V) \sim 0.01 mag., DM \sim
19.16 mag. for SMC. The discrepancy between such reddenings and the current
values based on BVI data is briefly discussed.Comment: 16 pages, 11 postscript figures, accepted for publication on Ap
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