2,122 research outputs found
Modeling Reverse-Phi Motion-Selective Neurons in Cortex: Double Synaptic-Veto Mechanism
Reverse-phi motion is the illusory reversal of perceived direction of movement when the stimulus contrast is reversed in successive frames. Livingstone, Tsao, and Conway (2000) showed that direction-selective cells in striate cortex of the alert macaque monkey showed reversed excitatory and inhibitory regions when two different contrast bars were flashed sequentially during a two-bar interaction analysis. While correlation or motion energy models predict the reverse-phi response, it is unclear how neurons can accomplish this. We carried out detailed biophysical simulations of a direction-selective cell model implementing a synaptic shunting scheme. Our results suggest that a simple synaptic-veto mechanism with strong direction selectivity for normal motion cannot account for the observed reverse-phi motion effect. Given the nature of reverse-phi motion, a direct interaction between the ON and OFF pathway, missing in the original shunting-inhibition model, it is essential to account for the reversal of response. We here propose a double synaptic-veto mechanism in which ON excitatory synapses are gated by both delayed ON inhibition at their null side and delayed OFF inhibition at their preferred side. The converse applies to OFF excitatory synapses. Mapping this scheme onto the dendrites of a direction-selective neuron permits the model to respond best to normal motion in its preferred direction and to reverse-phi motion in its null direction. Two-bar interaction maps showed reversed excitation and inhibition regions when two different contrast bars are presented
El desplazamiento inducido por desastres en el Caribe y el Pacífico
La gente de los pequeños Estados insulares en desarrollo es especialmente vulnerable al desplazamiento por desastres. Urge que los gobiernos del Caribe y del Pacífico realicen una mayor gestión de riesgos y una mayor planificación en vez de centrarse casi exclusivamente en la respuesta y la reubicación
Static vs. dynamical mean field theory of Mott antiferromagnets
Studying the antiferromagnetic phase of the Hubbard model by dynamical mean
field theory, we observe striking differences with static (Hartree-Fock) mean
field: The Slater band is strongly renormalized and spectral weight is
transferred to spin-polaron side bands. Already for intermediate values of the
interaction the overall bandwidth is larger than in Hartree-Fock, and the
gap is considerably smaller. Such differences survive any renormalization of
. Our photoemission experiments for Cr-doped VO show spectra
qualitatively well described by dynamical mean field theory.Comment: 6 pages, 5 figures - one figure added and further details about
quasiparticle dispersio
A learning rule for local synaptic interactions between excitation and shunting inhibition
The basic requirement for direction selectivity is a nonlinear interaction between two different inputs in space-time. In some models, the interaction is hypothesized to occur between excitation and inhibition of the shunting type in the neuron's dendritic tree. How can the required spatial specificity be acquired in an unsupervised manner? We here propose an activity-based, local learning model that can account for direction selectivity in visual cortex based on such a local veto operation and that depends on synaptically induced changes in intracellular calcium concentration. Our biophysical simulations suggest that a model cell with our learning algorithm can develop direction selectivity organically after unsupervised training. The learning rule is also applicable to a neuron with multiple-direction-selective subunits and to a pair of cells with opposite-direction selectivities and is stable under different starting conditions, delays, and velocities
Chemical Signatures of the First Galaxies: Criteria for One-Shot Enrichment
We utilize metal-poor stars in the local, ultra-faint dwarf galaxies (UFDs;
L_tot < 10^5 L_sun) to empirically constrain the formation process of the first
galaxies. Since UFDs have much simpler star formation histories than the halo
of the Milky Way, their stellar populations should preserve the fossil record
of the first supernova (SN) explosions in their long-lived, low-mass stars.
Guided by recent hydrodynamical simulations of first galaxy formation, we
develop a set of stellar abundance signatures that characterize the
nucleosynthetic history of such an early system if it was observed in the
present-day universe. Specifically, we argue that the first galaxies are the
product of chemical "one-shot" events, where only one (long-lived) stellar
generation forms after the first, Population III, SN explosions. Our abundance
criteria thus constrain the strength of negative feedback effects inside the
first galaxies. We compare the stellar content of UFDs with these one-shot
criteria. Several systems (Ursa Major II, and also Coma Berenices, Bootes I,
Leo IV, Segue 1) largely fulfill the requirements, indicating that their
high-redshift predecessors did experience strong feedback effects that shut off
star formation. We term the study of the entire stellar population of a dwarf
galaxy for the purpose of inferring details about the nature and origin of the
first galaxies "dwarf galaxy archaeology". This will provide clues to the
connection of the first galaxies, the surviving, metal-poor dwarf galaxies, and
the building blocks of the Milky Way.Comment: 9 pages (emulateapj), 2 figures, ApJ in pres
Reducing vortex density in superconductors using the ratchet effect
A serious obstacle that impedes the application of low and high temperature
superconductor (SC) devices is the presence of trapped flux. Flux lines or
vortices are induced by fields as small as the Earth's magnetic field. Once
present, vortices dissipate energy and generate internal noise, limiting the
operation of numerous superconducting devices. Methods used to overcome this
difficulty include the pinning of vortices by the incorporation of impurities
and defects, the construction of flux dams, slots and holes and magnetic
shields which block the penetration of new flux lines in the bulk of the SC or
reduce the magnetic field in the immediate vicinity of the superconducting
device. Naturally, the most desirable would be to remove the vortices from the
bulk of the SC. There is no known phenomenon, however, that could form the
basis for such a process. Here we show that the application of an ac current to
a SC that is patterned with an asymmetric pinning potential can induce vortex
motion whose direction is determined only by the asymmetry of the pattern. The
mechanism responsible for this phenomenon is the so called ratchet effect, and
its working principle applies to both low and high temperature SCs. As a first
step here we demonstrate that with an appropriate choice of the pinning
potential the ratchet effect can be used to remove vortices from low
temperature SCs in the parameter range required for various applications.Comment: 7 pages, 4 figures, Nature (in press
Gauge Invariant Two-Photon-Exchange Contributions in
The gauge invariant two-photon exchange (TPE) contributions in are discussed at hadronic level. The contact term is
added to keep the full amplitude gauge invariant by two methods: one is to
multiply form factors with the point-like particles scattering amplitude and
another is to construct a gauge invariant Lagangian. The practical calculations
show the TPE corrections by these two methods are almost the same, while the
later method is favored when extending the discussion to processes including
two charged finite-size particles like .Comment: 5 pages, 2 fugure
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