115,131 research outputs found
The mechanism underlying backward priming in a lexical decision task: Spreading activation versus semantic matching
Koriat (1981) demonstrated that an association from the target to a preceding prime, in the absence of an association from the prime to the target, facilitates lexical decision and referred to this effect as "backward priming". Backward priming is of relevance, because it can provide information about the mechanism underlying semantic priming effects. Following Neely (1991), we distinguish three mechanisms of priming: spreading activation, expectancy, and semantic matching/integration. The goal was to determine which of these mechanisms causes backward priming, by assessing effects of backward priming on a language-relevant ERP component, the N400, and reaction time (RT). Based on previous work, we propose that the N400 priming effect reflects expectancy and semantic matching/integration, but in contrast with RT does not reflect spreading activation. Experiment 1 shows a backward priming effect that is qualitatively similar for the N400 and RT in a lexical decision task. This effect was not modulated by an ISI manipulation. Experiment 2 clarifies that the N400 backward priming effect reflects genuine changes in N400 amplitude and cannot be ascribed to other factors. We will argue that these backward priming effects cannot be due to expectancy but are best accounted for in terms of semantic matching/integration
Recommendations for NASA research and development in artificial intelligence
Basic artificial intelligence (AI) research, AI applications, engineering, institutional management, and previously impractical missions enabled by AI are discussed
Nitrogen superfractionation in dense cloud cores
We report new calculations of interstellar 15N fractionation. Previously, we
have shown that large enhancements of 15N/14N can occur in cold, dense gas
where CO is frozen out, but that the existence of an NH + N channel in the
dissociative recombination of N2H+ severely curtails the fractionation. In the
light of recent experimental evidence that this channel is in fact negligible,
we have reassessed the 15N chemistry in dense cloud cores. We consider the
effects of temperatures below 10 K, and of the presence of large amounts of
atomic nitrogen. We also show how the temporal evolution of gas-phase isotope
ratios is preserved as spatial heterogeneity in ammonia ice mantles, as
monolayers deposited at different times have different isotopic compositions.
We demonstrate that the upper layers of this ice may have 15N/14N ratios an
order of magnitude larger than the underlying elemental value. Converting our
ratios to delta-values, we obtain delta(15N) > 3,000 per mil in the uppermost
layer, with values as high as 10,000 per mil in some models. We suggest that
this material is the precursor to the 15N `hotspots' recently discovered in
meteorites and IDPsComment: accepted by MNRA
The effect of beam-driven return current instability on solar hard X-ray bursts
The problem of electrostatic wave generation by a return current driven by a small area electron beam during solar hard X-ray bursts is discussed. The marginal stability method is used to solve numerically the electron and ion heating equations for a prescribed beam current evolution. When ion-acoustic waves are considered, the method appears satisfactory and, following an initial phase of Coulomb resistivity in which T sub e/T sub i rise, predicts a rapid heating of substantial plasma volumes by anomalous ohmic dissipation. This hot plasma emits so much thermal bremsstrahlung that, contrary to previous expectations, the unstable beam-plasma system actually emits more hard X-rays than does the beam in the purely collisional thick target regime relevant to larger injection areas. Inclusion of ion-cyclotron waves results in ion-acoustic wave onset at lower T sub e/T sub i and a marginal stability treatment yields unphysical results
Applying machine learning to the problem of choosing a heuristic to select the variable ordering for cylindrical algebraic decomposition
Cylindrical algebraic decomposition(CAD) is a key tool in computational
algebraic geometry, particularly for quantifier elimination over real-closed
fields. When using CAD, there is often a choice for the ordering placed on the
variables. This can be important, with some problems infeasible with one
variable ordering but easy with another. Machine learning is the process of
fitting a computer model to a complex function based on properties learned from
measured data. In this paper we use machine learning (specifically a support
vector machine) to select between heuristics for choosing a variable ordering,
outperforming each of the separate heuristics.Comment: 16 page
Mott transition, antiferromagnetism, and unconventional superconductivity in layered organic superconductors
The phase diagram of the layered organic superconductor
-(ET)Cu[N(CN)]Cl has been accurately measured from a
combination of H NMR and AC susceptibility techniques under helium gas
pressure. The domains of stability of antiferromagnetic and superconducting
long-range orders in the pressure {\it vs} temperature plane have been
determined. Both phases overlap through a first-order boundary that separates
two regions of inhomogeneous phase coexistence. The boundary curve is found to
merge with another first order line related to the metal-insulator transition
in the paramagnetic region. This transition is found to evolve into a crossover
regime above a critical point at higher temperature. The whole phase diagram
features a point-like region where metallic, insulating, antiferromagnetic and
non s-wave superconducting phases all meet.Comment: 4 pages, 6 figures, Revte
The Microcanonical Functional Integral. I. The Gravitational Field
The gravitational field in a spatially finite region is described as a
microcanonical system. The density of states is expressed formally as a
functional integral over Lorentzian metrics and is a functional of the
geometrical boundary data that are fixed in the corresponding action. These
boundary data are the thermodynamical extensive variables, including the energy
and angular momentum of the system. When the boundary data are chosen such that
the system is described semiclassically by {\it any} real stationary
axisymmetric black hole, then in this same approximation is shown to
equal 1/4 the area of the black hole event horizon. The canonical and grand
canonical partition functions are obtained by integral transforms of that
lead to "imaginary time" functional integrals. A general form of the first law
of thermodynamics for stationary black holes is derived. For the simpler case
of nonrelativistic mechanics, the density of states is expressed as a real-time
functional integral and then used to deduce Feynman's imaginary-time functional
integral for the canonical partition function.Comment: 29 pages, plain Te
The massless higher-loop two-point function
We introduce a new method for computing massless Feynman integrals
analytically in parametric form. An analysis of the method yields a criterion
for a primitive Feynman graph to evaluate to multiple zeta values. The
criterion depends only on the topology of , and can be checked
algorithmically. As a corollary, we reprove the result, due to Bierenbaum and
Weinzierl, that the massless 2-loop 2-point function is expressible in terms of
multiple zeta values, and generalize this to the 3, 4, and 5-loop cases. We
find that the coefficients in the Taylor expansion of planar graphs in this
range evaluate to multiple zeta values, but the non-planar graphs with crossing
number 1 may evaluate to multiple sums with roots of unity. Our
method fails for the five loop graphs with crossing number 2 obtained by
breaking open the bipartite graph at one edge
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