80 research outputs found
Two interacting particles in a random potential
We study the scaling of the localization length of two interacting particles
in a one-dimensional random lattice with the single particle localization
length. We obtain several regimes, among them one interesting weak Fock space
disorder regime. In this regime we derive a weak logarithmic scaling law.
Numerical data support the absence of any strong enhancement of the two
particle localization length
Automodulations in Kerr-lens Modelocked Solid-State Lasers
Nonstationary pulse regimes associated with self modulation of a Kerr-lens
modelocked Ti:sapphire laser have been studied experimentally and
theoretically. Such laser regimes occur at an intracavity group delay
dispersion that is smaller or larger than what is required for stable
modelocking and exhibit modulation in pulse amplitude and spectra at
frequencies of several hundred kHz. Stabilization of such modulations, leading
to an increase in the pulse peak power by a factor of ten, were accomplished by
weakly modulating the pump laser with the self-modulation frequency. The main
experimental observations can be explained with a round trip model of the fs
laser taking into account gain saturation, Kerr lensing, and second- and
third-order dispersion.Comment: 21 pages, 9 Postscript figures, TeX, the calculations are presented
on http://www.geocities.com/optomaple
Granular Solid Hydrodynamics
Granular elasticity, an elasticity theory useful for calculating static
stress distribution in granular media, is generalized to the dynamic case by
including the plastic contribution of the strain. A complete hydrodynamic
theory is derived based on the hypothesis that granular medium turns
transiently elastic when deformed. This theory includes both the true and the
granular temperatures, and employs a free energy expression that encapsulates a
full jamming phase diagram, in the space spanned by pressure, shear stress,
density and granular temperature. For the special case of stationary granular
temperatures, the derived hydrodynamic theory reduces to {\em hypoplasticity},
a state-of-the-art engineering model.Comment: 42 pages 3 fi
An Integrated Micro- and Macroarchitectural Analysis of the Drosophila Brain by Computer-Assisted Serial Section Electron Microscopy
A new software package allows for dense electron microscopy reconstructions of neuronal networks in the fruit fly brain, and reveals specific differences in microcircuits between insects and vertebrates
Dopamine, affordance and active inference.
The role of dopamine in behaviour and decision-making is often cast in terms of reinforcement learning and optimal decision theory. Here, we present an alternative view that frames the physiology of dopamine in terms of Bayes-optimal behaviour. In this account, dopamine controls the precision or salience of (external or internal) cues that engender action. In other words, dopamine balances bottom-up sensory information and top-down prior beliefs when making hierarchical inferences (predictions) about cues that have affordance. In this paper, we focus on the consequences of changing tonic levels of dopamine firing using simulations of cued sequential movements. Crucially, the predictions driving movements are based upon a hierarchical generative model that infers the context in which movements are made. This means that we can confuse agents by changing the context (order) in which cues are presented. These simulations provide a (Bayes-optimal) model of contextual uncertainty and set switching that can be quantified in terms of behavioural and electrophysiological responses. Furthermore, one can simulate dopaminergic lesions (by changing the precision of prediction errors) to produce pathological behaviours that are reminiscent of those seen in neurological disorders such as Parkinson's disease. We use these simulations to demonstrate how a single functional role for dopamine at the synaptic level can manifest in different ways at the behavioural level
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