422 research outputs found
Determination of the weak Hamiltonian in the SU(4) chiral limit through topological zero-mode wave functions
A new method to determine the low-energy couplings of the weak
Hamiltonian is presented. It relies on a matching of the topological poles in
of three-point correlators of two pseudoscalar densities and a
four-fermion operator, measured in lattice QCD, to the same observables
computed in the -regime of chiral perturbation theory. We test this
method in a theory with a light charm quark, i.e. with an SU(4) flavour
symmetry. Quenched numerical measurements are performed in a 2 fm box, and
chiral perturbation theory predictions are worked out up to next-to-leading
order. The matching of the two sides allows to determine the weak low-energy
couplings in the SU(4) limit. We compare the results with a previous
determination, based on three-point correlators containing two left-handed
currents, and discuss the merits and drawbacks of the two procedures.Comment: 38 pages, 9 figure
Weak low-energy couplings from topological zero-mode wavefunctions
We discuss a new method to determine the low-energy couplings of the weak Hamiltonian in the -regime. It relies on a matching of the
topological poles in of three-point functions of two pseudoscalar
densities and a four-fermion operator computed in lattice QCD, to the same
observables in the Chiral Effective Theory. We present the results of a NLO
computation in chiral perturbation theory of these correlation functions
together with some preliminary numerical results.Comment: 7 pages. Contribution to Lattice 200
K-->pipi amplitudes from lattice QCD with a light charm quark
We compute the leading-order low-energy constants of the DeltaS=1 effective
weak Hamiltonian in the quenched approximation of QCD with up, down, strange,
and charm quarks degenerate and light. They are extracted by comparing the
predictions of finite volume chiral perturbation theory with lattice QCD
computations of suitable correlation functions carried out with quark masses
ranging from a few MeV up to half of the physical strange mass. We observe a
large DeltaI=1/2 enhancement in this corner of the parameter space of the
theory. Although matching with the experimental result is not observed for the
DeltaI=1/2 amplitude, our computation suggests large QCD contributions to the
physical DeltaI=1/2 rule in the GIM limit, and represents the first step to
quantify the role of the charm quark-mass in K-->pipi amplitudes.Comment: 4 pages, 1 figure. Minor modifications. Final version to appear on
PR
Recruitment of visual cortex for language processing in blind individuals: A neurobiological model
After sensory deprivation, the visual cortex is functionally recruited into non-visual cognitive language and semantic processing. Why this functional organization takes place and how its underlying mechanisms work at the neuronal circuit level is still unclear. Here, we use a biologically constrained network model implementing anatomical structure, neurophysiological function and connectivity of the fronto-tempo-occipital cortex to simulate word-meaning acquisition in visually deprived and undeprived (âhealthy controlâ) brains. Whereas in the âundeprivedâ simulations only words denoting visual entities grew into the visual domain, the âblindâ models unexpectedly produced word-related neuronal circuits extending into visual cortex for all semantic categories (and especially for those carrying action-related meaning). Additionally, during word recognition, the blind model showed long-lasting spiking neural activity compared to the sighted model, a sign for enhanced verbal working memory due to the additional neural recruitment. Three factors are crucial for explaining this deprivation-related growth: (i) changes in the networkâs activity balance brought about by the absence of uncorrelated sensory input, (ii) the connectivity structure of the network, and (iii) Hebbian correlation learning. By offering a neurobiological account for neural changes of language processing due to visual deprivation, our model bridges the gap between cellular-level mechanisms and system-level language function in blind humans
A robust sound perception model suitable for neuromorphic implementation
Coath M, Sheik S, Chicca E, Indiveri G, Denham S, Wennekers T. A robust sound perception model suitable for neuromorphic implementation. Neuromorphic Engineering. 2014;7(278):1-10.We have recently demonstrated the emergence of dynamic feature sensitivity through exposure to formative stimuli in a real-time neuromorphic system implementing a hybrid analog/digital network of spiking neurons. This network, inspired by models of auditory processing in mammals, includes several mutually connected layers with distance-dependent transmission delays and learning in the form of spike timing dependent plasticity, which effects stimulus-driven changes in the network connectivity. Here we present results that demonstrate that the network is robust to a range of variations in the stimulus pattern, such as are found in naturalistic stimuli and neural responses. This robustness is a property critical to the development of realistic, electronic neuromorphic systems. We analyze the variability of the response of the network to ânoisyâ stimuli which allows us to characterize the acuity in information-theoretic terms. This provides an objective basis for the quantitative comparison of networks, their connectivity patterns, and learning strategies, which can inform future design decisions. We also show, using stimuli derived from speech samples, that the principles are robust to other challenges, such as variable presentation rate, that would have to be met by systems deployed in the real world. Finally we demonstrate the potential applicability of the approach to real sounds
On the determination of low-energy constants for transitions
We present our preliminary results for three-point correlation functions
involving the operators entering the effective Hamiltonian with
an active charm quark, obtained using overlap fermions in the quenched
approximation. This is the first computation carried out for valence quark
masses small enough so as to permit a matching to Quenched Chiral Perturbation
Theory in the -regime. The commonly observed large statistical
fluctuations are tamed by means of low-mode averaging techniques, combined with
restrictions to individual topological sectors. We also discuss the matching of
the resulting hadronic matrix elements to the effective low-energy constants
for transitions. This involves (a) finite-volume corrections
which can be evaluated at NLO in Quenched Chiral Perturbation Theory, and (b)
the short-distance renormalization of the relevant four-quark operators in
discretizations based on the overlap operator. We discuss perturbative
estimates for the renormalization factors and possible strategies for their
non-perturbative evaluation. Our results can be used to isolate the
long-distance contributions to the rule, coming from physics
effects around the intrinsic QCD scale.Comment: 11 pages, 2 figures, talks presented at Lattice 2005 (Weak matrix
elements
Correlation functions at small quark masses with overlap fermions
We report on recent work on the determination of low-energy constants
describing Delta{S}=1 weak transitions, in order to investigate the origins of
the Delta{I}=1/2 rule. We focus on numerical techniques designed to enhance the
statistical signal in three-point correlation functions computed with overlap
fermions near the chiral limit.Comment: Talk presented at Lattice2004(weak), Fermilab, 21-26 June 2004, 3
pages, 2 figure
Thinking in circuits: toward neurobiological explanation in cognitive neuroscience
Cognitive theory has decomposed human mental abilities into cognitive (sub) systems, and cognitive neuroscience succeeded in disclosing a host of relationships between cognitive systems and specific structures of the human brain. However, an explanation of why specific functions are located in specific brain loci had still been missing, along with a neurobiological model that makes concrete the neuronal circuits that carry thoughts and meaning. Brain theory, in particular the Hebb-inspired neurocybernetic proposals by Braitenberg, now offers an avenue toward explaining brainâmind relationships and to spell out cognition in terms of neuron circuits in a neuromechanistic sense. Central to this endeavor is the theoretical construct of an elementary functional neuronal unit above the level of individual neurons and below that of whole brain areas and systems: the distributed neuronal assembly (DNA) or thought circuit (TC). It is shown that DNA/TC theory of cognition offers an integrated explanatory perspective on brain mechanisms of perception, action, language, attention, memory, decision and conceptual thought. We argue that DNAs carry all of these functions and that their inner structure (e.g., core and halo subcomponents), and their functional activation dynamics (e.g., ignition and reverberation processes) answer crucial localist questions, such as why memory and decisions draw on prefrontal areas although memory formation is normally driven by information in the senses and in the motor system. We suggest that the ability of building DNAs/TCs spread out over different cortical areas is the key mechanism for a range of specifically human sensorimotor, linguistic and conceptual capacities and that the cell assembly mechanism of overlap reduction is crucial for differentiating a vocabulary of actions, symbols and concepts
Evolution of network structure by temporal learning
We study the effect of learning dynamics on network topology. A network of
discrete dynamical systems is considered for this purpose and the coupling
strengths are made to evolve according to a temporal learning rule that is
based on the paradigm of spike-time-dependent plasticity. This incorporates
necessary competition between different edges. The final network we obtain is
robust and has a broad degree distribution.Comment: revised manuscript in communicatio
Visual cortex recruitment during language processing in blind individuals is explained by Hebbian learning
In blind people, the visual cortex takes on higher cognitive functions, including language. Why this functional organisation mechanistically emerges at the neuronal circuit level is still unclear. Here, we use a biologically constrained network model implementing features of anatomical structure, neurophysiological function and connectivity of fronto-temporal-occipital areas to simulate word-meaning acquisition in visually deprived and undeprived brains. We observed that, only under visual deprivation, distributed word-related neural circuits âgrew intoâ the deprived visual areas, which therefore adopted a linguistic-semantic role. Three factors are crucial for explaining this deprivation-related growth: changes in the networkâs activity balance brought about by the absence of uncorrelated sensory input, the connectivity structure of the network, and Hebbian correlation learning. In addition, the blind model revealed long-lasting spiking neural activity compared to the sighted model during word recognition, which is a neural correlate of enhanced verbal working memory. The present neurocomputational model offers a neurobiological account for neural changes followed by sensory deprivation, thus closing the gap between cellular-level mechanisms, system-level linguistic and semantic function
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