2,837 research outputs found
A diagrammatic approach to study the information transfer in weakly non-linear channels
In a recent work we have introduced a novel approach to study the effect of
weak non-linearities in the transfer function on the information transmitted by
an analogue channel, by means of a perturbative diagrammatic expansion. We
extend here the analysis to all orders in perturbation theory, which allows us
to release any constraint concerning the magnitude of the expansion parameter
and to establish the rules to calculate easily the contribution at any order.
As an example we explicitly compute the information up to the second order in
the non-linearity, in presence of random gaussian connectivities and in the
limit when the output noise is not small. We analyze the first and second order
contributions to the mutual information as a function of the non-linearity and
of the number of output units. We believe that an extensive application of our
method via the analysis of the different contributions at distinct orders might
be able to fill a gap between well known analytical results obtained for linear
channels and the non trivial treatments which are required to study highly
non-linear channels.Comment: 17 pages, 3 figure
A replica free evaluation of the neuronal population information with mixed continuous and discrete stimuli: from the linear to the asymptotic regime
Recent studies have explored theoretically the ability of populations of
neurons to carry information about a set of stimuli, both in the case of purely
discrete or purely continuous stimuli, and in the case of multidimensional
continuous angular and discrete correlates, in presence of additional quenched
disorder in the distribution. An analytical expression for the mutual
information has been obtained in the limit of large noise by means of the
replica trick. Here we show that the same results can actually be obtained in
most cases without the use of replicas, by means of a much simpler expansion of
the logarithm. Fitting the theoretical model to real neuronal data, we show
that the introduction of correlations in the quenched disorder improves the
fit, suggesting a possible role of signal correlations-actually detected in
real data- in a redundant code. We show that even in the more difficult
analysis of the asymptotic regime, an explicit expression for the mutual
information can be obtained without resorting to the replica trick despite the
presence of quenched disorder, both with a gaussian and with a more realistic
thresholded-gaussian model. When the stimuli are mixed continuous and discrete,
we find that with both models the information seem to grow logarithmically to
infinity with the number of neurons and with the inverse of the noise, even
though the exact general dependence cannot be derived explicitly for the
thresholded gaussian model. In the large noise limit lower values of
information were obtained with the thresholded-gaussian model, for a fixed
value of the noise and of the population size. On the contrary, in the
asymptotic regime, with very low values of the noise, a lower information value
is obtained with the gaussian model.Comment: 34 pages, 5 figure
A perturbative approach to non-linearities in the information carried by a two layer neural network
We evaluate the mutual information between the input and the output of a two
layer network in the case of a noisy and non-linear analogue channel. In the
case where the non-linearity is small with respect to the variability in the
noise, we derive an exact expression for the contribution to the mutual
information given by the non-linear term in first order of perturbation theory.
Finally we show how the calculation can be simplified by means of a
diagrammatic expansion. Our results suggest that the use of perturbation
theories applied to neural systems might give an insight on the contribution of
non-linearities to the information transmission and in general to the neuronal
dynamics.Comment: Accepted as a preprint of ICTP, Triest
Replica symmetric evaluation of the information transfer in a two-layer network in presence of continuous+discrete stimuli
In a previous report we have evaluated analytically the mutual information
between the firing rates of N independent units and a set of multi-dimensional
continuous+discrete stimuli, for a finite population size and in the limit of
large noise. Here, we extend the analysis to the case of two interconnected
populations, where input units activate output ones via gaussian weights and a
threshold linear transfer function. We evaluate the information carried by a
population of M output units, again about continuous+discrete correlates. The
mutual information is evaluated solving saddle point equations under the
assumption of replica symmetry, a method which, by taking into account only the
term linear in N of the input information, is equivalent to assuming the noise
to be large. Within this limitation, we analyze the dependence of the
information on the ratio M/N, on the selectivity of the input units and on the
level of the output noise. We show analytically, and confirm numerically, that
in the limit of a linear transfer function and of a small ratio between output
and input noise, the output information approaches asymptotically the
information carried in input. Finally, we show that the information loss in
output does not depend much on the structure of the stimulus, whether purely
continuous, purely discrete or mixed, but only on the position of the threshold
nonlinearity, and on the ratio between input and output noise.Comment: 19 pages, 4 figure
Restoration of anterior teeth using an indirect composite technique. Case report
This article presents a case report of restoration of anterior teeth using an indirect technique with composite
veneers in order to restore the dental anatomy and to provide aesthetic and function of anterior teeth. A treatment of upper anterior teeth with indirect microhybrid composite restoration was proposed to a 40-year-old woman with old discolored and fractured ceramic veneers. Upper six anterior teeth were prepared and, after impressions with VPS of maxillary arche, the composite veneers were placed. There are several advantages offered by indirect techniques compared to direct techniques. The increased practice of veneering technique with indirect composites is due to improvement in the properties of composite materials in the last years. The results are very favourable and the patient is satisfied. The restorations with composite veneers have proved durable and aesthetic, protect tooth structure and aesthetic and function is predictably
re-established
A device to characterize optical fibres
ATLAS is a general purpose experiment approved for the LHC collider at CERN.
An important component of the detector is the central hadronic calorimeter; for
its construction more than 600,000 Wave Length Shifting (WLS) fibres
(corresponding to a total length of 1,120 Km) have been used.
We have built and put into operation a dedicated instrument for the
measurement of light yield and attenuation length over groups of 20 fibres at a
time.
The overall accuracy achieved in the measurement of light yield
(attenuation length) is 1.5% (3%).
We also report the results obtained using this method in the quality control
of a large sample of fibres.Comment: 17 pages 20 figeres submitted to NIM journa
Role of Atypical Chemokine Receptors in Microglial Activation and Polarization.
Inflammatory reactions occurring in the central nervous system (CNS), known as neuroinflammation, are key components of the pathogenic mechanisms underlying several neurological diseases. The chemokine system plays a crucial role in the recruitment and activation of immune and non-immune cells in the brain, as well as in the regulation of microglia phenotype and function. Chemokines belong to a heterogeneous family of chemotactic agonists that signal through the interaction with G protein-coupled receptors (GPCRs). Recently, a small subset of chemokine receptors, now identified as “atypical chemokine receptors” (ACKRs), has been described. These receptors lack classic GPCR signaling and chemotactic activity and are believed to limit inflammation through their ability to scavenge chemokines at the inflammatory sites. Recent studies have highlighted a role for ACKRs in neuroinflammation. However, in the CNS, the role of ACKRs seems to be more complex than the simple control of inflammation. For instance, CXCR7/ACKR3 was shown to control T cell trafficking through the regulation of CXCL12 internalization at CNS endothelial barriers. Furthermore, D6/ACKR2 KO mice were protected in a model of experimental autoimmune encephalomyelitis (EAE). D6/ACKR2 KO showed an abnormal accumulation of dendritic cells at the immunization and a subsequent impairment in T cell priming. Finally, CCRL2, an ACKR-related protein, was shown to play a role in the control of the resolution phase of EAE. Indeed, CCRL2 KO mice showed exacerbated, non- resolving disease with protracted inflammation and increased demyelination. This phenotype was associated with increased microglia and macrophage activation markers and imbalanced M1 vs. M2 polarization. This review will summarize the current knowledge on the role of the ACKRs in neuroinflammation with a particular attention to their role in microglial polarization and function
Thyroid nodules treated with percutaneous radiofrequency thermal ablation: a comparative study
Percutaneous radiofrequency thermal ablation (RTA) was reported as an effective tool for the management of thyroid nodules (TNs). The aim of this study was to investigate the effects of RTA and to establish whether they were treatment-related by comparison with a matched, untreated control group
Fixação biológica de nitrogênio em soja sob déficit hídrico.
Avaliaram-se em condições controladas os efeitos do déficit hídrico sobre a fixação biológica do nitrogênio (FBN) e componentes de rendimento em plantas de soja (Glycine max (L.) Merrill) (cv. BRS-232 e BRS-268) em três experimentos e em três estádios reprodutivos, em que as plantas foram submetidas a duas disponibilidades hídricas: condição normal de capacidade de campo e de déficit hídrico nos estádios reprodutivos. Durante os estádios R2 e R3 foram avaliados a condutância estomática; a massa e o número de nódulos; e os teores de ureídeos (ácido alantóico e alantoína) na seiva do xilema; além do rendimento de sementes por planta em R8. O déficit hídrico reduziu os atributos relacionados à FBN, com reflexos na redução dos componentes de rendimento de grãos das plantas. A condição de déficit hídrico durante 13 dias entre as fases reprodutivas de florescimento pleno e formação inicial de vagens reduziu a massa e o número de nódulos em soja, com consequente redução média no rendimento de grãos de soja por planta em 14%
Microstructural and morphological properties of homoepitaxial (001)ZnTe layers investigated by x-ray diffuse scattering
The microstructural and morphological properties of homoepitaxial (001)ZnTe
layers are investigated by x-ray diffuse scattering. High resolution reciprocal
space maps recorded close to the ZnTe (004) Bragg peak show different diffuse
scattering features. One kind of cross-shaped diffuse scattering streaks along
directions can be attributed to stacking faults within the epilayers.
Another kind of cross-shaped streaks inclined at an angle of about 80deg with
respect to the in-plane direction arises from the morphology of the
epilayers. (abridged version
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