Editors' Choice

Science  26 Oct 2012:
Vol. 338, Issue 6106, pp. 444
  1. Astronomy

    Blowing in the Solar Wind

    1. Maria Cruz

    Launched in 1977, the two Voyager spacecraft have traveled deep into space, but they are still transmitting data back to Earth. Having crossed the termination shock, the point where the solar wind is abruptly slowed down by the interstellar medium surrounding the solar system, both spacecraft are currently in the outermost layer of the heliosphere—the heliosheath. Richardson and Wang report recent data from the plasma instrument on Voyager 2, which crossed the termination shock in 2007 as solar activity was approaching its minimum. Previous data had shown a decrease in the solar wind density at the position of Voyager 2 6 months after it crossed the termination shock; now data from 2011 and 2012 reveal an increase in the plasma density back to the levels observed just after termination shock crossing, before the decrease was observed. These results may signal the end of solar minimum conditions in the heliosheath.

    Astrophys. J. 759, L19 (2012).

  2. Education

    Pay for Percentile

    1. Brad Wible

    What gets measured gets done. So claim advocates of high-stakes academic testing, arguing that paying teachers on the basis of student performance can improve education. Opponents of “pay for performance” argue that such a system pressures those being measured to game the system, which distorts the process being monitored. High-stakes assessments commonly use similar test topics, items, and formats to maintain consistency of the rating scale over time. This provides opportunity and incentive to “teach to the test,” rather than improving student understanding and achievement. Barlevy and Neal detail an approach to motivate teachers on the basis of student test performance, while limiting the opportunity to teach to the test by using completely new tests each time. With each new test, a student's achievement is reported as an ordinal ranking among a cohort of similarly achieving, similarly tested peers in the school system. A teacher's performance is measured as the sum of her/his students' individual percentile ranks among their respective peer comparison groups. The authors show how “contests” among teachers based on this summed ranking can elicit efficient teacher effort in every classroom.

    Am. Econ. Rev. 102, 1805 (2012).

  3. Neuroscience

    Countering Impaired Cognition

    1. Peter Stern

    Cognitive impairment is a cardinal feature of many psychiatric disorders, including schizophrenia. Blockade of 5-HT6 receptors is a potential strategy for correcting the cognitive deficits of schizophrenia and other central nervous system disorders. However, the molecular mechanisms underlying the deleterious impact of 5-HT6 receptors on cognitive function are still unknown. Using an unbiased proteomic approach to find protein partners and signaling mechanisms engaged by 5-HT6 receptors, Meffre et al. identified several members of the mammalian target of rapamycin (mTOR) signaling complex. mTOR has been implicated in the cognitive impairment associated with a number of genetic disorders. 5-HT6 receptor stimulation activated mTOR-dependent signaling, and the mTOR inhibitor rapamycin reversed some of the disruptive effects induced by a 5-HT6 receptor agonist in a recognition memory paradigm and in a social interaction test. In two developmental rodent models of schizophrenia, 5-HT6 receptor–elicited activation of the mTOR pathway was detected in the prefrontal cortex of adult rats, and rapamycin mimicked 5-HT6 antagonists in reversing the accompanying cognitive deficits. These results provide new insights into the molecular substrates mediating the negative impact of 5-HT6 receptors on cognition and into cellular events underlying cognitive deficits in schizophrenia.

    EMBO Mol. Med. 4, 1043 (2012).

  4. Geophysics

    The Core from Above

    1. Nicholas S. Wigginton

    Earth's core is split into a solid inner core and a fluid outer core. The convection and rotation of the outer core produce Earth's magnetic field and redistribute mass within the planet. Measured from orbiting satellites, variations in the magnetic field over time indicate changing outer core dynamics. Mass redistribution from fluid motion in the outer core should be detectable by gravity-sensing satellites, but surface processes such as the movement of water in the oceans and within river basins tend to dominate local gravity measurements. Mandea et al. reexamined global magnetic data to determine how the outer core changed over 8 years and correlated them with variations in gravitational data over the same time frame. Both sets of data show a distinct spatial feature centered under Africa, suggesting that core contributions to the gravity field are indeed measurable. A physical explanation for this feature may be related to density heterogeneity or interactions between the core and overlying mantle; however, more data collected from ongoing and future high-resolution satellites are needed to better understand the short-term dynamics of the outer core.

    Proc. Natl. Acad. Sci. U.S.A. 109, 10.1073/pnas.1207346109 (2012).

  5. Biochemistry

    A Viral Turn-Off

    1. Valda Vinson

    Chronic hepatitis C virus (HCV) infection is a major cause of liver failure. The NS3 protein of HCV has been aggressively pursued as a drug target because it is involved in viral polyprotein processing, through a serine protease domain, and in viral replication, through a helicase domain. Moreover, two drugs targeting the protease domain were approved in 2011, but more are needed. To identify new inhibitors, Saalau-Bethell et al. performed a fragment-based screen using crystals of the NS3 protein bound to NS4a (a peptide cofactor). Binding was detected at the interface of the protease and helicase domains, and the low-affinity fragments were elaborated into tight binding leads. The tightest binder had Kd = 0.02 µM, exhibited antiviral activity, and inhibited protease activity of the full-length protein, but not the isolated protease domain. Part of the NS3 helicase domain has been shown to bind the active site of the protease domain, and two species consistent with closed and open conformations have been suggested. The authors propose that inhibitors that bind at the NS3 domain interface shift the equilibrium toward the closed state, thereby inhibiting protease activity through a noncompetitive mechanism and probably helicase activity, too.

    Nat. Chem. Biol. 10.1038/NCHEMBIO.1081 (2012).

  6. Physics

    Topological Quasicrystals

    1. Jelena Stajic

    The discovery of topological order has changed the traditional view of phase transitions in condensed-matter physics. Topological phases are characterized by boundary states that are immune to certain types of perturbation and appear, for example, as edge states or surface states in two-dimensional (2D) and 3D systems, but are absent in 1D systems under most circumstances. Kraus et al. found evidence for topological boundary states in 1D quasicrystals—materials that can be viewed as a “projection” of periodic systems onto a lower-dimensional physical space. The experimental system consisted of parallel photonic waveguides engineered to realize a quasiperiodic Hamiltonian; when light was injected into a middle waveguide, it spilled over to its neighbors and beyond, whereas it stayed localized if injected into the leftmost guide, indicating a boundary state. In a modified setup, it was also possible to observe the adiabatic pumping of light between the boundaries. It is expected that the results can be generalized to 2D and 3D quasicrystals, which would have the topological properties of the higher dimensional spaces they are projected from.

    Phys. Rev. Lett. 109, 106402 (2012).

  7. Materials Science

    Feeling the Strain

    1. Marc S. Lavine

    Traditional strain gauges use a patterned metallic foil, in which the change in shape on deformation translates into a change in electrical conductivity. These devices typically have a gauge factor of 2 to 5; thus, for measuring smaller deformations, silicon gauges are used, which have a gauge factor closer to 100 but are more sensitive to temperature changes and mechanical deformation. Hempel et al. have developed a scalable deposition method for making strain gauges based on graphene, in which the sensitivity of the device can be tuned by varying the deposition conditions. Solutions containing graphene flakes were spray-coated onto a substrate to make a percolating network as the active part of the sensor. Thicker films had less electrical resistance but lower transparency, and it was shown that the gauge factor could be correlated to the initial resistance of the films. Unlike metal gauges, in which the metal itself deforms during strain, in these sensors the graphene flakes don't deform. Rather, the layers slide past each other, and it is the reduction in flake overlap that changes the electrical conductivity. Gauge factors between 10 and 150 were demonstrated, but in theory much higher values could be obtained. In addition, the sensors can be deposited onto existing surfaces, including curved ones such as light bulbs.

    Nano Lett. 10.1021/nl302959a (2012).

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