ScienceWeek

SCIENCEWEEK

March 9, 2007

Vol. 11 - Number 10

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The most modest research worker at his bench, pushing a probe into a neuron to measure the electric response when a light is flashed, is enmeshed in a large and intertwined network of theories that he carries into his work from the whole field of science, all the way from Ohm's law to Avogadro's number. He is not alone; he is sustained and held and in some sense imprisoned by the state of scientific theory in every branch. And what he finds is not a single fact either: It adds a thread to the network, ties a knot here and another there, and by these connections at once binds and enlarges the whole system.

-- Jacob Bronowski (1908-1974)

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Contents (full text below):

1. Neuroscience: Odor Cues During Slow-Wave Sleep Prompt Declarative Memory Consolidation

2. Physics: Watching Rush Hour in the World of Electrons

3. Evolutionary biology: The Elvis paradox

4. Planetary science: Water cycling on Mars

5. Neuroscience: Neuronal polarity -- From Extracellular Signals to Intracellular Mechanisms

6. Cognitive Science: Understanding Chimpanzee Facial Expression -- Insights into the Evolution of Communication

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1. Science 9 March 2007: Vol. 315. no. 5817, pp. 1426 - 1429

DOI: 10.1126/science.1138581

Odor Cues During Slow-Wave Sleep Prompt Declarative Memory Consolidation

Björn Rasch,1* Christian Büchel,2 Steffen Gais,1 Jan Born1*

Sleep facilitates memory consolidation. A widely held model assumes that this is because newly encoded memories undergo covert reactivation during sleep. We cued new memories in humans during sleep by presenting an odor that had been presented as context during prior learning, and so showed that reactivation indeed causes memory consolidation during sleep. Re-exposure to the odor during slow-wave sleep (SWS) improved the retention of hippocampus-dependent declarative memories but not of hippocampus-independent procedural memories. Odor re-exposure was ineffective during rapid eye movement sleep or wakefulness or when the odor had been omitted during prior learning. Concurring with these findings, functional magnetic resonance imaging revealed significant hippocampal activation in response to odor re-exposure during SWS.

1 Department of Neuroendocrinology, University of Lübeck, Ratzeburger Allee 160/23a, 23538 Lübeck, Germany. 2 NeuroImage Nord, Department of Systems Neuroscience, University Medical Center Hamburg-Eppendorf, Martinistrasse 52, 20246 Hamburg, Germany.

Sleep facilitates the consolidation of newly acquired memories for long-term storage (1–3). The prevailing model assumes that this consolidation relies on a covert reactivation of the novel neuronal memory representations during sleep after learning (3–6). In rats, hippocampal neuronal assemblies implicated in the encoding of spatial information during maze learning are reactivated in the same temporal order during slow-wave sleep (SWS) as during previous learning (7, 8). The consolidation of hippocampus-dependent memories benefits particularly from SWS (9–11), and reactivation of the hippocampus in SWS after spatial learning has also been seen in humans observed with positron emission tomography (12). However, none of these studies experimentally manipulated memory reactivation during sleep. Therefore, its causal role in memory consolidation is still unproven.

We used an odor to reactivate memories in humans during sleep, because odors are well known for their high potency as contextual retrieval cues not only for autobiographic memories, as delicately described in Marcel Proust's Remembrance of Things Past, but also for various other types of memory, including visuospatial memories (13, 14). Notably, in the brain, primary olfactory processing areas bypassing the thalamus project directly to higher-order regions, including the hippocampus (15), which enables them to modulate hippocampus-dependent declarative memories (16). The use of olfactory stimuli for cueing memories during sleep is particularly advantageous because odors, in contrast to other stimuli, can be presented without disturbing ongoing sleep (17).

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2. Science 9 March 2007: Vol. 315. no. 5817, pp. 1372 - 1373

DOI: 10.1126/science.1139805

Perspectives

PHYSICS: Watching Rush Hour in the World of Electrons

Jan Zaanen*

During the last decade, a revolution has been unfolding in our understanding of the behavior of electrons in solids. Quantum mechanics rules in this microscopic world, and researchers assumed that the smearing and averaging effects of the quantum motions would render this behavior exceedingly simple. In studying high-transition temperature (Tc) superconductivity in copper oxides, however, physicists found instead that the electron systems in these materials were exceptionally complex. On page 1380 of this issue, Kohsaka et al. (1) present an experimental breakthrough, studying the electron system on the surface of copper oxide superconductors by means of scanning tunneling spectroscopy. By cleverly exploiting the effects of the electron interactions, they manage to probe the electron traffic directly. They discover a world of amazing richness, shaped by the quantum motions of the electrons forming complex spatial patterns.

A main effect of strong interactions in the classical world is well known to anybody living in a metropolitan area: When the density of cars becomes too high during rush hour, the traffic comes to a standstill. The same phenomenon should occur with electrons in solids, but the weirdness of quantum physics interferes. Electrons should execute continual quantum motions, and these are so violent in conventional metals and superconductors that the effects of the interactions are washed away. In this regard, the electron systems found in high-Tc superconductors are exceptional. Due to the strong potentials exerted on the electrons by the crystal lattice of copper oxide planes, the quantum motions are hindered to a degree that the electron traffic might even get completely jammed. Copper oxides in their pristine state are thus insulators and, in order to turn them into (super)conductors, one has to remove electrons by chemical doping (that is, the addition of impurity elements).

The electron motions in these doped cuprates can be viewed as quantized stop-and-go traffic. Stop-and-go traffic in our world tends to develop complex collective patterns, and something similar happens with the electrons. In calculations, physicists found that electrons moving around in the copper oxide planes tend to arrange themselves in one-dimensional "highways" where they move rather easily, surrounded by insulating domains (2), and in recent years experimental support was found for the existence of such stripe patterns (3). There are indications that they occur in good superconductors as fluctuating patterns (3, 4), while in some cuprates they actually come to a standstill, likely due to imperfections in the crystal lattice having an effect similar to roadwork on our highways. These static stripes are clearly observed in both neutron-scattering (3) and resonant x-ray-scattering experiments (5), but these experiments only pick up average properties of the stripes. Researchers would like to view them in real space (as opposed to the reciprocal space of diffraction), and this is exactly what Kohsaka et al. claim to have accomplished.

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3. Nature 446, 147-149 (8 March 2007) | doi:10.1038/446147a; Published online 7 March 2007

Evolutionary biology: The Elvis paradox

Andrew Hendry1

Disagreement has long swirled around the relative importance of various forces that might drive evolution on timescales ranging from dozens to millions of generations. Writing in The American Naturalist, Estes and Arnold1 offer a provocative contribution to this debate: they propose that evolutionary changes on all timescales might be explained by a single, simple model of adaptation.

Much of the challenge can be distilled down to what has been called the 'paradox of stasis'2. For me, the most obvious manifestation of this paradox is that neo-darwinian theory, with its emphasis on the power of selection, predicts the potential for rapid adaptation, whereas most lineages of organisms instead show long-term stasis: that is, very little cumulative change over long periods of time3, 4. Several hypotheses have been advanced in the hope of resolving this seeming discontinuity between short- and long-term evolution2, 4, 5, but none has been convincing enough to resonate across the various camps.

Estes and Arnold1 point out that the best way to discriminate between the hypotheses is to confront the predictions of alternative evolutionary models with the reality of data. This sort of comparison has recently been made possible by compilations of data on phenotypic changes (such as in mean body size) within animal lineages at a variety of different timescales4, 6, 7. The pattern emerging from these data is that phenotypic changes over dozens of generations can range from small to large, and that this range remains roughly the same even over millions of generations (Fig. 1). This pattern thus affirms the original paradox — that phenotypic change can be dramatic on short timescales, but rarely accumulates into substantial evolutionary trends.

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4. Nature 446, 150-151 (8 March 2007) | doi:10.1038/446150b; Published online 7 March 2007

Planetary science: Water cycling on Mars

Victor R. Baker1

A succession of sophisticated spacecraft missions has led to spectacular advances in the understanding of Mars' global hydrology over the past few decades. One of many examples is the discovery of abundant hydrated sulphate salt minerals. These minerals are found at many locations on the planet — most notably at Meridiani Planum, the landing site of NASA's robotic Mars rover Opportunity — and prove that water must once have been abundant on the surface of Mars. On page 163 of this issue, Andrews-Hanna et al.1 use a numerical model to simulate the evolving global flow of subsurface groundwater early in Mars' geological history. They place their simulation in the context of the formation of the enormous volcanic uplift feature known as Tharsis.

One way of developing a model of martian hydrology comes from a comparison with what we know about Earth. Western science was painfully slow in achieving its understanding of Earth's hydrological cycle. Many, if not most, of Isaac Newton's scientific contemporaries held the view that Earth's rivers were ultimately fed from upland springs. The springs were presumed to discharge water from within the planet, in much the same way as blood flows from a cut in an artery of the human body. Water from the oceans was presumed to return to the land through subsurface veins.

By contrast, Eastern philosophical writings had long held that Earth's water flowed as part of a great cycle involving the atmosphere. About 3,000 years ago, the Vedas, Hindu texts of ancient India, explained Earth's water movements in terms of cyclical processes of evaporation, condensation, cloud formation, rainfall, river flow and water storage2. This concept of a global water cycle entered Western thought only late in the seventeenth century, when Edmond Halley, among others, showed that the evaporation from Earth's oceans supplied the rain clouds that led to a balancing run-off of water from land to the seas. Specifically, Halley compared evaporation from the Mediterranean Sea with estimates of its river inflow, thereby providing a modern scientific expression of the hydrological cycle.

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5. Nature Reviews Neuroscience 8, 194-205 (March 2007) | doi:10.1038/nrn2056

Neuronal polarity: from extracellular signals to intracellular mechanisms

Nariko Arimura1 and Kozo Kaibuchi1 About the authors

Summary

* Neurons are highly polarized cells, and most develop a single axon and several dendrites. These two compartments acquire specific characteristics that enable neurons to transmit electrical signals. Evolutionarily conserved signalling cascades participate in the initial events of neuronal polarization.

* Axonal outgrowth is the initial event of neuronal polarization in cultured hippocampal neurons. Each neurite has a dynamic motility during neuronal polarization, and the polarization mechanism involves positive and negative feedback.

* Extracellular signals, such as netrin and WNTs, regulate axonal orientation in Caenorhabditis elegans tissue. These extracellular signals might regulate the internal polarization programme.

* Signalling cascades downstream of phosphatidylinositol 3-kinase (PI3K) have been shown to have central roles in neuronal polarization. These signalling pathways involve Akt and glycogen synthase kinase 3beta (GSK3beta), the mitogen-activated protein kinase (MAPK) cascade and small GTPases.

* The PAR protein complex accumulates at the tips of axons, and mediates signalling from Cdc42 to Rac1, which influences the dynamics of actin filaments at the tip of the axon to enhance rapid axon outgrowth. Given that Rac1 can activate PI3K, this activation might form a positive feedback loop, which could sustain activity.

* Akt signalling to GSK3beta decreases the level of phosphorylation of microtubule-associated proteins, and thereby stabilizes microtubules.

* Overexpression of Crmp2 induces the formation of multiple axons. CRMP2 associates with tubulin dimers, numb and kinesin 1, and so might regulate the fate of axons and dendrites through a variety of mechanisms.

* Microtubule-based molecular transport might be a basic way of specifying axonal or dendritic fate. The directional transport of axonal molecules has essential roles in the initial event of neuronal polarization.

Author affiliations

1. Department of Cell Pharmacology, Graduate School of Medicine, Nagoya University, 65, Tsurumai, Showa, Nagoya, Aichi 466-8550, Japan.

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6. Social Cognitive and Affective Neuroscience 2006 1(3):221-228; doi:10.1093/scan/nsl031

Understanding chimpanzee facial expression: insights into the evolution of communication

Lisa A. Parr1 and Bridget M. Waller2

1Division of Psychiatry and Behavioral Sciences & Yerkes National Primate Research Center, Emory University, Atlanta, GA, USA and 2Department of Psychology, University of Portsmouth, Portsmouth UK

To understand the evolution of emotional communication, comparative research on facial expression similarities between humans and related species is essential. Chimpanzees display a complex, flexible facial expression repertoire with many physical and functional similarities to humans. This paper reviews what is known about these facial expression repertoires, discusses the importance of social organization in understanding the meaning of different expressions, and introduces a new coding system, the ChimpFACS, and describes how it can be used to determine homologies between human and chimpanzee facial expressions. Finally, it reviews previous studies on the categorization of facial expressions by chimpanzees using computerized tasks, and discusses the importance of configural processing for this skill in both humans and chimpanzees. Future directions for understanding the evolution of emotional communication will include studies on the social function of facial expressions in ongoing social interactions, the development of facial expression communication and more studies that examine the perception of these important social signals.

NATURAL ETHOLOGY OF FACIAL EXPRESSION: FORM AND FUNCTION

Many animal species communicate using a variety of highly conspicuous signals, including acoustic, tactile, olfactory and visual displays that have been tuned by natural selection to impact the listener in a reliable way (Smith, 1977; Dawkins and Krebs, 1978). Among primates, the visual and auditory domains have become the two most prominently involved in social communications. There is, for example, a general assumption that facial expressions convey a variety of information about an individual's motivation, intentions and emotions (van Hooff, 1967; Ekman, 1997; Parr, et al., 2002). As such, facial expressions are critically important for coordinating social interaction, facilitating group cohesion and maintaining individual social relationships (Parr et al., 2002). Despite the importance of facial expressions in the evolution of complex societies, there is little work comparing either the form or function of facial expression across distinct phylogenetic groups. Moreover, apart from a handful of excellent ethograms that describe the communication repertoires for a variety of species, including chimpanzees, bonobos, rhesus monkeys, capuchin monkeys and canids (Hinde and Rowell, 1962; van Hooff, 1962, 1967, 1973; Andrew, 1963; Goodall, 1968; Fox, 1969; Bolwig, 1978; Weigel, 1979; de Waal, 1988; Preuschoft and van Hooff, 1997; Redican, 1982; Parr, et al., 2005), there has been little attempt to standardize the description of their facial and vocal displays in a manner that facilitates comparative and evolutionary studies.

Fridlund (1994) has proposed that facial expressions are best understood as communicative signals and as such, researchers should focus on their functional consequences during social interaction, or how they impact the listener. Moreover, the only way to fully understand why facial expressions have evolved to convey a specific meaning is to compare similar facial expressions between evolutionarily related species, examining any factors that may have influenced their social function, including ecological pressures and social factors like dominance style and social organization (Preuschoft and van Hooff, 1997). Some facial expressions appear to be well represented across diverse taxonomic groups, making them good models for understanding social and emotional function, while others appear to be species-specific. The bared-teeth display, also referred to as the fear grin, or grimace, is one of the most conspicuous and well-studied facial expressions in ethology and has been reported in a variety of mammalian species from canids to primates. Research has shown, however, that the communicative function of this expression can differ quite broadly depending on the species, their type of social organization and social context. In wolves, for example, retraction of the lips horizontally over the teeth results in a ‘submissive grin’ which is used by cubs and subordinates when actively greeting adult conspecifics, or humans (Fox, 1969). Antithetical to this expression is a vertical lip retraction which is given by dominant animals during aggressive interactions, very similar facial movements but with vastly different social functions (Darwin, 1872; Fox, 1969).

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