23,216 research outputs found
Discontinuities in pattern inference
This paper deals with the inferrability of classes of E-pattern languages—also referred
to as extended or erasing pattern languages—from positive data in Gold’s
model of identification in the limit. The first main part of the paper shows that
the recently presented negative result on terminal-free E-pattern languages over binary
alphabets does not hold for other alphabet sizes, so that the full class of these
languages is inferrable from positive data if and only if the corresponding terminal
alphabet does not consist of exactly two distinct letters. The second main part yields
the insight that the positive result on terminal-free E-pattern languages over alphabets
with three or four letters cannot be extended to the class of general E-pattern
languages. With regard to larger alphabets, the extensibility remains open.
The proof methods developed for these main results do not directly discuss the
(non-)existence of appropriate learning strategies, but they deal with structural
properties of classes of E-pattern languages, and, in particular, with the problem
of finding telltales for these languages. It is shown that the inferrability of classes
of E-pattern languages is closely connected to some problems on the ambiguity
of morphisms so that the technical contributions of the paper largely consist of
combinatorial insights into morphisms in word monoids
Discontinuities in pattern inference
This paper deals with the inferrability of classes of E-pattern languages—also referred
to as extended or erasing pattern languages—from positive data in Gold’s
model of identification in the limit. The first main part of the paper shows that
the recently presented negative result on terminal-free E-pattern languages over binary
alphabets does not hold for other alphabet sizes, so that the full class of these
languages is inferrable from positive data if and only if the corresponding terminal
alphabet does not consist of exactly two distinct letters. The second main part yields
the insight that the positive result on terminal-free E-pattern languages over alphabets
with three or four letters cannot be extended to the class of general E-pattern
languages. With regard to larger alphabets, the extensibility remains open.
The proof methods developed for these main results do not directly discuss the
(non-)existence of appropriate learning strategies, but they deal with structural
properties of classes of E-pattern languages, and, in particular, with the problem
of finding telltales for these languages. It is shown that the inferrability of classes
of E-pattern languages is closely connected to some problems on the ambiguity
of morphisms so that the technical contributions of the paper largely consist of
combinatorial insights into morphisms in word monoids
Using prior information to identify boundaries in disease risk maps
Disease maps display the spatial pattern in disease risk, so that high-risk
clusters can be identified. The spatial structure in the risk map is typically
represented by a set of random effects, which are modelled with a conditional
autoregressive (CAR) prior. Such priors include a global spatial smoothing
parameter, whereas real risk surfaces are likely to include areas of smooth
evolution as well as discontinuities, the latter of which are known as risk
boundaries. Therefore, this paper proposes an extension to the class of CAR
priors, which can identify both areas of localised spatial smoothness and risk
boundaries. However, allowing for this localised smoothing requires large
numbers of correlation parameters to be estimated, which are unlikely to be
well identified from the data. To address this problem we propose eliciting an
informative prior about the locations of such boundaries, which can be combined
with the information from the data to provide more precise posterior inference.
We test our approach by simulation, before applying it to a study of the risk
of emergency admission to hospital in Greater Glasgow, Scotland
Non-stationary patterns of isolation-by-distance: inferring measures of local genetic differentiation with Bayesian kriging
Patterns of isolation-by-distance arise when population differentiation
increases with increasing geographic distances. Patterns of
isolation-by-distance are usually caused by local spatial dispersal, which
explains why differences of allele frequencies between populations accumulate
with distance. However, spatial variations of demographic parameters such as
migration rate or population density can generate non-stationary patterns of
isolation-by-distance where the rate at which genetic differentiation
accumulates varies across space. To characterize non-stationary patterns of
isolation-by-distance, we infer local genetic differentiation based on Bayesian
kriging. Local genetic differentiation for a sampled population is defined as
the average genetic differentiation between the sampled population and fictive
neighboring populations. To avoid defining populations in advance, the method
can also be applied at the scale of individuals making it relevant for
landscape genetics. Inference of local genetic differentiation relies on a
matrix of pairwise similarity or dissimilarity between populations or
individuals such as matrices of FST between pairs of populations. Simulation
studies show that maps of local genetic differentiation can reveal barriers to
gene flow but also other patterns such as continuous variations of gene flow
across habitat. The potential of the method is illustrated with 2 data sets:
genome-wide SNP data for human Swedish populations and AFLP markers for alpine
plant species. The software LocalDiff implementing the method is available at
http://membres-timc.imag.fr/Michael.Blum/LocalDiff.htmlComment: In press, Evolution 201
Gap Filling of 3-D Microvascular Networks by Tensor Voting
We present a new algorithm which merges discontinuities in 3-D images of tubular structures presenting undesirable gaps. The application of the proposed method is mainly associated to large 3-D images of microvascular networks. In order to recover the real network topology, we need to fill the gaps between the closest discontinuous vessels. The algorithm presented in this paper aims at achieving this goal. This algorithm is based on the skeletonization of the segmented network followed by a tensor voting method. It permits to merge the most common kinds of discontinuities found in microvascular networks. It is robust, easy to use, and relatively fast. The microvascular network images were obtained using synchrotron tomography imaging at the European Synchrotron Radiation Facility. These images exhibit samples of intracortical networks. Representative results are illustrated
Dispersal and population structure at different spatial scales in the subterranean rodent Ctenomys australis
This study was funded by grants from Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET, PIP5838), Agencia de Promoción Científica y Tecnológica de la Argentina (PICTO1-423, BID-1728/OC-AR), and the programme ECOS-Sud France/Argentina (A05B01).Background: The population genetic structure of subterranean rodent species is strongly affected by demographic (e.g. rates of dispersal and social structure) and stochastic factors (e.g. random genetic drift among subpopulations and habitat fragmentation). In particular, gene flow estimates at different spatial scales are essential to understand genetic differentiation among populations of a species living in a highly fragmented landscape. Ctenomys australis (the sand dune tuco-tuco) is a territorial subterranean rodent that inhabits a relatively secure, permanently sealed burrow system, occurring in sand dune habitats on the coastal landscape in the south-east of Buenos Aires province, Argentina. Currently, this habitat is threatened by urban development and forestry and, therefore, the survival of this endemic species is at risk. Here, we assess population genetic structure and patterns of dispersal among individuals of this species at different spatial scales using 8 polymorphic microsatellite loci. Furthermore, we evaluate the relative importance of sex and habitat configuration in modulating the dispersal patterns at these geographical scales. Results: Our results show that dispersal in C. australis is not restricted at regional spatial scales (similar to 4 km). Assignment tests revealed significant population substructure within the study area, providing support for the presence of two subpopulations from three original sampling sites. Finally, male-biased dispersal was found in the Western side of our study area, but in the Eastern side no apparent philopatric pattern was found, suggesting that in a more continuous habitat males might move longer distances than females. Conclusions: Overall, the assignment-based approaches were able to detect population substructure at fine geographical scales. Additionally, the maintenance of a significant genetic structure at regional (similar to 4 km) and small (less than 1 km) spatial scales despite apparently moderate to high levels of gene flow between local sampling sites could not be explained simply by the linear distance among them. On the whole, our results support the hypothesis that males disperse more frequently than females; however they do not provide support for strict philopatry within females.Publisher PDFPeer reviewe
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