Complexity Digest 2000.50

11-Dec-2000

For individual e-mail subscriptions go to Subscriptions.
Previous issue 2000.49 | Next issue 2000.51

Content

  1. Immune Molecules Prune Neural Links, Science
  2. Synaptic Efficacy and the Transmission of Complex Firing Patterns Between Neurons, J. Neurophysiol.
  3. Maze Navigation by Honeybees: Learning Path Regularity, Learn. Mem.
  4. Gene Mutation Extends Lifespan In "I'm Not Dead Yet" Fruitflies, Natl. Inst. Aging/Science Daily
  5. Europe's Meeting Of Unequals, Financial Times
  6. Have We Overdone Deregulation and Privatization?, HBS Working Knowledge.
  7. Unchained Value: The New Logic of Digital Business, HBS Working Knowledge.
  8. Common Knowledge: How Companies Thrive by Sharing What They Know, HBS Working Knowledge
  9. The Causes of 20th Century Warming, Science
  10. Low Clouds and Cosmic Rays, Science/ Phys.Rev. Let.
  11. Models Of Division Of Labor In Social Insects, Annu. Rev. Entomol.
  12. Earth's Continental Land Masses Created In Short, Fast Bursts, Science Daily
  13. Diet And The Evolution Of = The Earliest Human Ancestors, PNAS
  14. Testing A Biosynthetic = Theory Of The Genetic Code: Fact Or Artifact?, PNAS
  15. RNA Editing Process Plays Essential Role In Embryo Development, Science Daily
  16. High-Resolution Inkjet Printing of All-Polymer Transistor Circuits, Science
  17. Synchronization of Randomly Multiplexed Chaotic Systems with Application to Communication, Phys.Rev.Lett.
  18. Network Robustness and Fragility: Percolation on Random Graphs, Phys.Rev.Lett.
  19. Theoretical Analysis of a Dripping Faucet, Phys.Rev.Lett.

  1. Immune Molecules Prune Neural Links, Science Next Article Bookmark and Share

    Summary: (1) In work described on page 2155, a team of neuroscientists suggests that theclass I major histocompatibility complex proteins, previously known for their role in controlling immune responses, also play a role in the nervous system. They are necessary for the formation of normal neuronal connectionsin a visual area of the brain during development, and later in life, they're called into play in the hippocampus, a brain area involved in memory and learning. The work shows a completely unexpected function for the immune system molecules.Brain-Building MHCs In some ways, the nervous system and the immune system solve similar problems: They both have to distinguish and respond to an extremely large array of input from the external world, and both are exceedingly complex. Huh et al. (p. 2155; see the news story by Helmuth) show that class I majorhisto compatibility complex (MHC) molecules, used by the immune system to respond to antigens, are also necessary for accurate assembly of the brain.In mice genetically deficient for class I MHC molecules, the neural connections between the retina and their targets in the central nervous system are abnormal. Long-term potentiation, a form of cellular learning, is enhanced, and another form, long-term depression, is eliminated. The diversity and specificity of class I MHC molecules makes them attractive candidates for a role in establishing neural connections.

    (2) Brain-Building MHCs

    In some ways, the nervous system and the immune system solve similar problems: They both have to distinguish and respond to an extremely large array of input from the external world, and both are exceedingly complex. Huh et al. (p. 2155; see the news story by Helmuth) show that class I major histocompatibility complex (MHC) molecules, used by the immune system to respond to antigens, are also necessary for accurate assembly of the brain.

    In mice genetically deficient for class I MHC molecules, the neural connections between the retina and their targets in the central nervous system are abnormal. Long-term potentiation, a form of cellular learning, is enhanced, and another form, long-term depression, is eliminated. The diversity and specificity of class I MHC molecules makes them attractive candidates for a role in establishing neural connections.


  2. Synaptic Efficacy and the Transmission of Complex Firing Patterns Between Neurons, J. Neurophysiol. Next Article Bookmark and Share

    Abstract: In central neurons, the summation of inputs from presynaptic cells combined with the unreliability of synaptic transmission produces incessant variations of the membrane potential termed synaptic noise (SN). These fluctuations, which depend on both the unpredictable timing of afferent activities and quantal variations of postsynaptic potentials, have defied conventional analysis. We show here that, when applied to SN recorded from the Mauthner (M) cell of teleosts, a simple method of nonlinear analysis reveals previously undetected features of this signal including hidden periodic components. The phase relationship between these components is compatible with the notion that the temporal organization of events comprising this noise is deterministic rather than random and that it is generated by presynaptic interneurons behaving as coupled periodic oscillators. Furthermore a model of the presynaptic network shows how SN is shaped both by activities in incoming inputs and by the distribution of their synaptic weights expressed as mean quantal contents of the activated synapses. In confirmation we found experimentally that long-term tetanic potentiation (LTP), which selectively increases some of these synaptic weights, permits oscillating temporal patterns to be transmitted more effectively to the postsynaptic cell. Thus the probabilistic nature of transmitter release, which governs the strength of synapses, may be critical for the transfer of complex timing information within neuronal assemblies.

  3. Maze Navigation by Honeybees: Learning Path Regularity, Learn. Mem. Next Article Bookmark and Share

    Abstract: We investigated the ability of honeybees to learn mazes of four types: constant-turn mazes, in which the appropriate turn is always in the same direction in each decision chamber; zig-zag mazes, in which the appropriate turn is alternately left and right in successive decision chambers; irregular mazes, in which there is no readily apparent pattern to the turns; and variable irregular mazes, in which the bees were trained to learn several irregular mazes simultaneously. The bees were able to learn to navigate all four types of maze. Performance was best in the constant-turn mazes, somewhat poorer in the zig-zag mazes, poorer still in the irregular mazes, and poorest in the variable irregular mazes. These results demonstrate that bees do not navigate such mazes simply by memorizing the entire sequence of appropriate turns. Rather, performance in the various configurations depends on the existence of regularity in the structure of the maze and on the ease with which this regularity is recognized and learned.

  4. Gene Mutation Extends Lifespan In "I'm Not Dead Yet" Fruitflies, Natl. Inst. Aging/Science Daily Next Article Bookmark and Share

    Excerpt: Mutating a single gene can double the lifespan of fruitflies from 37 days to between 69 and 71 days, while maintaining a high level of functioning and fertility. This finding of a research team led by Stephen L. Helfand was supported in part by the National Institute on Aging (NIA), part of the National Institutes of Health. Their study is reported in the December 14 issue of Science. The gene complex was named Indy as a joking reference to the tag line from Monty Python and the Holy Grail, "I'm not dead yet." This is the third mutation in the fruitfly genome that is reported to extend lifespan. According to Helfand, the Indy gene is associated with the way that the body stores and uses energy.

    The gene is named "Indy" in homage to Monty Python and the Holy Grail's tag line, "I'm not dead yet," uttered by a supposed plague victim being hauled off for burial while still alive. (JPEG file available on request.)

    The researchers speculate that the way the Indy gene mutation works to extend life and health may be via changes in the normal metabolism of food. This link between altered metabolism and life-span extension became the focus of Helfand's studies when other laboratories showed that research animals receiving full nutrition but lowered calorie intake, or caloric restriction, lived longer. Although the mechanism by which caloric restriction benefits longevity is not understood, Dr. Helfand suggests that it is likely to involve changes in energy utilization. The Indy fruitfly differs from other long-lived fruitflies by the direct, rather than indirect, action of the altered gene on metabolism and the use of food energy.

    "What is interesting about this line of research is the recurrence of the link between metabolism, caloric restriction and longevity. This study points to the possibility that if you genetically alter metabolism, you can alter lifespan," said Dr. David Finkelstein, research director for metabolic regulation research at the National Institute on Aging.

    "While there is an 80 percent homology between the fruitfly and human genomes, we are still many steps away from understanding how caloric restriction may affect human lifespan," Finkelstein said.


  5. Europe's Meeting Of Unequals, Financial Times Next Article Bookmark and Share

    The problem of how to come to a legitimate method to determine the democratic decision of the people of a modern society has not only been painfully demonstrated in the pitiful processes related to the counting of votes. The idealistic "one person, one vote" principle has not been implemented not only in the US. There, due to the antiquated electoral system e.g. a vote for president in Alaska counts three times as much as a vote in Massachusetts. But not only the US uses thresholds to introduce non-linearites in the vote-counting process. The same problem causes trouble in Europe where European matters are decided by votes assigned to countries and not to people. That raises the problem how to take into account the different population sizes in different countries.

    Excerpt: As for the twin aims of efficiency and legitimacy, the complex solutions proposed at the summit seem more likely to hinder than help. With all the variations of voting weights for big and small, increased thresholds necessary to reach a qualified majority decision on any issue, and failure to extend significantly the subjects on which majority voting can be used, future decision-making is likely to be more difficult, not less. By opting for greater complexity in order to reconcile their differences, the EU leaders will make their system less transparent, and less open to democratic control, whether by the European parliament, or by national parliaments. Simplification of the rules should have been the order of the day. It was not."


  6. Have We Overdone Deregulation and Privatization?, HBS Working Knowledge. Next Article Bookmark and Share

    Excerpt: During the Jimmy Carter administration, Congress enacted legislation that had become known as "the Federal Express bill." It was designed to test the idea of deregulation by allowing air freight carriers to fly planes of any size on any routes, without federal price controls.

    The bill accommodated the persistent lobbying activities of one Fred Smith, the young CEO of Federal Express, who had been required under previous regulation to use small, inefficient aircraft to transport freight or else submit to stringent government regulation. It was regarded by Congress as an experiment carried out in a small, obscure industry which, if unsuccessful, would have little economic impact.

    Little did Congress realize that true believers in deregulation, like Alfred Kahn, a Cornell economics professor whom Carter had appointed as chairman of the Civil Aeronautics Board, would champion the extension of the idea to the entire airline industry, and then into areas such as brokerage fees and other professional services.

    In recent years, the deregulation movement has spread to industries with which consumers interact daily, such as electric power and telephone service, whose dependability and equitable pricing were generally left to government and not thought much about.


  7. Unchained Value: The New Logic of Digital Business, HBS Working Knowledge. Next Article Bookmark and Share

    Excerpt: In Unchained Value, Internet expert Mary Cronin introduces a radically new strategic model for organization that she calls the "digital value system." It is focused not on static, internally focused "chains" but on dynamic, external webs of relationships that take full advantage of the power, flexibility, and opportunity of the digital arena.

    One of the keys to the new model, she writes, is an understanding that the old strategy of hoarding information to maximize its value is no longer appropriate. "The Internet undermines competitive strategy based on information scarcity," she writes, "by making every Web site into a potential channel for free distribution of anything and everything that can be transmitted digitally."


  8. Common Knowledge: How Companies Thrive by Sharing What They Know, HBS Working Knowledge Next Article Bookmark and Share

    Excerpt: Pervading the idea of knowledge sharing are three myths. Perhaps myth is the wrong term—maybe they are just assumptions that seem reasonable at first glance, but when acted on send organizations to a dead end. Many of the organizations I studied started with one or more of these assumptions and then had to make corrections to get back on track. The three myths are (1) build it and they will come, (2) technology can replace face-to-face, and (3) first you have to create a learning culture.

    Managers who want to make the knowledge in their organizations more available often have a mental image of a large warehouse that contains all of that knowledge. They envision those who are looking for knowledge going to the warehouse and taking out what they need. The idea has a lot of intuitive appeal. Knowledge seems so amorphous that the notion of its being documented and located in a central place offers a comforting sense of control and manageability.


  9. The Causes of 20th Century Warming, Science Next Article Bookmark and Share

    Summary: Global air surface temperatures increased by about 0.6°C during the 20th century, but as Zwiers and Weaver discuss in their Perspective, the warming was not continuous. Two distinct periods of warming, from 1910 to 1945 and since 1976, were separated by a period of very gradual cooling. The authors highlight the work by Stott et al., who have performed the most comprehensive simulation of 20th century climate to date. The agreement between observed and simulated temperature variations strongly suggests that forcing from anthropogenic activities, moderated by variations in solar and volcanic forcing, has been the main driver of climate change during the past century.

    Global annual mean near-surface air temperature increased during the 20th century in two major steps, the first between roughly 1910 and 1940 and the second (which is still continuing) after about 1975. It has been difficult to understand the causes of this overall rise, partly because anthropogenic forcing by fossil fuel combustion has grown steadily during that interval and partly because it was not as important a forcing factor in the first half of the century as in the second. Stott et al. (p. 2133; see the Perspective by Zwiers and Weaver) have used a state-of-the-art climate model, HadCM3, to examine the reasons for this increase. An ensemble of four simulations of the last 140 years indicates that a combination of natural climate variations and human-induced variability can explain the observed temperature rise, andthat most of the multidecadal-scale global variations are not due to internal variability of Earth's climate system, but are externally forced.


  10. Low Clouds and Cosmic Rays, Science/ Phys.Rev. Let. Next Article Bookmark and Share

    Summary: If the study of global climate change were a card game, one of the wild cards would be the role of clouds. Clouds are a primary influence on the energy budget of Earth's surface and atmosphere because of their effects on the reflection and absorption of solar radiation and their trapping of outgoing long-wave radiation. Clouds differ in their radiative properties, however, and the complexity of cloud formation is greater than our understanding of all of the factors that control their distribution and composition. Solar cosmic rays may influence global cloud cover because they can ionize atmospheric particles and thus create condensation nuclei for cloud droplet formation. The terrestrial cosmic ray flux depends on solar output and is modulated by Earth's magnetic field; both of these quantities are known to vary.

    Marsh and Svensmark have measured global average monthly cloud anomalies for lower, middle, and upper troposphere, and correlated them with changes in the cosmic ray flux. They found, surprisingly, that cloud cover at altitudes of less than 3.2 kilometers covaries with cosmic ray fluxes from 1980 to 1995, but no correlation was seen for higher altitude clouds. If this relation is systematic, cosmic ray variability could have a significant effect on the evolution of climate.

    • Low Clouds And Cosmic Rays, H. Jesse Smith, Science, 290(5499): 12/15/00, p. 2033b
    • Marsh and Svensmark, Phys. Rev. Lett. 85, 5004 (2000

  11. Models Of Division Of Labor In Social Insects, Annu. Rev. Entomol. Next Article Bookmark and Share

    Abstract: Division of labor is one of the most basic and widely studied aspects of colony behavior in social insects. Studies of division of labor are concerned with the integration of individual worker behavior into colony level task organization and with the question of how regulation of division of labor may contribute to colony efficiency.

    Here we describe and critique the current models concerned with the proximate causes of division of labor in social insects. The models have identified various proximate mechanisms to explain division of labor, based on both internal and external factors. On the basis of these factors, we suggest a classification of the models. We first describe the different types of models and then review the empirical evidence supporting them.

    The models to date may be considered preliminary and exploratory; they have advanced our understanding by suggesting possible mechanisms for division of labor and by revealing how individual and colony-level behavior may be related. They suggest specific hypotheses that can be tested by experiment and so may lead to the development of more powerful and integrative explanatory models.


  12. Earth's Continental Land Masses Created In Short, Fast Bursts, Science Daily Next Article Bookmark and Share

    Excerpts: Scientists believe they have unraveled one of geology's most enduring mysteries about how the Earth's continental crust was built, and they say it happened in a relative blink of an eye.

    According to Alexander Cruden, associate professor of geology at the University of Toronto and second author of the paper to appear in the Dec. 6 issue of Nature, the way that granite forms - a rock that makes up about 70 to 80 per cent of the Earth's continental crust - is not the sluggish, multi-million year process that scientists previously believed. In fact, Cruden and his co-authors argue that the process occurs in rapid, dynamic and possibly catastrophic events that take between 1,000 and 100,000 years, depending on the size of the granite intrusion. And that's changing how scientists look at the formation of the Earth's continents. (…)

    The researchers used experimental studies that involved melting rock samples to understand how granite magma initially forms in the upper mantle and lower crust and how fast it can move. That data was then applied to theoretical models to determine its method and rate of ascension. New models for the emplacement stage - where the granite is intruded into older rock in the upper crust - are based on a combination of theoretical studies and fieldwork in areas such as the Canadian Shield, Sweden, the Sierra Nevada of California, Greenland and the Andes of South America. A unique aspect of the research is that the three main stages of granite formation - generation, ascent and emplacement - are regarded together as a system. Historically, these processes have been studied by different geological specialists in isolation from each other.

    Cruden likens the granite formation process to subterranean volcanic eruptions. Like Lego blocks built on top of one another, large parts of the Earth's continental land masses were created by tens of thousands of quick eruptions or bursts of molten magma that were transferred rapidly from the mantle and lower-most crust and then injected as large horizontal sheets into the upper crust. These sheets then cooled and crystallized to form the large granite intrusions that we see exposed at the surface of all continents today, he says.

    The Earth's continents began forming approximately four billion years ago, Cruden explains. "This research has important implications for how we understand the basic physics and chemistry of crust formation processes as well as the formation of economic ore deposits - gold and copper, for example - many of which are associated with granite intrusions."


  13. Diet And The Evolution Of = The Earliest Human Ancestors, PNAS Next Article Bookmark and Share

    Abstract: Over the past decade, discussions of the evolution of the earliest human ancestors have focused on the locomotion of the australopithecines. Recent discoveries in a broad range of disciplines have raised important questions about the influence of ecological factors in early human evolution. Here we trace the cranial and dental traits of the early australopithecines through time, to show that between 4.4 million and 2.3 million years ago, the dietary capabilities of the earliest hominids changed dramatically, leaving them well suited for life in a variety of habitats and able to cope with significant changes in resource availability associated with long-term and short-term climatic fluctuations.


  14. Testing A Biosynthetic = Theory Of The Genetic Code: Fact Or Artifact?, PNAS Next Article Bookmark and Share

    Abstract: It has long been conjectured that the canonical genetic code evolved from a simpler primordial form that encoded fewer amino acids [e.g., Crick, F. H. C. (1968) J. Mol. Biol. 38, 367-379]. The most influential form of this idea, "code coevolution" [Wong, J. T.-F. (1975) Proc. Natl. Acad. Sci. USA 72, 1909-1912], proposes that the genetic code coevolved with the invention of biosynthetic pathways for new amino acids. It further proposes that a comparison of modern codon assignments with the conserved metabolic pathways of amino acid biosynthesis can inform us about this history of code expansion. Here we re-examine the biochemical basis of this theory to test the validity of its statistical support. We show that the theory's definition of "precursor-product" amino acid pairs is unjustified biochemically because it requires the energetically unfavorable reversal of steps in extant metabolic pathways to achieve desired relationships. In addition, the theory neglects important biochemical constraints when calculating the probability that chance could assign precursor-product amino acids to contiguous codons. A conservative correction for these errors reveals a surprisingly high 23% probability that apparent patterns within the code are caused purely by chance. Finally, even this figure rests on post hoc assumptions about primordial codon assignments, without which the probability rises to 62% that chance alone could explain the precursor-product pairings found within the code. Thus we conclude that coevolution theory cannot adequately explain the structure of the genetic code.

  15. RNA Editing Process Plays Essential Role In Embryo Development, Science Daily Next Article Bookmark and Share

    Excerpt: In a new study, scientists at The Wistar Institute report the first direct evidence that RNA editing is essential to mammalian embryo development. RNA editing is a normal but not yet fully understood process in which small nucleotide changes occur after DNA has been transcribed into RNA. The process makes it possible for one gene to be translated into multiple proteins with different structures or functions.

    The researchers repeatedly attempted to delete, or knock out, in mice a gene known to be involved in RNA editing called ADAR1 in order to study its function. Certain target genes in the brain are known to be subjected to RNA editing by the ADAR1 enzyme, including glutamate receptor ion channels, critical for memory formation, and serotonin receptors, which regulate emotional behaviors. The investigators expected that deletion of the ADAR1 gene would therefore lead to significant changes in brain functions.

    Unexpectedly, however, they found that the knockout mouse embryos died midterm due to an inability to make mature red blood cells. At the least, the results suggest that ADAR1 and RNA editing are critical to the development of mature red blood cells, an essential step in mammalian embryo development. The new findings were published in the December 1 issue of Science.

    "The inability of mice with a defective RNA editing system to make mature red blood cells is likely just the tip of the iceberg," says Wistar professor Kazuko Nishikura, Ph.D., senior author on the study. "The ADAR1 gene is expressed in many tissues throughout the body in addition to the brain and is probably involved in the RNA editing of a number of target genes that have not yet been identified."

    As scientists prepare to enter the post-genomic era, the role of RNA editing in determining protein structure and function may become an increasingly important consideration in genetic research. Investigations such as Nishikura's indicate that RNA editing is fundamental to key biological processes and point to the complexity of predicting protein structures and functions from gene sequences alone.

    Before the discovery of RNA editing in mammals in the 1990s, it was believed that the path from DNA to protein was fairly straightforward: DNA in a cell's nucleus is transcribed to RNA, and then sometimes shortened to splice out noncoding sections to form mature messenger RNA. The mature messenger RNA is transported to the cell's cytoplasm, where translation to protein occurs.

    But researchers learned that mammalian protein production can be more complicated; some RNA is edited prior to translation. Single or multiple nucleotides may change before the mature messenger RNA moves into the cytoplasm, leading ultimately to the production of a protein that does not fully reflect the original genetic instructions in the DNA. RNA editing is, in a sense, an economical system, enabling one gene to produce a number of proteins with different structures or functions. Scientists believe that the known target genes represent only a fraction of those that are subjected to RNA editing.

    Nishikura and her co-investigators aimed to produce a mouse line lacking the gene ADAR1, which is part of a small gene family that produces enzymes involved in the RNA editing of a number of target genes. Midway through development, however, the mouse embryos lacking the ADAR1 gene died, which surprised the researchers; often, gene knockout mice are born alive because the mother supplies necessary biological functions to its embryos. Further study revealed that the embryos died due to an inability to produce mature red blood cells.


  16. High-Resolution Inkjet Printing of All-Polymer Transistor Circuits, Science Next Article Bookmark and Share

    Abstract: Although inkjet printing has been used to fabricate low-cost organic light emitting diodes and displays, its application to more complex thin film transistors (TFTs) has been hampered by its resolution limit (~50 micrometers). In order to maintain sufficient drive-current and switching times and reasonable turn-on voltages in polymer TFTs, channel lengths lessthan 10 micrometers are needed. Sirringhaus et al. (p. 2123) introduce a method using a prepatterned substrate in which the hydrophobic properties of polyimide are used to define the critical features. They demonstrate the ability to fabricate TFTs with 5-micrometer channel lengths and on-off ratios in excess of 105 at operating voltages of 10 volts.


  17. Synchronization of Randomly Multiplexed Chaotic Systems with Application to Communication, Phys.Rev.Lett. Next Article Bookmark and Share

    Abstract: Synchronized chaotic systems have recently been applied tothe area of secure communications in a variety of ways. At the same time, there have also been significant advances in deciphering messages masked by chaotic signals. It is important, therefore, to explore more secure approaches to using chaos in communication. We show that multiple chaotic systems can be synchronized through a scalar coupling which carries a stochastic signal generated by random multiplexing of the source systems. This approach, which is a variant of the active-passive decomposition method, promises enhanced security in chaos-based communication.


  18. Network Robustness and Fragility: Percolation on Random Graphs, Phys.Rev.Lett. Next Article Bookmark and Share

    Abstract: Recent work on the Internet, social networks, and the power grid has addressed the resilience of these networks to either random or targeted deletion of network nodes or links. Such deletions include, for example, the failure of Internet routers or power transmission lines. Percolation models on random graphs provide a simple representation of this process but have typically been limited to graphs with Poisson degree distribution at their vertices. Such graphs are quite unlike real-world networks, which often possess power-law or other highly skewed degree distributions. In this paper we study percolation on graphs with completely general degree distribution, giving exact solutions for a variety of cases, including site percolation, bond percolation, and models in which occupation probabilities depend on vertex degree. We discuss the application of our theory to the understanding of network resilience.


  19. Theoretical Analysis of a Dripping Faucet, Phys.Rev.Lett. Bookmark and Share

    A NAME=19>
    Rob Shaw's classic experiment on chaotic systems from the early 80's has been revisited in great detail:

    Abstract: While previous studies of continuous emission of drops from a faucet have shown the richness of the system's nonlinear response, a theory of dripping has heretofore been lacking. Long-time behavior of dripping is simulated computationally by tracking the formation of up to several hundred drops in a sequence, rather than the usual single drop, at a given flow rate Q and verified by experiments. As Q increases, the system evolves from a period-1 system through a number of period doubling (halving) bifurcations as dripping ultimately gives way to jetting. That hysteresis can occur is also demonstrated.


Also available in: Simple HTML format | TXT format | TXT format with links | Print