Editors' Choice

Science  28 Nov 2003:
Vol. 302, Issue 5650, pp. 1480

    Hearing with Mirrors

    1. Peter R. Stern

    Anatomical and electrophysiological studies in macaque monkeys have indicated that the primate auditory system is organized in the form of tonotopic maps, which means that neighboring neurons on the auditory cortical surface will be responsive to sounds in neighboring frequency bands. Recently, several groups have investigated the organization of the primary auditory cortex in humans with functional neuroimaging methods and have outlined an organization in humans similar to the one described in the monkey.

    Formisano et al. have used ultrahigh field magnetic resonance imaging (with high spatial resolution) to describe the detailed layout of the tonotopic maps in the human primary auditory cortex. Their results provide further evidence for functional subparcellation and confirm the existence of two tonotopically organized, mirror-symmetric areas in the core of the human auditory cortex. The identification of primary auditory subdivisions represents an important advance in understanding the cortical substrate that deals with the human perception of sounds, including speech and music. — PRS

    Neuron 40, 859 (2003).


    It's Crowded Inside

    1. Gilbert J. Chin

    The lipid bilayer structure of biological membranes is well defined conceptually—hydrophilic and charged head groups on the outside and nonpolar hydrocarbon tails on the inside—but not as well described at the atomic scale. Marrink and Mark have carried out computer simulations of vesicle formation and vesicle fusion in a system holding about a thousand dipalmitoyl phosphatidylcholine (DPPC) molecules and hundreds of thousands of water molecules. In general terms, the lipids behave pretty much as expected, coalescing first into micelles and then aggregating into a vesicle, with a higher initial lipid concentration yielding two micelles and a hemifused vesicle. Replacing a quarter of the DPPC with dipalmitoyl phosphatidylethanolamine produced vesicles with an excess of the smaller ethanolamine head group in the inner monolayer, consistent with its known preference for negative curvature domains. Close examination of the nonpolar core revealed that some of the hydrocarbon tails of the outer lipids extended across the midway point, whereas others folded back on themselves and occupied the extra space available in the head group region of the outer layer. Although these vesicles are small and thus more curved than most cellular membranes, these packing modes may nevertheless be relevant to the lipid gymnastics during membrane fusion. — GJC

    J. Am. Chem. Soc. 10.1021/ja0352092 (2003).


    The Difference Is Clear

    1. Stella M. Hurtley

    The microtubules that make up the mitotic spindle are connected at the pole to the centrosome and at the equator to the central region of the chromosome, the kinetochore. The dynamic behavior of microtubules is controlled largely by what goes on at their ends. O'Toole et al. examined the structure and placement of microtubule ends in relation to their centrosome or kinetochore targets during mitosis. Using high-voltage electron tomography, they found that the ends of the microtubules that were associated with the centrosome could be either closed (conical) or open (flared), and they suggest that these conformations correspond to the absence or presence of components that would promote microtubule dynamics. Most of the microtubules had closed ends and were distributed uniformly around the centrioles in the centrosome. Open-ended microtubules were more likely to be aimed toward kinetochores. Further work will be required to define more precisely the functional consequences of these differences. — SMH

    J. Cell Biol. 163, 451 (2003).


    Speedier Screening

    1. Ian S. Osborne

    The generation and detection of terahertz radiation (or T-rays), which lies in the frequency range between short radio waves (100 GHz) and the far infrared (10 THz), is a technology presently being developed for large-area imaging, with a view toward security, because of its non-ionizing energy and ability to penetrate objects such as material packaging and clothing. However, the techniques that provide sufficient output power for large-scale imaging are constrained to operate at cryogenic operation, to emit only monochromatic radiation, or are untunable—all of which limit somewhat their general application.

    Baker et al. present a broadband T-ray imaging system in which the radiation is generated during the interaction between femtosecond pulses from a 1.06-μm laser and the photoconductive semiconductor InGaAs. Coupling the optical pump pulses with the generated T-rays allows them to employ phase-sensitive detection techniques, thereby raising the possibility of real-time imaging of biological and non-biological samples alike. — ISO

    Appl. Phys. Lett. 83, 4113 (2003).


    Toughened by Worms

    1. Marc S. Lavine

    Epoxy resins, particularly those that are flame retardant, are of special importance to the microelectronic and aircraft industries, where their use is mandated. Unfortunately, many epoxies are intrinsically brittle, and this tendency to fracture is worsened by the brominated monomers that are added to improve resistance to fire. Attempts to make these materials tougher often lead to a loss of modulus and temperature stability.

    Previous work has shown that block copolymers can form micelles or vesicles within the epoxy and increase its toughness without affecting other key properties. Dean et al. find that when the block copolymers form wormlike micelles, the toughness is enhanced even more, and, surprisingly, the glass transition temperature of the composite, and hence its useful operating range, is increased. Control over which morphology forms is governed by the ratio of the two polymers that form the diblock component. The most dramatic increase was seen in a blend that contained 50% brominated monomer: With the addition of just 5% wormlike micelle-forming polymer, the virtually useless fragile glassy material was transformed into a tough resistant plastic. — MSL

    Macromolecules 10.1021/ma034807y (2003).


    Going with the Flow

    1. Julia Fahrenkamp-Uppenbrink

    Many industrial-scale catalytic processes use a fixed bed reactor, in which a solution containing the reactants flows continuously over a heterogeneous catalyst that is immobilized on a solid porous support; the product emerges in the output stream. Hillerich and Plenio have adapted this reactor for use with homogeneous catalysts. In their biphasic system of two immiscible liquids, a polar tag is attached to the catalyst to keep it in the polar solvent (dimethyl sulfoxide), while the reactants and products reside primarily in the nonpolar solvent (n-heptane). The lower-density, reactant-containing n-heptane is fed into the bottom of a glass tube filled with the higher-density catalyst phase. Vigorous stirring creates small droplets that rise slowly to the top of the tube. During transit upward, the reactants partition into the catalyst phase, where they are converted into products that are extracted back into the buoyant droplets. At the top of the tube, the product-containing effluent is drawn off. Filling the tube with various sizes of glass beads provides an easy way to control the transit speed of the reactants. This method, which does not require expensive solvents or pressurized equipment, should be applicable to a range of organic reactions. — JFU

    Chem. Comm. 10.1039/b310504k (2003).


    Extending a Helping Hand

    1. Andrew M. Sugden

    Invasive species introduced deliberately or accidentally by humans have become a major nuisance and threat to ecosystems worldwide. It is commonly thought that invasive alien plants are often able to overwhelm native plants because of superior competitive ability. In a study of the interaction between invasive and native species in California grasslands, Seabloom et al. show that this is not necessarily the case.

    More than 9 million hectares of these grasslands have become dominated by southern European annual grasses and forbs, with important consequences for ecosystem processes such as nutrient leaching, carbon storage, and fire dynamics. Experimental alteration of parameters such as seed quantity, availability of nitrate and water, and disturbance produced conditions under which the native plants reinvaded the grasslands, edging out the usurpers. These results suggest that the exotics are not always superior competitors, and that native grassland can be restored by increasing the amount and dispersal of native seeds. As well as providing optimism that the invaded grasslands of California can be restored at least partially, this study suggests a potential approach for studying and alleviating the effects of invasions in other systems. — AMS

    Proc. Natl. Acad. Sci. U.S.A. 100, 13384 (2003).