arXiv subjects

S. Panzeri

Publications and source records attributed to S. Panzeri.

10 recordsLinked to original sources

arXiv2000

A unified approach to the study of temporal, correlational and rate coding

We demonstrate that the information contained in the spike occurrence times of a population of neurons can be broken up into a series of terms, each of which reflect something about potential coding mechanisms. This is possible in the coding r{é}gime in which few spikes are emitted in the relevant time window. This approach allows us to study the additional information contributed by spike timing beyond that present in the spike counts; to examine the contributions to the whole information of different statistical properties of spike trains, such as firing rates and correlation functions; and forms the basis for a new quantitative procedure for the analysis of simultaneous multiple neuron recordings. It also provides theoretical constraints upon neural coding strategies. We find a transition between two coding r{é}gimes, depending upon the size of the relevant observation timescale. For time windows shorter than the timescale of the stimulus-induced response fluctuations, there exists a spike count coding phase, where the purely temporal information is of third order in time. For time windows much longer than the characteristic timescale, there can be additional timing information of first order, leading to a temporal coding phase in which timing information may affect the instantaneous information rate. We study the relative contributions of the dynamic firing rate and correlation variables to the full temporal information; the interaction of signal and noise correlations in temporal coding; synergy between spikes and between cells; and the effect of refractoriness. We illustrate the utility of the technique by analysis of a few cells from the rat barrel cortex.

physics.bio-ph↗

arXiv1996

Finite Temperature Lattice QCD in the Large N Limit

Our aim is to give a self-contained review of recent advances in the analytic description of the deconfinement transition and determination of the deconfinement temperature in lattice QCD at large N. We also include some new results, as for instance in the comparison of the analytic results with Montecarlo simulations. We first review the general set-up of finite temperature lattice gauge theories, using asymmetric lattices, and develop a consistent perturbative expansion in the coupling $β_s$ of the space-like plaquettes. We study in detail the effective models for the Polyakov loop obtained, in the zeroth order approximation in $β_s$, both from the Wilson action (symmetric lattice) and from the heat kernel action (completely asymmetric lattice). The distinctive feature of the heat kernel model is its relation with two-dimensional QCD on a cylinder; the Wilson model, on the other hand, can be exactly reduced to a twisted one-plaquette model via a procedure of the Eguchi-Kawai type. In the weak coupling regime both models can be related to exactly solvable Kazakov-Migdal matrix models. The instability of the weak coupling solution is due in both cases to a condensation of instantons; in the heat kernel case, it is directly related to the Douglas-Kazakov transition of QCD2. A detailed analysis of these results provides rather accurate predictions of the deconfinement temperature. In spite of the zeroth order approximation they are in good agreement with the Montecarlo simulations in 2+1 dimensions, while in 3+1 dimensions they only agree with the Montecarlo results away from the continuum limit.

hep-th↗

arXiv1996

Effective actions for finite temperature Lattice Gauge Theories

We consider a lattice gauge theory at finite temperature in ($d$+1) dimensions with the Wilson action and different couplings $β_t$ and $β_s$ for timelike and spacelike plaquettes. By using the character expansion and Schwinger-Dyson type equations we construct, order by order in $β_s$, an effective action for the Polyakov loops which is exact to all orders in $β_t$. As an example we construct the first non-trivial order in $β_s$ for the (3+1) dimensional SU(2) model and use this effective action to extract the deconfinement temperature of the model.

hep-lat↗

arXiv1996

Toward an analytic determination of the deconfinement temperature in SU(2) L.G.T.

We consider the SU(2) lattice gauge theory at finite temperature in (d+1) dimensions, with different couplings $β_t$ and $β_s$ for timelike and spacelike plaquettes. By using the character expansion of the Wilson action and performing the integrals over space-like link variables, we find an effective action for the Polyakov loops which is exact to all orders in $β_t$ and to the first non-trivial order in $β_s$. The critical coupling for the deconfinement transition is determined in the (3+1) dimensional case, by the mean field method, for different values of the lattice size $N_t$ in the compactified time direction and of the asymmetry parameter $ρ= \sqrt{β_t/β_s}$. We find good agreement with Montecarlo simulations in the range $1\leq N_t \leq 5$, and good qualitative agreement in the same range with the logarithmic scaling law of QCD. Moreover the dependence of the results from the parameter $ρ$ is in excellent agreement with previous theoretical predictions.

hep-lat↗

arXiv1994

Deconfinement Transition in Large N Lattice Gauge Theory

We study analytically the phase diagram of the pure $SU(N)$ lattice gauge theory at finite temperature, and we attempt to estimate the critical deconfinement temperature. We apply large $N$ techniques to the Wilson and to the Heat Kernel action, and we study the resulting models both in the strong coupling and in the weak coupling limits. Using the Heat Kernel action, we establish an interesting connection between the Douglas-Kazakov phase transition of two-dimensional QCD and the deconfining transition in $d$ dimensions. The analytic results obtained for the critical temperature compare well with Montecarlo simulations of the full theory in $(2+1)$ and in $(3+1)$ dimensions.

hep-lat↗

arXiv1993

Two-dimensional QCD on the sphere and on the cylinder

The partition functions of QCD2 on simple surfaces admit representations in terms of exponentials of the inverse coupling, that are modular transforms of the usual character expansions. We review the construction of such a representation in the case of the cylinder, and show how it leads to a formulation of QCD2 as a $c=1$ matrix model of the Kazakov-Migdal type. The eigenvalues describe the positions of $N$ Sutherland fermions on a circle, while their discretized momenta label the representations in the corresponding character expansion. Using this language, we derive some new results: we give an alternative description of the Douglas-Kazakov phase transition on the sphere, and we argue that an analogous phase transition exists on the cylinder. We calculate the large $N$ limit of the partition function on the cylinder with boundary conditions given by semicircular distributions of eigenvalues, and we find an explicit expression for the large $N$ limit of the Itzykson-Zuber integral with the same boundary conditions. (Talk given at the ``Workshop on high energy physics and cosmology'' at Trieste, July 1993.)

hep-th↗

arXiv1993

Two dimensional QCD is a one dimensional Kazakov-Migdal model

We calculate the partition functions of QCD in two dimensions on a cylinder and on a torus in the gauge $\partial_{0} A_{0} = 0$ by integrating explicitly over the non zero modes of the Fourier expansion in the periodic time variable. The result is a one dimensional Kazakov-Migdal matrix model with eigenvalues on a circle rather than on a line. We prove that our result coincides with the standard expansion in representations of the gauge group. This involves a non trivial modular transformation from an expansion in exponentials of $g^2$ to one in exponentials of $1/g^2$. Finally we argue that the states of the $U(N)$ or $SU(N)$ partition function can be interpreted as a gas of N free fermions, and the grand canonical partition function of such ensemble is given explicitly as an infinite product.

hep-th↗

arXiv1992

The Kazakov-Migdal Model as a High Temperature Lattice Gauge Theory

We show that the Kazakov-Migdal (K-M) induced gauge model in $d$ dimensions describes the high temperature limit of ordinary lattice gauge theories in $d+1$ dimensions. The matter fields are related to the Polyakov loops, while the spatial gauge variables become the gauge fields of the K-M model. This interpretation of the K-M model is in agreement with some recent results in high temperature lattice QCD.

hep-th↗

arXiv1992

Kazakov-Migdal Induced Gauge Theory and the Coupling of 2D Quantum Gravity to D=1 Matter

Recently Kazakov and Migdal proposed a new approach to the large $N$ limit of SU(N) gauge theories which could hopefully describe the asymptotically free fixed point of QCD in 4 dimensions. In this contribution we review the exact solution of their model in the case of a $d=1$ compactified lattice and its connection with the partition function of the vortex-free sector of the d=1 compactified bosonic string. Some new results on the addition of a linear term in the K-M model and on the SU(N) restriction of the general U(N) model are also presented.

hep-lat↗

arXiv1992

Exact solution of d=1 Kazakov-Migdal induced gauge theory

We give the exact solution of the Kazakov-Migdal induced gauge model in the case of a D=1 compactified lattice with a generic number $S$ of sites and for any value of N. Due to the peculiar features of the model, the partition function that we obtain also describes the vortex-free sector of the D=1 compactified bosonic string, and it coincides in the continuum limit with the one obtained by Boulatov and Kazakov in this context.

hep-th↗