ScienceWeek

SCIENCEWEEK

December 22, 2006

Vol. 10 - Number 50

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The pursuit of science has often been compared to the scaling of mountains, high and not so high. But who amongst us can hope, even in imagination, to scale the Everest and reach its summit when the sky is blue and the air is still, and in the stillness of the air survey the entire Himalayan range in the dazzling white of the snow stretching to infinity? None of us can hope for a comparable vision of nature and of the universe around us. But there is nothing mean or lowly in standing in the valley below and awaiting the Sun to rise over Kinchinjunga.

-- Subrahmanyan Chandrasekhar (1910-1995)

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Contents (full text below):

1. Neuroscience: On Processing of Stimuli Out of Awareness
2. Physics: A New Type of Insulator
3. Optics: On the Transfer of Momentum by Light
4. Medical Biology: On a Malaria Vaccine

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Also Noted:

The Science of Orgasm. Barry R. Komisaruk, Carlos Beyer-Flores, and Beverly Whipple. Johns Hopkins University Press, Baltimore, 2006. Hardback: 371 pp., illus. ISBN 080188490X. More information at: http://www.amazon.com/exec/obidos/ASIN/080188490X/scienceweek

The Origins of the Future. Ten Questions for the Next Ten Years. John Gribbin. Yale University Press, New Haven, CT, 2006. Hardback: 308 pp. ISBN 0300119984. More information at: http://www.amazon.com/exec/obidos/ASIN/0300119984/scienceweek

The Man Who Tried to Clone Himself. And Other True Stories of the World's Most Bizarre Research and the Ig Nobel Prizes. Marc Abrahams. Plume (Penguin Group USA), New York, 2006. Paperback: 271 pp., illus. ISBN 0452287723. More information at: http://www.amazon.com/exec/obidos/ASIN/0452287723/scienceweek

Mastering Your PhD. Survival and Success in the Doctoral Years and Beyond. P. Gosling and B. Noordam. Springer, Berlin, 2006. Paperback: 166 pp. ISBN 3540333878. More information at: http://www.amazon.com/exec/obidos/ASIN/3540333878/scienceweek

Directory of Physics, Astronomy and Geophysics Staff. 2006 Edition. American Institute of Physics. American Institute of Physics, College Park, MD, 2006. Paperback: 660 pp. ISBN 073540335X. More information at: http://www.amazon.com/exec/obidos/ASIN/073540335X/scienceweek

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1. NEUROSCIENCE: ON PROCESSING OF STIMULI OUT OF AWARENESS

The following points are made by Petra Stoerig (Science 2006 314:1694):

1) Sigmund Freud's groundbreaking work first demonstrated that a large part of our conscious mental life is governed by motives and memories that are not, or are no longer, accessible to conscious insight. Events that can no longer be consciously remembered also guide the behavior of neurological patients, as famously shown by Édouard Claparède's amnesic patient. The patient could not recall that her doctor had once painfully pricked her with a needle when shaking her hand, but nonetheless refused to shake his hand again (1). Information that fails to reach awareness can also guide behavior in neurological patients who, as a result of lesions of the brain's sensory cortices, are completely unaware of its existence. Evidence for such "implicit" processing has come from studies of vision, audition, touch, emotion, and action (2), and has also been well documented in healthy people. New work (3) adds a new twist to the slowly unfolding story of the impact of implicit processes and offers a brain-based explanation.

2) Imagine performing a task that requires full attention. If you had to do so in a normal office environment, rather than a sound-proof lab, you would likely predict that telephones ringing and people rushing by your desk would distract you more than would imperceptible stimuli irrelevant to your task. Tsushima et al (3) investigated the influence of the strength of distracting stimuli on task performance. Contrary to expectation, they report that subthreshold distracting stimuli -- that is, weak stimuli that are irrelevant to the task being performed -- have a greater impact than strong, easily noticeable distractors. The authors used a rapid visual presentation task in which healthy participants had to detect two digits that appeared very briefly in a central stream of six letters. This stream of alphanumeric symbols was surrounded by an annulus of randomly moving dots. Different proportions of these dots -- 0 to 20% -- moved coherently in the same direction (the "signal") during the trials. The larger the proportion of these coherently moving dots (the larger the motion coherence ratio of the task-irrelevant background), the more motion one perceives. Although higher motion coherence should have impaired digit identification more than lower coherence -- after all, motion tends to capture attention, even inadvertently -- performance was significantly worse at 5% coherence ratios than at 20%, where task performance was statistically similar to that observed at 0% coherence. At 5% coherence, the task-irrelevant motion had the most pronounced effect on digit identification, although the subjects were unable to discriminate the direction of the motion signal at this low coherence ratio, even when this was their only task.

3) Moreover, magnetic resonance imaging revealed that the human motion complex hMT+, the brain's visual cortical area that responds preferentially to moving stimuli, was more strongly activated at 5% coherence than at higher coherence ratios. Because stronger stimuli commonly evoke both earlier and stronger cortical responses than weak ones (4,5), this result is surprising. The authors relate it to the inverse relationship they observed between activity in hMT+ and in the lateral prefrontal cortex, an area thought to play an important role in inhibiting the influence of irrelevant signals. They suggest that because of the weakness of the 5% coherent motion signal, this cortical region fails to effectively inhibit the hMT+ response. Because it fails to "notice" the hMT+ response, the lateral prefrontal cortex fails to intervene, so that the weak motion distractor can exert its disruptive effect on performance, unchecked.

References (abridged):

1. É. Claparède, Arch. Psychol. 11, 79 (1911).

2. B. de Gelder, E. de Haan, C. Heywood, Eds., Out of Mind: Varieties of Unconscious Processes (Oxford Univ. Press, Oxford, 2001).

3. Y. Tsushima, Y. Sasaki, T. Watanabe, Science 314, 1786 (2006).

4. J. H. Maunsell, J. R. Gibson, J. Neurophysiol. 68, 1332 (1992).

5. W. T. Newsome, E. B. Pare, J. Neurosci. 8, 2201 (1988).

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2. PHYSICS: A NEW TYPE OF INSULATOR

The following points are made by C.L. Kane and E.J. Mele (Science 2006 314:1692):

1) Electrical insulators are usually appreciated for their ability to do nothing. Such materials either trap or restrict the motion of free charges in matter. This is useful in all kinds of applications, ranging from the wiring in your home to directing the flow of electrons in the tiny circuits of your iPod. Now new work (1) has proposed a new kind of two-dimensional insulator, which permits the flow of charge only at its edges. This may lead to the development of a new kind of solid-state electronic device.

2) An insulator has an energy gap separating filled and empty bands of electronic states, and thus is electrically inert because a finite energy is required to dislodge an electron. In the 1960s, Kohn (2) characterized the insulating state in terms of the sensitivity of electrons inside the material to effects on the sample boundary. His insight was that the electrons of an insulator can be regarded as occupying localized orbitals, so that they are insensitive to perturbations on the boundary.

3) The presence of a bulk energy gap does not guarantee that electrons have this "nearsighted" property. A counter example is provided by the quantum Hall state of a two-dimensional electron gas in a perpendicular magnetic field. In the quantum Hall effect, an energy gap results from the quantization of the closed circular orbits that electrons follow in a magnetic field. The inside of a quantum Hall system is thus inert like an insulator. However, at the boundary of the material a different type of motion occurs, which allows charge to flow in one-dimensional edge states. These edge states are unique in that they allow for charge to flow in one direction only. This makes them insensitive to scattering from impurities and explains the observed precise quantization of the Hall resistance.

4) Because both have a bulk energy gap, the insulating state and the quantum Hall state appear similar. The difference was explained by Thouless et al (3), who generalized Kohn's notion of boundary sensitivity to show that an occupied band is characterized by an integer topological index. This index, n, distinguishes the insulating state (n = 0) from the quantum Hall state (n not 0) in a manner similar to the way that the mathematical "genus" of a solid body -- which counts the number of holes -- distinguishes a marble from a donut or a coffee cup. For quantum Hall states, the conducting edge states are a consequence of this topological structure.

5) Recently a new class of topological insulators has been predicted to be possible at zero magnetic field. This occurs because electrons have a quantum property called spin, which can have two possible polarizations, "up" and "down". In 2005, it was shown theoretically that a single two-dimensional sheet of graphite, called graphene, has a small energy gap that arises from the interaction between the electrons' spin and orbital degrees of freedom (4). The resulting electronic state is inert in the bulk like an insulator, but has conducting edge states. The authors found that a new topological invariant distinguishes this state from a conventional insulator and guarantees the presence of those edge states (5). In the simplest picture, the edge states are spin-filtered in that electrons with spin-up propagate in one direction, whereas electrons with spin-down propagate in the opposite direction. In this sense, this state exhibits a quantum spin Hall effect.


References:

1. B. A. Bernevig, T. L. Hughes, S.-C. Zhang, Science 314, 1757 (2006).

2. W. Kohn, Phys. Rev. 133, A171 (1964).

3. D. J. Thouless, M. Kohmoto, M. P. Nightingale, M. den Nijs, Phys. Rev. Lett. 49, 405 (1982).

4. C. L. Kane, E. J. Mele, Phys. Rev. Lett. 95, 226801 (2005).

5. C. L. Kane, E. J. Mele, Phys. Rev. Lett. 95, 146802 (2005).

Science http://www.sciencemag.org

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3. OPTICS: ON THE TRANSFER OF MOMENTUM BY LIGHT

The following points are made by Ulf Leonhardt (Nature 2006 444:823):

1) One of Rudolf Peierls' Surprises in Theoretical Physics (1) is the difficulty of assessing the momentum of light in transparent materials such as glass or water. Despite its being a seemingly basic concept, rival theories (2,3) predict values for the momentum that differ by considerable factors, and that have been hotly debated for many decades. Experimental tests (4,5) have been rare. New work (6,7) now brings out the big guns to break the impasse: Dereli, Gratus and Tucker (6) resort to Einstein's general relativity for the requisite theory; and Campbell et al (7) use ultracold atoms in an experimental test.

2) Light's momentum describes the degree to which light sets other things in motion when they absorb or reflect it. The force of light is usually rather weak, but visual evidence for its existence can be seen, for example, in a comet's tail when the action of sunlight transfers momentum onto the particles of the tail, pushing dust away from the comet. The debated issue is the ratio between the momentum, p, and the energy, E, of light in a medium. This ratio depends on the degree to which a material slows light from its speed in a vacuum, c. The reduced speed is often written c/n, where n is the refractive index of the medium. In 1908, Hermann Minkowski (2) proposed that the fundamental relationship between all these quantities is (p=nE/c); a year later, Max Abraham (3) postulated [p=E/(nc)]. These rivalling momenta differ by n^(2), which is a sizeable factor in most media: in water, n alone is 1.33, and in glass it is 1.46.

3) It might seem that this dispute could easily be settled by experiment: all it should take is some water and a laser beam illuminating its surface. The surface partially reflects the beam; the rest of the incident light enters the water. The water must take up any imbalance in momentum, so the surface should rise or fall, depending on the momentum of light in water compared with that in air. In Minkowski's case, the momentum in water is higher and the water should rise; in Abraham's case, the reverse is true and the water level should fall. An experiment in the mid-1970s found that the water rises (4). In a second experiment, the light pressure on a mirror suspended in water was measured (5), and the same result emerged. Thus, in both cases, Minkowski is the winner. Case closed.

4) Not so fast. Sometimes, one can directly calculate light forces without considering the momentum transfer. Careful calculation of the forces acting behind the scenes reveals that the momentum of light is in fact chameleonic: its precise value depends on the type of experiment performed. In fact, according to the calculations, the first of the experiments (4) should have observed Abraham's momentum in action, whereas the second should indeed see Minkowski's momentum. The problem with the first experiment was that the beam did not uniformly illuminate the surface. This imbalance created lateral light forces that curved and lifted the water to a greater extent than the momentum could push it, resulting in a false interpretation of the result.

References (abridged):

1. Peierls, R. More Surprises in Theoretical Physics (Princeton Univ. Press, 1991).

2. Minkowski, H. Nachr. Ges. Wiss. Göttn Math.-Phys. Kl. 53 (1908).

3. Abraham, M. Rend. Circ. Matem. Palermo 28, 1 (1909).

4. Askin, A. & Dziedzic, J. M. Phys. Rev. Lett. 30, 139–142 (1973).

5. Jones, R. V. & Leslie, B. Proc. R. Soc. Lond. A 360, 347–363

6. Dereli, T., Gratus, J. & Tucker, R. W. Phys. Lett. A 361, 190–193 (2007).

7. Campbell, G. K. et al. Phys. Rev. Lett. 94, 170403 (2005). (1978).

Nature http://www.nature.com/nature

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4. MEDICAL BIOLOGY: ON A MALARIA VACCINE

The following points are made by Stephen L. Hoffman (Nature 2006 444:824):

1) A vaccine against malaria would be the ideal means of preventing the hundreds of millions of cases of the disease that occur annually across the globe (1). But no malaria vaccine has yet been licensed, and there is little consensus on how to develop one. Initial success in inducing an immune response against the malaria parasite (Plasmodium falciparum) came more than 30 years ago (2-4), with the discovery that human volunteers could be completely protected against malaria by exposure to radiation-weakened sporozoites -- the infectious parasite cells passed on by mosquitoes. But until recently it has not been considered practicable to manufacture and administer such "attenuated" sporozoites as part of a mass immunization strategy (5) -- not least because it took more than 1000 bites from mosquitoes infected with the irradiated cells to fully protect most of the volunteers against the disease (4).

2) Considerable effort has therefore gone into trying to define which of the thousands of proteins expressed by the parasite are involved in the protective immunity elicited by the attenuated sporozoites, with the aim of eventually making vaccines against them. New work (6) reports that one of these proteins, the circumsporozoite protein, might elicit much of the protective immunity induced by the attenuated sporozoites. However, it seems that a protective response can occur in the absence of this protein too.

3) The first clinical trials using a purified-protein malaria vaccine began 20 years ago, but so far only volunteers immunized with vaccines based on the circumsporozoite protein -- the major sporozoite surface protein -- have been reproducibly protected against P. falciparum sporozoite challenges. However, the protective immunity induced by the best circumsporozoite-protein vaccine is far lower than that induced by radiation-attenuated P. falciparum sporozoites (4). One explanation for this dramatic difference is that the whole-parasite vaccine induces protective immune responses against tens, hundreds or even thousands of parasite proteins, whereas the single-protein vaccine can elicit an immune response only against that protein.

4) Accordingly, there are significant efforts to identify additional target proteins expressed in sporozoites, and in the parasite during the liver stages of infection, to construct a more effective vaccine. If one, or a few proteins, like the circumsporozoite protein, is responsible for the protection, efforts to identify other target proteins could be curtailed, and resources focused on optimizing immunization with these few proteins. If, however, strong immunity depends on immune responses against many proteins, it may be difficult, or even impossible, to construct an effective vaccine based on only parts of the parasite. So, resolution of whether protective immunity relies on one, a few or many proteins is crucial.

References (abridged):

1. Breman, J. G., Egan, A. & Keusch, G. T. Am. J. Trop. Med. Hyg. 64 (Suppl. 1–2), iv–vii (2001).

2. Clyde, D. F., Most, H., McCarthy, V. C. & Vanderberg, J. P. Am. J. Med. Sci. 266, 169–177 (1973).

3. Rieckmann, K. H., Carson, P. E., Beaudoin, R. L., Cassells, J. S. & Sell, K. W. Trans. R. Soc. Trop. Med. Hyg. 68, 258–259 (1974).

4. Hoffman, S. L. et al. J. Infect. Dis. 185, 1155–1164 (2002).

5. Luke, T. C. & Hoffman, S. L. J. Exp. Biol. 206, 3803–3808 (2003).

6. Kumar, K. A. et al. Nature 444, 937–940 (2006).

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