575 research outputs found
Direct magnetic resonance arthrography of the knee: utility of axial traction
The purpose of this study was to determine the impact of axial traction during acquisition of direct magnetic resonance (MR) arthrography examination of the knee in terms of joint space width and amount of contrast material between the cartilage surfaces. Direct knee MR arthrography was performed in 11 patients on a 3-T MR imaging unit using a T1-weighted isotropic gradient echo sequence in a coronal plane with and without axial traction of 15kg. Joint space widths were measured at the level of the medial and the lateral femorotibial joint with and without traction. The amount of contrast material in the medial and lateral femorotibial joint was assessed independently by two musculoskeletal radiologists in a semiquantitative manner using three grades (‘absence of surface visualization, ‘partial surface visualization or ‘complete surface visualization'). With traction, joint space width increased significantly at the lateral femorotibial compartment (mean = 0.55mm, p = 0.0105) and at the medial femorotibial compartment (mean = 0.4mm, p = 0.0124). There was a trend towards an increased amount of contrast material in the femorotibial compartment with axial traction. Direct MR arthrography of the knee with axial traction showed a slight and significant increase of the width of the femorotibial compartment with a trend towards more contrast material between the articular cartilage surface
Plasticity facilitates sustainable growth in the commons
In the commons, communities whose growth depends on public goods, individuals
often rely on surprisingly simple strategies, or heuristics, to decide whether
to contribute to the common good (at risk of exploitation by free-riders).
Although this appears a limitation, here we show how four heuristics lead to
sustainable growth by exploiting specific environmental constraints. The two
simplest ones --contribute permanently or switch stochastically between
contributing or not-- are first shown to bring sustainability when the public
good efficiently promotes growth. If efficiency declines and the commons is
structured in small groups, the most effective strategy resides in contributing
only when a majority of individuals are also contributors. In contrast, when
group size becomes large, the most effective behavior follows a minimal-effort
rule: contribute only when it is strictly necessary. Both plastic strategies
are observed in natural systems what presents them as fundamental social motifs
to successfully manage sustainability
Effects of inhomogeneous activity of players and noise on cooperation in spatial public goods games
We study the public goods game in the noisy case by considering the players
with inhomogeneous activity teaching on a square lattice. It is shown that the
introduction of the inhomogeneous activity of teaching of the players can
remarkably promote cooperation. By investigating the effects of noise on
cooperative behavior in detail, we find that the variation of cooperator
density with the noise parameter displays several different
behaviors: monotonically increases (decreases) with ;
firstly increases (decreases) with and then it decreases (increases)
monotonically after reaching its maximum (minimum) value, which depends on the
amount of the multiplication factor , on whether the system is homogeneous
or inhomogeneous, and on whether the adopted updating is synchronous or
asynchronous. These results imply that the noise plays an important and
nontrivial role in the evolution of cooperation.Comment: 4 pages, 4 figure
Promote cooperation by localised small-world communication
The emergence and maintenance of cooperation within sizable groups of
unrelated humans offer many challenges for our understanding. We propose that
the humans' capacity of communication, such as how many and how far away the
fellows can build up mutual communications, may affect the evolution of
cooperation. We study this issue by means of the public goods game (PGG) with a
two-layered network of contacts. Players obtain payoffs from five-person public
goods interactions on a square lattice (the interaction layer). Also, they
update strategies after communicating with neighbours in learning layer, where
two players build up mutual communication with a power law probability
depending on their spatial distance. Our simulation results indicate that the
evolution of cooperation is indeed sensitive to how players choose others to
communicate with, including the amount as well as the locations. The tendency
of localised communication is proved to be a new mechanism to promote
cooperation.Comment: 6 pages, 4 figure
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Community projects: an experimental analysis of a fair implementation process
We define and experimentally test a public provision mechanism that meets three basic ethical requirements and allows community members to influence, via monetary bids, which of several projects is implemented. For each project, participants are assigned personal values, which can be positive or negative. We provide either public or private information about personal values. This produces two distinct public provision games, which are experimentally implemented and analyzed for various projects. In spite of the complex experimental task, participants do not rely on bidding their own personal values as an obvious simple heuristic whose general acceptance would result in fair and efficient outcomes. Rather, they rely on strategic underbidding. Although underbidding is affected by projects’ characteristics, the provision mechanism mostly leads to the implementation of the most efficient project
Evolutionary dynamics for persistent cooperation in structured populations
The emergence and maintenance of cooperative behavior is a fascinating topic in evolutionary biology and social science. The public goods game (PGG) is a paradigm for exploring cooperative behavior. In PGG, the total resulting payoff is divided equally among all participants. This feature still leads to the dominance of defection without substantially magnifying the public good by a multiplying factor. Much effort has been made to explain the evolution of cooperative strategies, including a recent model in which only a portion of the total benefit is shared by all the players through introducing a new strategy named persistent cooperation. A persistent cooperator is a contributor who is willing to pay a second cost to retrieve the remaining portion of the payoff contributed by themselves. In a previous study, this model was analyzed in the framework of well-mixed populations. This paper focuses on discussing the persistent cooperation in lattice-structured populations. The evolutionary dynamics of the structured populations consisting of three types of competing players (pure cooperators, defectors, and persistent cooperators) are revealed by theoretical analysis and numerical simulations. In particular, the approximate expressions of fixation probabilities for strategies are derived on one-dimensional lattices. The phase diagrams of stationary states, and the evolution of frequencies and spatial patterns for strategies are illustrated on both one-dimensional and square lattices by simulations. Our results are consistent with the general observation that, at least in most situations, a structured population facilitates the evolution of cooperation. Specifically, here we find that the existence of persistent cooperators greatly suppresses the spreading of defectors under more relaxed conditions in structured populations compared to that obtained in well-mixed populations
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