1,653 research outputs found
Variability in non-invasive brain stimulation studies: reasons and results
Non-invasive brain stimulation techniques (NIBS), such as Theta Burst Stimulation (TBS), Paired Associative Stimulation (PAS) and transcranial Direct Current Stimulation (tDCS), are widely used to probe plasticity in the human motor cortex (M1). Although TBS, PAS and tDCS differ in terms of physiological mechanisms responsible for experimentally-induced cortical plasticity, they all share the ability to elicit long-term potentiation (LTP) and depression (LTD) in M1. However, NIBS techniques are all affected by relevant variability in intra- and inter-subject responses. A growing number of factors contributing to NIBS variability have been recently identified and reported. In this review, we have readdressed the issue of variability in human NIBS studies. We have first briefly discussed the physiological mechanisms responsible for TBS, PAS and tDCS-induced cortical plasticity. Then, we have provided statistical measures of intra- and inter-subject variability, as calculated in previous studies. Finally, we have reported in detail known sources of variability by categorizing them into physiological, technical and statistical factors. Improving knowledge about sources of variability could lead to relevant advances in designing new tailored NIBS protocols in physiological and pathological conditions
On quantum and relativistic mechanical analogues in mean field spin models
Conceptual analogies among statistical mechanics and classical (or quantum)
mechanics often appeared in the literature. For classical two-body mean field
models, an analogy develops into a proper identification between the free
energy of Curie-Weiss type magnetic models and the Hamilton-Jacobi action for a
one dimensional mechanical system. Similarly, the partition function plays the
role of the wave function in quantum mechanics and satisfies the heat equation
that plays, in this context, the role of the Schrodinger equation in quantum
mechanics. We show that this identification can be remarkably extended to
include a wide family of magnetic models classified by normal forms of suitable
real algebraic dispersion curves. In all these cases, the model turns out to be
completely solvable as the free energy as well as the order parameter are
obtained as solutions of an integrable nonlinear PDE of Hamilton-Jacobi type.
We observe that the mechanical analog of these models can be viewed as the
relativistic analog of the Curie-Weiss model and this helps to clarify the
connection between generalised self-averaging and in statistical thermodynamics
and the semi-classical dynamics of viscous conservation laws.Comment: Dedicated to Sandro Graffi in honor of his seventieth birthda
La palabra Rafue como imaginario para una poética de los mitos uitotos
Rafue es entendido como palabra-acción y de sabiduría y es la base de cuanto existe en la cultura uitoto. En este estudio se explica la importancia que tiene rafue dentro de la comunidad y en la creación de su imaginario. También se ilustra del papel que cumple dentro de la mitología y su relevancia en la conformación de la simbología y de la estructura de los mitos, estableciendo una base para una poética de los mitos uitotos. Se explica la relación que existe entre esta poética y el imaginario, teniendo a rafue como base y elemento común. Adicional a esto, se establece un puente entre la cultura occidental y la uitoto, teniendo a la Palabra y a rafue como base de este. Esta relación se hace sobre todo en el plano del sistema de creencias uitotos y católico-cristianas.Rafue is understood as word of action and wisdom and is the basis of all of what's in the uitoto culture. In this study, the importance of rafue within the community and the creation of its imaginary are explained. It also illustrates the role within the mythology and its relevance in the constitution of the symbolism and structure of myths, establishing a basis for a poetics of uitoto myths. The relationship between this poetics and imaginary, having rafue as a base and common element is explained. Additional to this, a bridge is established between Western and uitoto cultures, taking Word and rafue as the basis of this. This relationship is mostly about uitoto and Christian beliefs.Profesional en Estudios LiterariosPregrad
Graphene on h-BN: to align or not to align?
The contact strength, adhesion and friction, between graphene and an
incommensurate crystalline substrate such as {\it h}-BN depends on their
relative alignment angle . The well established Novaco-McTague (NM)
theory predicts for a monolayer graphene on a hard bulk {\it h}-BN crystal face
a small spontaneous misalignment, here \,\,0.45 degrees
which if realized would be relevant to a host of electronic properties besides
the mechanical ones. Because experimental equilibrium is hard to achieve, we
inquire theoretically about alignment or misalignment by simulations based on
dependable state-of-the-art interatomic force fields. Surprisingly at first, we
find compelling evidence for , i.e., full energy-driven alignment
in the equilibrium state of graphene on {\it h}-BN. Two factors drive this
deviation from NM theory. First, graphene is not flat, developing on {\it h}-BN
a long-wavelength out-of-plane corrugation. Second, {\it h}-BN is not hard,
releasing its contact stress by planar contractions/expansions that accompany
the interface moir\'e structure. Repeated simulations by artificially forcing
graphene to keep flat, and {\it h}-BN to keep rigid, indeed yield an
equilibrium misalignment similar to as expected. Subsequent
sliding simulations show that friction of graphene on {\it h}-BN, small and
essentially independent of misalignments in the artificial frozen state,
strongly increases in the more realistic corrugated, strain-modulated, aligned
state
Meta-stable states in the hierarchical Dyson model drive parallel processing in the hierarchical Hopfield network
In this paper we introduce and investigate the statistical mechanics of
hierarchical neural networks: First, we approach these systems \`a la Mattis,
by thinking at the Dyson model as a single-pattern hierarchical neural network
and we discuss the stability of different retrievable states as predicted by
the related self-consistencies obtained from a mean-field bound and from a
bound that bypasses the mean-field limitation. The latter is worked out by
properly reabsorbing fluctuations of the magnetization related to higher levels
of the hierarchy into effective fields for the lower levels. Remarkably, mixing
Amit's ansatz technique (to select candidate retrievable states) with the
interpolation procedure (to solve for the free energy of these states) we prove
that (due to gauge symmetry) the Dyson model accomplishes both serial and
parallel processing. One step forward, we extend this scenario toward multiple
stored patterns by implementing the Hebb prescription for learning within the
couplings. This results in an Hopfield-like networks constrained on a
hierarchical topology, for which, restricting to the low storage regime (where
the number of patterns grows at most logarithmical with the amount of neurons),
we prove the existence of the thermodynamic limit for the free energy and we
give an explicit expression of its mean field bound and of the related improved
boun
Topological properties of hierarchical networks
Hierarchical networks are attracting a renewal interest for modelling the
organization of a number of biological systems and for tackling the complexity
of statistical mechanical models beyond mean-field limitations. Here we
consider the Dyson hierarchical construction for ferromagnets, neural networks
and spin-glasses, recently analyzed from a statistical-mechanics perspective,
and we focus on the topological properties of the underlying structures. In
particular, we find that such structures are weighted graphs that exhibit high
degree of clustering and of modularity, with small spectral gap; the robustness
of such features with respect to link removal is also studied. These outcomes
are then discussed and related to the statistical mechanics scenario in full
consistency. Lastly, we look at these weighted graphs as Markov chains and we
show that in the limit of infinite size, the emergence of ergodicity breakdown
for the stochastic process mirrors the emergence of meta-stabilities in the
corresponding statistical mechanical analysis
Hierarchical neural networks perform both serial and parallel processing
In this work we study a Hebbian neural network, where neurons are arranged
according to a hierarchical architecture such that their couplings scale with
their reciprocal distance. As a full statistical mechanics solution is not yet
available, after a streamlined introduction to the state of the art via that
route, the problem is consistently approached through signal- to-noise
technique and extensive numerical simulations. Focusing on the low-storage
regime, where the amount of stored patterns grows at most logarithmical with
the system size, we prove that these non-mean-field Hopfield-like networks
display a richer phase diagram than their classical counterparts. In
particular, these networks are able to perform serial processing (i.e. retrieve
one pattern at a time through a complete rearrangement of the whole ensemble of
neurons) as well as parallel processing (i.e. retrieve several patterns
simultaneously, delegating the management of diff erent patterns to diverse
communities that build network). The tune between the two regimes is given by
the rate of the coupling decay and by the level of noise affecting the system.
The price to pay for those remarkable capabilities lies in a network's capacity
smaller than the mean field counterpart, thus yielding a new budget principle:
the wider the multitasking capabilities, the lower the network load and
viceversa. This may have important implications in our understanding of
biological complexity
From Dyson to Hopfield: Processing on hierarchical networks
We consider statistical-mechanical models for spin systems built on
hierarchical structures, which provide a simple example of non-mean-field
framework. We show that the coupling decay with spin distance can give rise to
peculiar features and phase diagrams much richer that their mean-field
counterpart. In particular, we consider the Dyson model, mimicking
ferromagnetism in lattices, and we prove the existence of a number of
meta-stabilities, beyond the ordered state, which get stable in the
thermodynamic limit. Such a feature is retained when the hierarchical structure
is coupled with the Hebb rule for learning, hence mimicking the modular
architecture of neurons, and gives rise to an associative network able to
perform both as a serial processor as well as a parallel processor, depending
crucially on the external stimuli and on the rate of interaction decay with
distance; however, those emergent multitasking features reduce the network
capacity with respect to the mean-field counterpart. The analysis is
accomplished through statistical mechanics, graph theory, signal-to-noise
technique and numerical simulations in full consistency. Our results shed light
on the biological complexity shown by real networks, and suggest future
directions for understanding more realistic models
Preventive strategies in oral health for special needs patients
As regards to the most common oral disease in
pediatric patients, intellectual disability is not a
risk factor for caries disease itself, but it rather
reduces the individual capability to self-care and
therefore to his own oral care. Children suffering
of systemic pathologies and/or with different
stages of disability are to be considered at high
risk for dental caries development. According to
recent guidelines for oral health prevention in
childhood, individual additional strategies for a
preventive care should be applied for these patients.
All the health providers, family and caregivers
should be involved with the aim of being
aware, motivated and informed on oral health issues,
and a better access system to the dental
care structure, both logistic, professional and
economical should be assured
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