This Week in Science

Science  13 Apr 2001:
Vol. 292, Issue 5515, pp. 165
  1. Single-Molecule Moves in Melts

    CREDIT: L. A. DESCHENES AND D. A. VANDEN BOUT

    A critical issue in understanding the dynamics of glassy states is the origin of nonexponential relaxation of molecular motion. Such nonexponential kinetics could be inherent or could arise from a population of molecules that relax with exponential kinetics along a distribution of time scales. Deschenes and Vanden Bout (p. 255; see the Perspective by Ediger and Skinner) examined the rotational orientation of individual dye molecules in polymer films just above their glass transition temperature. They find that heterogenous dynamics result from dye molecules diffusing first on one time scale and then abruptly switching their relaxation time scale in response to changes in the glassy medium.

  2. Tuning Josephson Junctions

    In a Josephson junction, superconducting Cooper pairs of electrons cross a narrow insulating region barrier between two superconductors even at zero bias. Josephson junctions are one of the keystone structures for new superconducting devices, but one limitation has been that the properties of the junctions are more or less fixed once they have been fabricated. Schön et al. (p. 252; see the Perspective by Wendin and Shumeiko;) now describe the fabrication of tunable Josephson junctions in organic materials in which the superconductivity can be induced by the application of an electric field.

  3. Egging-on Crystallization

    Crystal nuclei will spontaneously form in a liquid when molecular fluctuations cause a local set of atoms to associate temporarily. The stability of the nuclei is highly temperature dependent because the surface energy, which drives shrinkage of the nuclei, competes with the bulk energy, which drives their growth. The size and length scale associated with the initial stages of crystallization prevents direct observation in most materials. Gasser et al. (p. 258) used confocal microscopy in a colloidal system to observe the size, shape, and dynamics of nuclei. The nuclei that formed were ellipsoidal, as opposed to the spherical shape assumed by most theories, and the lattice packing of the grown crystal matched that of the nuclei.

  4. Twin Peeks at Ocean Warming

    Although global warming is often framed in terms of atmospheric temperature, the atmosphere contains only a small fraction of the heat supplied by the sun. From an energetic point of view, the oceans are far more important, and it has been shown recently that the heat content of the upper 3000 meters of the ocean has increased during the past 50 years. Two studies show that this observation agrees quite well with what we should expect for an Earth warmed by greenhouse gas emissions (see the news story by Kerr). Using different climate models, Levitus et al. (p. 267) and Barnett et al. (p. 270) show that changes in the distribution of heat in the world ocean are accurately reproduced when forcing by anthropogenic greenhouse gases and sulfate aerosols are incorporated. The agreement between these different models, which have both successfully simulated observed changes in the climate system that have not been reproduced previously, lends greater confidence to these approaches.

  5. The Ice Sheets Cometh

    The recent dramatic changes in Earth's climate, marked by the growth and retreat of large ice sheets about every 100,000 years, are thought to have been driven by small changes in Earth's orbit through a yet-unknown amplification mechanism. Such orbital variations have occurred throughout Earth's history—not just during recent glacial times—but clear climatic effects have been difficult to resolve. Zachos et al. (p. 274; see the news story by Kerr) now present a detailed deep-sea record of climate change from about 23 million years ago that seems to show such an effect. A major glaciation corresponds with a time when cycles in the obliquity and eccentricity of Earth's axis were synchronized, and the record over several million years shows an orbital pacing. This congruence may have had a longer lasting effect, as it is about the time that large and persistent ice sheets appeared in Antarctica.

  6. Be Subthreshold and Multiply

    The auditory system of the barn owl has provided numerous valuable models for neurobiology. Peña and Konishi (p. 249; see the cover and the news story by Helmuth) used intracellular recording from many identified space-specific neurons in the inferior colliculus of the barn owl to determine what computational processes could account for the emergence of space specificity. They compared multiplication and addition in the space-specific cells and present clear evidence in favor of multiplication of subthreshold inputs. The need for multiplicative combination was predicted by previous modeling studies but not shown conclusively. The results in this study will allow the mechanisms in those models to be tested experimentally.

  7. Chip Off the Old Clock

    The main circadian clock in mammals is located in the brain, but the cells of many peripheral tissues also contain clocks. Yagita et al. (p. 278) report that the molecular components that make up these peripheral clocks, at least those in fibroblasts, are the same as those that form the brain's master clock in the mouse. Transcriptional and translation feedback loops consisting of Per2, Bmal1, Clock, and Cry1 and their respective proteins interact to produce a circadian 24-hour rhythm in cultured fibroblasts. The oscillations and period length are dependent on the presence of Cry1, as in the brain. Thus, the causes of the distinctive features of peripheral oscillators—a tendency to damp and insensitivity to light—must be sought elsewhere.

  8. Flow Coated

    CREDIT: I. COHEN ET AL.

    The coating or encapsulation of microparticles is a critical step in developing drug delivery vehicles and in protecting cells, tissues, or catalysts. Current methods tend to produce particles of the same final diameter, so that particles of different diameter have a non-uniform coat thickness or produce uniform coats but are restricted in the choice of polymers and their further processing. Cohen et al. (p. 265) can coat particles uniformly by selectively withdrawing them from a water-oil interface, and they can cure the coatings via light, heat, or chemical reaction.

  9. Molecular Trigger of Infectivity

    The protozoan parasite Leishmania causes severe and disfiguring diseases and is transmitted between humans and companion or wild animals by insect (sand fly) vectors. The factors that regulate the parasite's exit and entry between the various life cycle stages in the different hosts have been difficult to pinpoint. Cunningham et al. (p. 285) show that tetrahydrobiopterin (H4B), through control of pteridine metabolism, restrains the differentiation of noninfective procyclic forms of the parasites within the sand fly into highly infective metacyclic forms. Mutant Leishmania that lack the key enzyme pteridine reductase 1 had low levels of H4B and were hypervirulent in mice.

  10. Insulin-Like Pathways and Aging

    In the nematode Caenorhabditis elegans, mutations in the daf pathway, which normally regulates an inactive hibernation-like life phase, can prolong life-span dramatically. The daf pathway is homologous to the insulin pathway of higher organisms, and the ability of caloric restriction to increase the life-span of rodents suggests further links. Fabrizio et al. (p. 288; see the news story by Strauss, 6 April) take this demonstration a step further by screening for mutants that control life-span in nonreplicating yeast. They identify Sch9, a kinase similar to the Akt/PKB kinase in the insulin signaling pathway of mammals, as well as the yeast homolog of adenylate cyclase. The insulin-like signaling pathway appears to be a prime regulator of life-span, likely through its control of whole-organism metabolism.

  11. "What" and "Where" in What We Hear

    Compared to other sensory modalities, our understanding of the organization of primate auditory cortex is still rather incomplete. Tian et al. (p. 290) analyzed neuronal responses in the lateral belt of the auditory cortex and found evidence for distinct processing pathways. Different areas of the lateral belt respond with different degrees of specificity for location and range of monkey calls. There may be different processing streams in the auditory cortex similar to the pathways described in the visual system.

  12. G Proteins and Presynaptic Inhibition

    Several neurotransmitters modulate synaptic release from presynaptic terminals by activating G protein-coupled receptors. Blackmer et al. (p. 293) investigated the basic mechanisms underlying this modulation of the exocytotic machinery in neurons. Microinjection of G-protein βγ-subunits (Gβγ) into lamprey reticulospinal axons inhibited synaptic transmission without decreasing the action potential-dependent influx of Ca2+. This result suggests a potentially new effector role for Gβγ through directly modulating the vesicle fusion machinery in the synaptic terminal.

  13. Functional Segregation of Visual Processing Streams

    Whether separate functional streams connect the magno- and parvocellular retino-geniculo-cortical pathways to different cortical systems, particularly the dorsal and ventral cortical stream of the visual processing system, is still not resolved. Yabuta et al. (p. 297; see the Perspective by Levitt) combined laser scanning photostimulation and whole-cell voltage clamp recordings in slices of macaque visual cortex to identify neurons providing excitatory input to layer 4B of area V1. Layer 4B pyramidal neurons receive inputs from both 4Cα and 4Cβ, whereas spiny stellate cells receive input from 4C4Cα but not 4Cβ.

  14. Caught in an Excited State

    Many chemical reactions proceed through short-lived photoexcited states whose structures are often not well characterized unless the molecules form crystals suitable for time-resolved x-ray diffraction studies. Chen et al. (p. 262) show that a less direct structural method, x-ray absorption fine-structure spectroscopy, can be applied with nanosecond time resolution to molecules in solution. They determined the coordination of a photoexcited nickel porphyrin molecule as it reattached piperidine ligands from solution.

  15. Farm Futures

    Global change is about more than the effects of greenhouse gases on climate. Tilman et al. (p. 281) recast agriculturally driven changes in global nitrogen and phosphorus fertilization, in pesticide production and trade, and in the extent of agricultural land, for next 50 years. To arrive at their forecasts, they extrapolate the linear increase of these parameters with time, global population, and per capita Gross Domestic Product during the past half-century. They forecast at least twofold increases in most parameters by 2050.

  16. Object Processing in the Infant Brain

    Csibra et al. (Reports, 24 Nov. 2000, p. 1582) found that 8-month-old infants showed electroencephalogram responses in the gamma band (40 Hz) when presented with Kanizsa figures, illusory-square diagrams known to be perceived by infants of that age, but that 6-month-olds, who do not perceive the squares, did not show such gamma-band activity (GBA). The study thus underscored the link between GBA and the process by which “the brain ‘binds' together separately coded stimulus features to form unitary representations of objects.” Comments by Müller and by Herrmann and Friederici point out significant differences between the Csibra et al. infant study and studies in adults—particularly with respect to the location of the GBA increase in the brain and the detailed patterns of event-related potential (ERP) data presented by Csibra et al.—and question the proposed relation between the ERPs and perceptual binding. Csibra and Johnson respond that “in light of … variable findings in adults, we need to keep an open mind when interpreting the data from infants.” The full text of these comments can be seen at www.sciencemag.org/cgi/content/full/292/5515/163a

Log in to view full text

Via your Institution

Log in through your institution

Log in through your institution