9,373 research outputs found
In Synch but Not in Step: Circadian Clock Circuits Regulating Plasticity in Daily Rhythms
The suprachiasmatic nucleus (SCN) is a network of neural oscillators that program daily rhythms in mammalian behavior and physiology. Over the last decade much has been learned about how SCN clock neurons coordinate together in time and space to form a cohesive population. Despite this insight, much remains unknown about how SCN neurons communicate with one another to produce emergent properties of the network. Here we review the current understanding of communication among SCN clock cells and highlight a collection of formal assays where changes in SCN interactions provide for plasticity in the waveform of circadian rhythms in behavior. Future studies that pair analytical behavioral assays with modern neuroscience techniques have the potential to provide deeper insight into SCN circuit mechanisms
Recent Results on the Decay of Metastable Phases
We review some aspects of current knowledge regarding the decay of metastable
phases in many-particle systems. In particular we emphasize recent theoretical
and computational developments and numerical results regarding homogeneous
nucleation and growth in kinetic Ising and lattice-gas models. An introductory
discussion of the droplet theory of homogeneous nucleation is followed by a
discussion of Monte Carlo and transfer-matrix methods commonly used for
numerical study of metastable decay, including some new algorithms. Next we
discuss specific classes of systems. These include a brief discussion of recent
progress for fluids, and more exhaustive considerations of ferromagnetic Ising
models ({\it i.e.}, attractive lattice-gas models) with weak long-range
interactions and with short-range interactions. Whereas weak-long-range-force
(WLRF) models have infinitely long-lived metastable phases in the
infinite-range limit, metastable phases in short-range-force (SRF) models
eventually decay, albeit extremely slowly. Recent results on the finite-size
scaling of metastable lifetimes in SRF models are reviewed, and it is pointed
out that such effects may be experimentally observable.Comment: 34 pages, LaTex, 8 ps figs. on request, preprint FSU-SCRI-94-6
Mobility of Dislocations in Aluminum
The velocities of individual dislocations of edge and mixed types in pure aluminum single crystals were determined as a function of appliedâresolved shear stress and temperature. The dislocation velocities were determined from measurements of the displacements of individual dislocations produced by stress pulses of known duration. The BergâBarrett xâray technique was employed to observe the dislocations, and stress pulses of 15 to 108 ÎŒsec duration were applied by propagating torsional waves along the axes of [111]âoriented cylindrical crystals. Resolved shear stresses up to 16Ă10^6 dynesâcm^2 were applied at temperatures ranging from â150° to +70°C, and dislocation velocities were found to vary from 10 to 2800 cmâsec over these ranges of stress and temperature. The experimental conditions were such that the dislocation velocities were not significantly influenced by impurities, dislocation curvature, dislocationâdislocation interactions, or longârange internal stress fields in the crystals. The velocity of dislocations is found to be linearly proportional to the appliedâresolved shear stress, and to decrease with increasing temperature. Qualitative comparison of these results with existing theories leads to the conclusion that the mobility of individual dislocations in pure aluminum is governed by dislocationâphonon interactions. The phononâviscosity theory of dislocation mobility can be brought into agreement with the experimental results by reasonable choices of the values of certain constants appearing in the theory
Evaluating Frame-of-Reference Rater Training Effectiveness via Performance Schema Accuracy
Frame-of-reference (FOR) training has been shown to be an effective intervention for improving the accuracy of performance ratings (e.g., Woehr & Huffcutt, 1994). Despite evidence in support of the effectiveness of FOR training, few studies have empirically addressed the ultimate goal of FOR training, which is to train raters to share a common conceptualization of performance (Athey & McIntyre, 1987; Woehr, 1994). The present study tested the hypothesis that FOR-trained raters would possess schemas of performance after training that are more similar to an expert schema than would control-trained raters. It was also hypothesized that schema accuracy would be positively related to rating accuracy. Results supported these hypotheses. Implications for FOR training research and practice are discussed
User acceptance of intelligent avionics: A study of automatic-aided target recognition
User acceptance of new support systems typically was evaluated after the systems were specified, designed, and built. The current study attempts to assess user acceptance of an Automatic-Aided Target Recognition (ATR) system using an emulation of such a proposed system. The detection accuracy and false alarm level of the ATR system were varied systematically, and subjects rated the tactical value of systems exhibiting different performance levels. Both detection accuracy and false alarm level affected the subjects' ratings. The data from two experiments suggest a cut-off point in ATR performance below which the subjects saw little tactical value in the system. An ATR system seems to have obvious tactical value only if it functions at a correct detection rate of 0.7 or better with a false alarm level of 0.167 false alarms per square degree or fewer
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