1,294 research outputs found
Does Neuronal Synchrony Underlie Visual Feature Grouping?
SummaryPrevious research suggests that synchronous neural activity underlies perceptual grouping of visual image features. The generality of this mechanism is unclear, however, as previous studies have focused on pairs of neurons with overlapping or collinear receptive fields. By sampling more broadly and employing stimuli that contain partially occluded objects, we have conducted a more incisive test of the binding by synchrony hypothesis in area MT. We find that synchrony in spiking activity shows little dependence on feature grouping, whereas gamma band synchrony in field potentials can be significantly stronger when features are grouped. However, these changes in gamma band synchrony are small relative to the variability of synchrony across recording sites and do not provide a robust population signal for feature grouping. Moreover, these effects are reduced when stimulus differences nearby the receptive fields are eliminated using partial occlusion. Our findings suggest that synchrony does not constitute a general mechanism of visual feature binding
Structural Estimation of the New-Keynesian Model: A Formal Test of Backward- and Forward-Looking Behavior
Loss of ultracold 87Rb133Cs molecules via optical excitation of long-lived two-body collision complexes
We show that the lifetime of ultracold ground-state 87Rb133Cs molecules in an optical trap is limited by fast optical excitation of long-lived two-body collision complexes. We partially suppress this loss mechanism by applying square-wave modulation to the trap intensity, such that the molecules spend 75% of each modulation cycle in the dark. By varying the modulation frequency, we show that the lifetime of the collision complex is 0.53 0.06 ms in the dark. We find that the rate of optical excitation of the collision complex is 3ĂŸ4 â2 Ă 103 Wâ1 cm2 sâ1 for λ ÂŒ 1550 nm, leading to a lifetime of < 100 ns for typical trap intensities. These results explain the two-body loss observed in experiments on nonreactive bialkali molecules
Linear Sigma Models of H and KK Monopoles
We propose a gauged linear sigma model of k H-monopoles. We also consider the
T-dual of this model describing KK-monopoles and clarify the meaning of
"winding coordinate" studied recently in hep-th/0507204.Comment: 13 pages, lanlmac; V3:added argument on the nature of disk instanto
Robust storage qubits in ultracold polar molecules
Quantum states with long-lived coherence are essential for quantum computation, simulation and metrology. The nuclear spin states of ultracold molecules prepared in the singlet rovibrational ground state are an excellent candidate for encoding and storing quantum information. However, it is important to understand all sources of decoherence for these qubits, and then eliminate them, to reach the longest possible coherence times. Here we fully characterize the dominant mechanisms of decoherence for a storage qubit in an optically trapped ultracold gas of RbCs molecules using high-resolution Ramsey spectroscopy. Guided by a detailed understanding of the hyperfine structure of the molecule, we tune the magnetic field to where a pair of hyperfine states have the same magnetic moment. These states form a qubit, which is insensitive to variations in magnetic field. Our experiments reveal a subtle differential tensor light shift between the states, caused by weak mixing of rotational states. We demonstrate how this light shift can be eliminated by setting the angle between the linearly polarized trap light and the applied magnetic field to a magic angle of arccos(1/3ââ)â55â. This leads to a coherence time exceeding 5.6âs at the 95% confidence level
Current-Carrying Cosmic Strings in Scalar-Tensor Gravities
We study the modifications on the metric of an isolated self-gravitating
bosonic superconducting cosmic string in a scalar-tensor gravity in the
weak-field approximation. These modifications are induced by an arbitrary
coupling of a massless scalar field to the usual tensorial field in the
gravitational Lagrangian. The metric is derived by means of a matching at the
string radius with a most general static and cylindrically symmetric solution
of the Einstein-Maxwell-scalar field equations. We show that this metric
depends on five parameters which are related to the string's internal structure
and to the solution of the scalar field. We compare our results with those
obtained in the framework of General Relativity.Comment: 23 pages, no figures, LATEX fil
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A tillering inhibition gene influences root-shoot carbon partitioning and pattern of water use to improve wheat productivity in rainfed environments
Genetic modification of shoot and root morphology has potential to improve water and nutrient
19 uptake of wheat crops in rainfed environments. Near-isogenic lines (NILs) varying for a tillering
20 inhibition (tin) gene and representing multiple genetic backgrounds were investigated in contrasting
21 controlled environments for shoot and root growth. Leaf area, shoot and root biomass were similar
22 until tillering whereupon reduced tillering in tin-containing NILs produced reductions of up to 60% in
23 total leaf area and biomass, and increases in total root length of up to 120% and root biomass to
24 145%. Together, root-to-shoot ratio increased two-fold with the tin gene. The influence of tin on shoot
25 and root growth was greatest in the cv. Banks genetic background, particularly in the biculm-selected
26 NIL, and was typically strongest in cooler environments. A separate de-tillering study confirmed
27 greater root-to-shoot ratios with regular tiller removal in non-tin containing genotypes. In validating
28 these observations in a rainfed field study, the tin allele had a negligible effect on seedling growth but
29 was associated with significantly (P<0.05) reduced tiller number (-37%), leaf area index (-26%) and
30 spike number (-35%) to reduce plant biomass (-19%) at anthesis. Root biomass, root-to-shoot ratio at
31 early stem elongation and root depth at maturity were increased in tin-containing NILs. Soil water use
32 was slowed in tin-containing NILs resulting in greater water availability, greater stomatal
33 conductance, cooler canopy temperatures and maintenance of green leaf area during grain-filling.
34 Together these effects contributed to increases in harvest index and grain yield. In both the controlled
35 and field environments, the tin gene was commonly associated with increased root length and biomass
36 but the significant influence of genetic background and environment suggests careful assessment of
37 tin-containing progeny in selection for genotypic increases in root growth
On the CSFT approach to localized closed string tachyons
We compute the potential for localized closed string tachyons in bosonic
string theory on the orbifold C/Z_4 using level-truncated closed string field
theory. The critical points of the potential exhibit features which agree with
their conjectured identification as lower-order orbifolds. However this case
also raises some questions regarding the quantitative predictions associated
with these conjectures.Comment: 20 pages, 3 figures, v2: The relation between the flat space and
orbifold gravitational constants has been corrected. This resolves the puzzle
of multiple predictions, but worsens the agreement between the depth of the
potential and the change in the deficit angl
Localized Tachyons and the Quantum McKay Correspondence
The condensation of closed string tachyons localized at the fixed point of a
C^d/\Gamma orbifold can be studied in the framework of renormalization group
flow in a gauged linear sigma model. The evolution of the Higgs branch along
the flow describes a resolution of singularities via the process of tachyon
condensation. The study of the fate of D-branes in this process has lead to a
notion of a ``quantum McKay correspondence.'' This is a hypothetical
correspondence between fractional branes in an orbifold singularity in the
ultraviolet with the Coulomb and Higgs branch branes in the infrared. In this
paper we present some nontrivial evidence for this correspondence in the case
C^2/Z_n by relating the intersection form of fractional branes to that of
``Higgs branch branes,'' the latter being branes which wrap nontrivial cycles
in the resolved space.Comment: 25 pages; harvma
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