389 research outputs found
Stimulus competition by inhibitory interference
When two stimuli are present in the receptive field of a V4 neuron, the
firing rate response is between the weakest and strongest response elicited by
each of the stimuli alone (Reynolds et al, 1999, Journal of Neuroscience
19:1736-1753). When attention is directed towards the stimulus eliciting the
strongest response (the preferred stimulus), the response to the pair is
increased, whereas the response decreases when attention is directed to the
other stimulus (the poor stimulus). These experimental results were reproduced
in a model of a V4 neuron under the assumption that attention modulates the
activity of local interneuron networks. The V4 model neuron received
stimulus-specific asynchronous excitation from V2 and synchronous inhibitory
inputs from two local interneuron networks in V4. Each interneuron network was
driven by stimulus-specific excitatory inputs from V2 and was modulated by a
projection from the frontal eye fields. Stimulus competition was present
because of a delay in arrival time of synchronous volleys from each interneuron
network. For small delays, the firing rate was close to the rate elicited by
the preferred stimulus alone, whereas for larger delays it approached the
firing rate of the poor stimulus. When either stimulus was presented alone the
neuron's response was not altered by the change in delay. The model suggests
that top-down attention biases the competition between V2 columns for control
of V4 neurons by changing the relative timing of inhibition rather than by
changes in the degree of synchrony of interneuron networks. The mechanism
proposed here for attentional modulation of firing rate - gain modulation by
inhibitory interference - is likely to have more general applicability to
cortical information processing.Comment: 20 pages, 7 figures, 1 tabl
Anisotropy induced Feshbach resonances in a quantum dipolar gas of magnetic atoms
We explore the anisotropic nature of Feshbach resonances in the collision
between ultracold magnetic submerged-shell dysprosium atoms, which can only
occur due to couplings to rotating bound states. This is in contrast to
well-studied alkali-metal atom collisions, where most Feshbach resonances are
hyperfine induced and due to rotation-less bound states. Our novel
first-principle coupled-channel calculation of the collisions between
open-4f-shell spin-polarized bosonic dysprosium reveals a striking correlation
between the anisotropy due to magnetic dipole-dipole and electrostatic
interactions and the Feshbach spectrum as a function of an external magnetic
field. Over a 20 mT magnetic field range we predict about a dozen Feshbach
resonances and show that the resonance locations are exquisitely sensitive to
the dysprosium isotope.Comment: 5 pages, 4 figure
Feshbach spectroscopy and analysis of the interaction potentials of ultracold sodium
We have studied magnetic Feshbach resonances in an ultracold sample of Na
prepared in the absolute hyperfine ground state. We report on the observation
of three s-, eight d-, and three g-wave Feshbach resonances, including a more
precise determination of two known s-wave resonances, and one s-wave resonance
at a magnetic field exceeding 200mT. Using a coupled-channels calculation we
have improved the sodium ground-state potentials by taking into account these
new experimental data, and derived values for the scattering lengths. In
addition, a description of the molecular states leading to the Feshbach
resonances in terms of the asymptotic-bound-state model is presented.Comment: 11 pages, 4 figure
Collisional cooling of ultra-cold atom ensembles using Feshbach resonances
We propose a new type of cooling mechanism for ultra-cold fermionic atom
ensembles, which capitalizes on the energy dependence of inelastic collisions
in the presence of a Feshbach resonance. We first discuss the case of a single
magnetic resonance, and find that the final temperature and the cooling rate is
limited by the width of the resonance. A concrete example, based on a p-wave
resonance of K, is given. We then improve upon this setup by using both
a very sharp optical or radio-frequency induced resonance and a very broad
magnetic resonance and show that one can improve upon temperatures reached with
current technologies.Comment: 4 pages, 3 figure
Mechanisms for Phase Shifting in Cortical Networks and their Role in Communication through Coherence
In the primate visual cortex, the phase of spikes relative to oscillations in the local field potential (LFP) in the gamma frequency range (30–80 Hz) can be shifted by stimulus features such as orientation and thus the phase may carry information about stimulus identity. According to the principle of communication through coherence (CTC), the relative LFP phase between the LFPs in the sending and receiving circuits affects the effectiveness of the transmission. CTC predicts that phase shifting can be used for stimulus selection. We review and investigate phase shifting in models of periodically driven single neurons and compare it with phase shifting in models of cortical networks. In a single neuron, as the driving current is increased, the spike phase varies systematically while the firing rate remains constant. In a network model of reciprocally connected excitatory (E) and inhibitory (I) cells phase shifting occurs in response to both injection of constant depolarizing currents and to brief pulses to I cells. These simple models provide an account for phase-shifting observed experimentally and suggest a mechanism for implementing CTC. We discuss how this hypothesis can be tested experimentally using optogenetic techniques
Multichannel quantum-defect theory for slow atomic collisions
We present a multichannel quantum-defect theory for slow atomic collisions
that takes advantages of the analytic solutions for the long-range potential,
and both the energy and the angular-momentum insensitivities of the short-range
parameters. The theory provides an accurate and complete account of scattering
processes, including shape and Feshbach resonances, in terms of a few
parameters such as the singlet and the triplet scattering lengths. As an
example, results for Na-Na scattering are presented and compared
close-coupling calculations.Comment: 8 pages, 3 figure
Comparison of numerical methods for the calculation of cold atom collisions
Three different numerical techniques for solving a coupled channel
Schroedinger equation are compared. This benchmark equation, which describes
the collision between two ultracold atoms, consists of two channels, each
containing the same diagonal Lennard-Jones potential, one of positive and the
other of negative energy. The coupling potential is of an exponential form. The
methods are i) a recently developed spectral type integral equation method
based on Chebyshev expansions, ii) a finite element expansion, and iii) a
combination of an improved Numerov finite difference method and a Gordon
method. The computing time and the accuracy of the resulting phase shift is
found to be comparable for methods i) and ii), achieving an accuracy of ten
significant figures with a double precision calculation. Method iii) achieves
seven significant figures. The scattering length and effective range are also
obtained.Comment: 22 pages, 3 figures, submitted to J. Comput. Phys. documentstyle
[thmsa,sw20aip]{article} in .te
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