578 research outputs found
Novel type of phase transition in a system of self-driven particles
A simple model with a novel type of dynamics is introduced in order to
investigate the emergence of self-ordered motion in systems of particles with
biologically motivated interaction. In our model particles are driven with a
constant absolute velocity and at each time step assume the average direction
of motion of the particles in their neighborhood with some random perturbation
() added. We present numerical evidence that this model results in a
kinetic phase transition from no transport (zero average velocity, ) to finite net transport through spontaneous symmetry breaking of the
rotational symmetry. The transition is continuous since is
found to scale as with
Group selection models in prebiotic evolution
The evolution of enzyme production is studied analytically using ideas of the
group selection theory for the evolution of altruistic behavior. In particular,
we argue that the mathematical formulation of Wilson's structured deme model
({\it The Evolution of Populations and Communities}, Benjamin/Cumings, Menlo
Park, 1980) is a mean-field approach in which the actual environment that a
particular individual experiences is replaced by an {\it average} environment.
That formalism is further developed so as to avoid the mean-field approximation
and then applied to the problem of enzyme production in the prebiotic context,
where the enzyme producer molecules play the altruists role while the molecules
that benefit from the catalyst without paying its production cost play the
non-altruists role. The effects of synergism (i.e., division of labor) as well
as of mutations are also considered and the results of the equilibrium analysis
are summarized in phase diagrams showing the regions of the space of parameters
where the altruistic, non-altruistic and the coexistence regimes are stable. In
general, those regions are delimitated by discontinuous transition lines which
end at critical points.Comment: 22 pages, 10 figure
Dynamic instabilities induced by asymmetric influence: Prisoners' dilemma game on small-world networks
A two-dimensional small-world type network, subject to spatial prisoners'
dilemma dynamics and containing an influential node defined as a special node
with a finite density of directed random links to the other nodes in the
network, is numerically investigated. It is shown that the degree of
cooperation does not remain at a steady state level but displays a punctuated
equilibrium type behavior manifested by the existence of sudden breakdowns of
cooperation. The breakdown of cooperation is linked to an imitation of a
successful selfish strategy of the influential node. It is also found that
while the breakdown of cooperation occurs suddenly, the recovery of it requires
longer time. This recovery time may, depending on the degree of steady state
cooperation, either increase or decrease with an increasing number of long
range connections.Comment: 5 pages, 6 figure
Traffic Instabilities in Self-Organized Pedestrian Crowds
In human crowds as well as in many animal societies, local interactions among
individuals often give rise to self-organized collective organizations that
offer functional benefits to the group. For instance, flows of pedestrians
moving in opposite directions spontaneously segregate into lanes of uniform
walking directions. This phenomenon is often referred to as a smart collective
pattern, as it increases the traffic efficiency with no need of external
control. However, the functional benefits of this emergent organization have
never been experimentally measured, and the underlying behavioral mechanisms
are poorly understood. In this work, we have studied this phenomenon under
controlled laboratory conditions. We found that the traffic segregation
exhibits structural instabilities characterized by the alternation of organized
and disorganized states, where the lifetime of well-organized clusters of
pedestrians follow a stretched exponential relaxation process. Further analysis
show that the inter-pedestrian variability of comfortable walking speeds is a
key variable at the origin of the observed traffic perturbations. We show that
the collective benefit of the emerging pattern is maximized when all
pedestrians walk at the average speed of the group. In practice, however, local
interactions between slow- and fast-walking pedestrians trigger global
breakdowns of organization, which reduce the collective and the individual
payoff provided by the traffic segregation. This work is a step ahead toward
the understanding of traffic self-organization in crowds, which turns out to be
modulated by complex behavioral mechanisms that do not always maximize the
group's benefits. The quantitative understanding of crowd behaviors opens the
way for designing bottom-up management strategies bound to promote the
emergence of efficient collective behaviors in crowds.Comment: Article published in PLoS Computational biology. Freely available
here:
http://www.ploscompbiol.org/article/info%3Adoi%2F10.1371%2Fjournal.pcbi.100244
Structure of Fat Jets at the Tevatron and Beyond
Boosted resonances is a highly probable and enthusiastic scenario in any
process probing the electroweak scale. Such objects when decaying into jets can
easily blend with the cornucopia of jets from hard relative light QCD states.
We review jet observables and algorithms that can contribute to the
identification of highly boosted heavy jets and the possible searches that can
make use of such substructure information. We also review previous studies by
CDF on boosted jets and its measurements on specific jet shapes.Comment: invited review for a special "Top and flavour physics in the LHC era"
issue of The European Physical Journal C, we invite comments regarding
contents of the review; v2 added references and institutional preprint
number
The Affective Impact of Financial Skewness on Neural Activity and Choice
Few finance theories consider the influence of “skewness” (or large and asymmetric but unlikely outcomes) on financial choice. We investigated the impact of skewed gambles on subjects' neural activity, self-reported affective responses, and subsequent preferences using functional magnetic resonance imaging (FMRI). Neurally, skewed gambles elicited more anterior insula activation than symmetric gambles equated for expected value and variance, and positively skewed gambles also specifically elicited more nucleus accumbens (NAcc) activation than negatively skewed gambles. Affectively, positively skewed gambles elicited more positive arousal and negatively skewed gambles elicited more negative arousal than symmetric gambles equated for expected value and variance. Subjects also preferred positively skewed gambles more, but negatively skewed gambles less than symmetric gambles of equal expected value. Individual differences in both NAcc activity and positive arousal predicted preferences for positively skewed gambles. These findings support an anticipatory affect account in which statistical properties of gambles—including skewness—can influence neural activity, affective responses, and ultimately, choice
Nonlinear deterministic equations in biological evolution
We review models of biological evolution in which the population frequency
changes deterministically with time. If the population is self-replicating,
although the equations for simple prototypes can be linearised, nonlinear
equations arise in many complex situations. For sexual populations, even in the
simplest setting, the equations are necessarily nonlinear due to the mixing of
the parental genetic material. The solutions of such nonlinear equations
display interesting features such as multiple equilibria and phase transitions.
We mainly discuss those models for which an analytical understanding of such
nonlinear equations is available.Comment: Invited review for J. Nonlin. Math. Phy
Charge Solitons in 1-D Arrays of Serially Coupled Josephson Junctions
We study a 1-D array of Josephson coupled superconducting grains with kinetic
inductance which dominates over the Josephson inductance. In this limit the
dynamics of excess Cooper pairs in the array is described in terms of charge
solitons, created by polarization of the grains. We analyze the dynamics of
these topological excitations, which are dual to the fluxons in a long
Josephson junction, using the continuum sine-Gordon model. We find that their
classical relativistic motion leads to saturation branches in the I-V
characteristic of the array. We then discuss the semi-classical quantization of
the charge soliton, and show that it is consistent with the large kinetic
inductance of the array. We study the dynamics of a quantum charge soliton in a
ring-shaped array biased by an external flux through its center. If the
dephasing length of the quantum charge soliton is larger than the circumference
of the array, quantum phenomena like persistent current and coherent current
oscillations are expected. As the characteristic width of the charge soliton is
of the order of 100 microns, it is a macroscopic quantum object. We discuss the
dephasing mechanisms which can suppress the quantum behaviour of the charge
soliton.Comment: 26 pages, LaTex, 7 Postscript figure
Agent based modelling helps in understanding the rules by which fibroblasts support keratinocyte colony formation
Background: Autologous keratincoytes are routinely expanded using irradiated mouse fibroblasts and bovine serum for clinical use. With growing concerns about the safety of these xenobiotic materials, it is desirable to culture keratinocytes in media without animal derived products. An improved understanding of epithelial/mesenchymal interactions could assist in this.
Methodology/Principal Findings: A keratincyte/fibroblast o-culture model was developed by extending an agent-based keratinocyte colony formation model to include the response of keratinocytes to both fibroblasts and serum. The model was validated by comparison of the in virtuo and in vitro multicellular behaviour of keratinocytes and fibroblasts in single and co-culture in Greens medium. To test the robustness of the model, several properties of the fibroblasts were changed to investigate their influence on the multicellular morphogenesis of keratinocyes and fibroblasts. The model was then used to generate hypotheses to explore the interactions of both proliferative and growth arrested fibroblasts with keratinocytes. The key predictions arising from the model which were confirmed by in vitro experiments were that 1) the ratio of fibroblasts to keratinocytes would critically influence keratinocyte colony expansion, 2) this ratio needed to be optimum at the beginning of the co-culture, 3) proliferative fibroblasts would be more effective than irradiated cells in expanding keratinocytes and 4) in the presence of an adequate number of fibroblasts, keratinocyte expansion would be independent of serum.
Conclusions: A closely associated computational and biological approach is a powerful tool for understanding complex biological systems such as the interactions between keratinocytes and fibroblasts. The key outcome of this study is the finding that the early addition of a critical ratio of proliferative fibroblasts can give rapid keratinocyte expansion without the use of irradiated mouse fibroblasts and bovine serum
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