21,073 research outputs found
Neural Sampling by Irregular Gating Inhibition of Spiking Neurons and Attractor Networks
A long tradition in theoretical neuroscience casts sensory processing in the
brain as the process of inferring the maximally consistent interpretations of
imperfect sensory input. Recently it has been shown that Gamma-band inhibition
can enable neural attractor networks to approximately carry out such a sampling
mechanism. In this paper we propose a novel neural network model based on
irregular gating inhibition, show analytically how it implements a Monte-Carlo
Markov Chain (MCMC) sampler, and describe how it can be used to model networks
of both neural attractors as well as of single spiking neurons. Finally we show
how this model applied to spiking neurons gives rise to a new putative
mechanism that could be used to implement stochastic synaptic weights in
biological neural networks and in neuromorphic hardware
All-optical switching of photonic entanglement
Future quantum optical networks will require the ability to route entangled
photons at high speeds, with minimal loss and added in-band noise, and---most
importantly---without disturbing the photons' quantum state. Here we present an
all-optical switch which fulfills these requirements and characterize its
performance at the single photon level. It exhibits a 200-ps switching window,
120:1 contrast, 1.5-dB loss, and induces no measurable degradation in the
switched photons' entangled-state fidelity (< 0.002). As a proof-of-principle
demonstration of its capability, we use the switch to demultiplex a single
quantum channel from a dual-channel, time-division-multiplexed entangled photon
stream. Furthermore, because this type of switch couples the temporal and
spatial degrees of freedom, it provides an important new tool with which to
encode multiple-qubit quantum states on a single photon
Characterizing Pixel and Point Patterns with a Hyperuniformity Disorder Length
We introduce the concept of a hyperuniformity disorder length that controls
the variance of volume fraction fluctuations for randomly placed windows of
fixed size. In particular, fluctuations are determined by the average number of
particles within a distance from the boundary of the window. We first
compute special expectations and bounds in dimensions, and then illustrate
the range of behavior of versus window size by analyzing three
different types of simulated two-dimensional pixel pattern - where particle
positions are stored as a binary digital image in which pixels have value
zero/one if empty/contain a particle. The first are random binomial patterns,
where pixels are randomly flipped from zero to one with probability equal to
area fraction. These have long-ranged density fluctuations, and simulations
confirm the exact result . Next we consider vacancy patterns, where a
fraction of particles on a lattice are randomly removed. These also display
long-range density fluctuations, but with for small . For a
hyperuniform system with no long-range density fluctuations, we consider
Einstein patterns where each particle is independently displaced from a lattice
site by a Gaussian-distributed amount. For these, at large , approaches
a constant equal to about half the root-mean-square displacement in each
dimension. Then we turn to grayscale pixel patterns that represent simulated
arrangements of polydisperse particles, where the volume of a particle is
encoded in the value of its central pixel. And we discuss the continuum limit
of point patterns, where pixel size vanishes. In general, we thus propose to
quantify particle configurations not just by the scaling of the density
fluctuation spectrum but rather by the real-space spectrum of versus
. We call this approach Hyperuniformity Disorder Length Spectroscopy
Enhancement of synchronization in a hybrid neural circuit by spike timing dependent plasticity
Synchronization of neural activity is fundamental for many functions of the brain. We demonstrate that spike-timing dependent plasticity (STDP) enhances synchronization (entrainment) in a hybrid circuit composed of a spike generator, a dynamic clamp emulating an excitatory plastic synapse, and a chemically isolated neuron from the Aplysia abdominal ganglion. Fixed-phase entrainment of the Aplysia neuron to the spike generator is possible for a much wider range of frequency ratios and is more precise and more robust with the plastic synapse than with a nonplastic synapse of comparable strength. Further analysis in a computational model of HodgkinHuxley-type neurons reveals the mechanism behind this significant enhancement in synchronization. The experimentally observed STDP plasticity curve appears to be designed to adjust synaptic strength to a value suitable for stable entrainment of the postsynaptic neuron. One functional role of STDP might therefore be to facilitate synchronization or entrainment of nonidentical neurons
All-Pole Recursive Digital Filters Design Based on Ultraspherical Polynomials
A simple method for approximation of all-pole recursive digital filters, directly in digital domain, is described. Transfer function of these filters, referred to as Ultraspherical filters, is controlled by order of the Ultraspherical polynomial, nu. Parameter nu, restricted to be a nonnegative real number (nu ≥ 0), controls ripple peaks in the passband of the magnitude response and enables a trade-off between the passband loss and the group delay response of the resulting filter. Chebyshev filters of the first and of the second kind, and also Legendre and Butterworth filters are shown to be special cases of these allpole recursive digital filters. Closed form equations for the computation of the filter coefficients are provided. The design technique is illustrated with examples
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