Complexity Digest 2001.26

25-Jun-2001

For individual e-mail subscriptions go to Subscriptions.
Previous issue 2001.25 | Next issue 2001.27

Content

  1. Risk And Uncertainty, Nature
  2. Shift From Forest To Crops Lowers Daytime Temperatures, NCAR/Science Daily
  3. Finding New Ways To Fight Plant Diseases, Science
    1. Genetically Modified Canola Becoming A Weed, CBC News Online
  4. Life Or Death In Cells: 'Critical Effectors' Of Apoptosis, The Scientist
    1. Drug Makers On The Apoptotic Trail, The Scientist
  5. The Zombie Within, Nature
    1. Introspective Physicalism As An Approach To The Science Of Consciousness, CogPrints
  6. Finger-Tapping Reveals Internal Mechanisms, Helping Us Respond To Subliminal Changes In Stimuli, APA/Science Daily
  7. Play's The Thing, New Scientist
  8. Non-Stick Water, Nature
    1. Liquid Marbles, Nature
  9. Plasticity Near And Far, Science
    1. Synchronizing The Brain's Signals, Science
    2. Precise And Reliable Brain Trains, Science
  10. Ionic Mechanism Of Hearing, Science
  11. Neural Details Of How Barn Owls Locate Sound Sources, Physics Today
  12. Neuroscience: Awareness Of Space, Nature
    1. Spatial Awareness Is A Function Of The Temporal Not The Posterior Parietal Lobe, Nature
  13. Single Neurons In Prefrontal Cortex Encode Abstract Rules, Nature
  14. Parallel-distributed Processing In Olfactory Cortex, Chem. Senses
  15. Nanopore: High-Speed. Single-Molecule DNA Sequencing, Harvard University Website
  16. A Continuous Model Of Computation, arXiv
  17. Statistical Mechanics Of Complex Networks, arXiv
  18. Acoustic Oscillations In Accord, Science
  19. Cosmology: Math Trick May Cause Tension Headache, Science
  20. Links & Snippets
    1. Announcements

  1. Risk And Uncertainty, Nature Next Article Bookmark and Share

    Excerpts: Insofar as uncertainty enters, it is because the underlying laws, although known, manifest themselves in a sufficiently complicated way that only probabilistic statements can be made. Examples range from roulette wheels to the chaotic dynamics of local weather prediction. Given this conception, or misconception, of what science is all about, it is not surprising that people expect scientists to give them clear and unambiguous advice when new and worrying problems appear.

    Difficulties arise when the uncertainties in scientific advice to policy-makers are not caused by probabilistic predictions, (…)


  2. Shift From Forest To Crops Lowers Daytime Temperatures, NCAR/Science Daily Next Article Bookmark and Share

    Excerpt: The large-scale conversion of forests to croplands in the midwestern United States over the last century has led to a measurable cooling of the region's climate, according to Gordon Bonan of the National Center for Atmospheric Research (NCAR). The new study, which appears in the June issue of the Journal of Climate, is the first to document the link between regional climate change and a major change in temperate forest cover. NCAR's primary sponsor is the National Science Foundation.

    "Human uses of land, especially clearing of forest for agriculture and reforestation of abandoned farmland, are an important cause of regional climate change," concludes Bonan. The cooling is the result of the changeover of the region to crops, which reflect more sunlight back into space than forests.

    The impact of land-use changes on climate is currently one of the most uncertain factors contributing to climate warming, according to the latest report from the United Nations/World Meteorological Organization Intergovernmental Panel on Climate Change, issued earlier this year. Most of the work to date on this subject has been with computer models and has focused on deforestation in the tropics in areas such as the Amazon. Bonan's is one of the first observational studies on the effect of temperate forest changes on regional climate.


  3. Finding New Ways To Fight Plant Diseases, Science Next Article Bookmark and Share

    Summary: Many plant pathogens and the insects that often spread them are exploding worldwide as they overcome the pesticides, agricultural practices, and biocontrols that once held them in check. At the same time, some effective chemicals are being banned because of environmental concerns. And efficient global travel is spreading viral, bacterial, and fungal plant pathogens into new areas, while global warming is allowing insect vectors to expand their ranges. Researchers therefore must continually search for better ways to protect plants against infectious diseases, using everything from high-tech genomics and genetic engineering to new agronomic practices aimed at disrupting plant-pest interactions.

    1. Genetically Modified Canola Becoming A Weed, CBC News Online Next Article Bookmark and Share

      Excerpts: Western farmers are struggling with a new pest in their fields - a crop that was supposed to make their lives easier.

      Genetically modified (GM) canola is appearing in farmers' fields where it wasn't planted, and because the plant has been engineered to resist conventional herbicides, it's tough to kill. (…)

      But the alternative chemicals can kill farmers' intended crops, and in some cases, the GM canola appears to be resistant to the other chemicals.

      Monsanto, which created one of the GM canola strains, says that if farmers' call the company, they'll send out a team to manually pull up the weeds. But Martin Phillipson, a University of Saskatchewan law professor, said that Monsanto may be liable for damages if their GM canola continues to spread.


  4. Life Or Death In Cells: 'Critical Effectors' Of Apoptosis, The Scientist Next Article Bookmark and Share

    Excerpts: If the proteins were redundant (both serving the same regulatory function), then the mice would show evidence of a disrupted apoptotic pathway. And indeed, "when we bred the Bak-/- and Bax-/- mice together," Thompson says, "there were problems." Fewer than 10 percent of the double knockout mice reached adulthood, and those that did displayed severe phenotypic abnormalities, the most striking of which was inter-digital webbing on their paws.3 Inter-digital webbing is a tell-tale sign of the absence of apoptosis, notes Thompson. The double knockout mice also exhibited several other anatomic and neurological abnormalities associated with dysfunctional apoptotic pathways.3 Furthermore, cells that were isolated from the double knockout mice were resistant to apoptosis in response to various external cell death stimuli, including radiation, one of the classic assays for apoptosis.

    1. Drug Makers On The Apoptotic Trail, The Scientist Next Article Bookmark and Share

      Excerpt: Apoptosis, a key process in the development of embryonic tissue differentiation, later helps to regulate the normal cellular life cycle by destroying damaged cells. When something goes awry, too little apoptosis can make cancer cells resistant to chemotherapy and even death-defiant. At the other extreme, premature or excessive apoptosis has been linked to neurodegenerative diseases, such as Alzheimer's, and to nerve cell loss in strokes.

      Not surprisingly, many major pharmaceutical companies recognize the value of apoptosis research. Understanding and controlling programmed cell death is expected to yield new drug discoveries and therapeutics.

      • Drug Makers On The Apoptotic Trail, Big Pharma Contributes To The "Absolute Explosion" In Cell Death Research, Ted Agres, The Scientist 15[13]:18, Jun. 25, 2001


  5. The Zombie Within, Nature Next Article Bookmark and Share

    Excerpts: (…) what is the difference between the neuronal pathways that subserve zombie agents and the neural networks that give rise to specific, conscious perception? Both probably involve the cerebral cortex and the thalamus. Are they based on activity in different subsets of neurons, segregated according to brain areas? (…) Or might the difference be the type of neural activity involved? We hypothesized earlier that consciousness involves synchronized firing of neurons at the millisecond level, whereas uncorrelated firing can influence behavior without generating that special buzz in the head.

    1. Introspective Physicalism As An Approach To The Science Of Consciousness, CogPrints Next Article Bookmark and Share

      Abstract: Most 'theories of consciousness' are based on vague speculations about the properties of conscious experience. We aim to provide a more solid basis for a science of consciousness. We argue that a theory of consciousness should provide an account of the very processes that allow us to acquire and use information about our own mental states - the processes underlying introspection. This can be achieved through the construction of information processing models that can account for 'Type-C' processes. Type-C processes can be specified experimentally by identifying paradigms in which awareness of the stimulus is necessary for an intentional action. The Shallice (1988b) framework is put forward as providing an initial account of Type-C processes, which can relate perceptual consciousness to consciously performed actions. Further, we suggest that this framework may be refined through the investigation of the functions of prefrontal cortex. The formulation of our approach requires us to consider fundamental conceptual and methodological issues associated with consciousness. The most significant of these issues concerns the scientific use of introspective evidence. We outline and justify a conservative methodological approach to the use of introspective evidence, with attention to the difficulties historically associated with its use in psychology.

      Contributing Editor's Note: It seems that the study of consciousness faces a similar "problem" as Artificial Intelligence faced in its origins, when people tried to define intelligence, in the sense that there is no agreement in what consciousness is. But, as with Artificial Intelligence, researchers do not stop just because there is not a sharp definition of their subject of study. Some philosophers (Wittgensteing, for example) have argued that people should not "speculate" by speaking about things which are not well-defined. But if we would stop speaking about them, they would never become clearer. As Feyerabend would say, progress in science can be made precisely because people speak about things that are not well-defined.


  6. Finger-Tapping Reveals Internal Mechanisms, Helping Us Respond To Subliminal Changes In Stimuli, APA/Science Daily Next Article Bookmark and Share

    Excerpt: - Keeping up with the beat: People are quite good at it, even when the timing changes at a nearly imperceptible level. Using the well-known experimental procedure of finger-tapping to an auditory beat, researcher Bruno Repp of Haskins Laboratories in New Haven, Conn. observed that people correctly adjusted their tapping when the beat changed in a barely detectable manner, suggesting that an internal mechanism automatically guides motor actions in response to stimuli that change without our even being aware of it. This finding is reported in the June issue of the Journal of Experimental Psychology: Human Perception and Performance, published by the American Psychological Association (APA).

    Repp studied how eight people adjusted their tapping in response to 1-2 percent changes (which are very difficult to detect, and therefore considered subliminal) in a series of pulsing tones from a digital piano. Subjects tapped a key on a silent piano keyboard in or out of synchrony with the tones, as instructed, and made successful adjustments -- a process called "phase correction" -- in response to subliminal timing changes. They had not been warned that the timing might change; thus subjects stayed with the beat, even without a conscious perception of change. Thus, Repp concludes that the agent guiding corrections of this motor behavior is a brain mechanism that responds to stimulus changes below the perceptual threshold.


  7. Play's The Thing, New Scientist Next Article Bookmark and Share

    Excerpt: But if play is not simply a developmental hiccup, as biologists once thought, why did it evolve? There are scores of theories, but none is totally convincing. The latest idea is perhaps the most audacious--it suggests that play has evolved to build big brains. In other words, playing makes you intelligent.

    Playfulness is quite a rare trait. It is common only among the mammals, although a few of the larger-brained birds such as magpies and crows also indulge. Animals at play often use unique signals--tail-wagging in dogs, for example--to indicate that activity superficially resembling adult behaviour is not really in earnest.


  8. Non-Stick Water, Nature Next Article Bookmark and Share

    Excerpts: Adding a simple powder to a drop of water gives it remarkable properties: the powder-coated drop no longer sticks to surfaces, and moves by rolling, much as a solid sphere would.

    (…) questions relevant to scientists in diverse fields: from chemists and engineers interested in the dynamics of drops, to astronomers interested in the stability of rotating stars and planets. How and when can liquid droplets actually roll on a surface? How fast can they move? Can they be controlled? And to what end?


    1. Liquid Marbles, Nature Next Article Bookmark and Share

      Excerpts: The transport of a small amount of liquid on a solid is not a simple process, owing to the nature of the contact between the two phases. Setting a liquid droplet in motion requires non-negligible forces (…) and often results in the deposition of liquid behind the drop. (…) Here we propose a simple alternative, which consists of encapsulating an aqueous liquid droplet with a hydrophobic powder. The resulting 'liquid marbles' are found to behave like a soft solid, and show dramatically reduced adhesion to a solid surface.

      • Liquid Marbles,Pascale Aussillous, David Quéré, Nature 411, 924-927 (2001).

  9. Plasticity Near And Far, Science Next Article Bookmark and Share

    Excerpts: The linkage between cellular activities at the synapse, such as long-term potentiation (LTP), and the changes in neuronal connectivity that reflect higher levels of cortical plasticity during development has been debated.

    Di Cristo et al. (p. 2337) show that modulators of extracellular signal-regulated protein kinase (ERK) function affect both cellular LTP and experience-dependent plasticity. The ERK signal transduction cascade couples the plasticity shown in the visual cortex during the critical period for establishment of ocular dominance to the phenomenon of activity-dependent changes in synaptic strength as seen in vitro at individual synapses.


    1. Synchronizing The Brain's Signals, Science Next Article Bookmark and Share

      Excerpts: Sometimes neurons get so excited that they fire in harmony, but neuroscientists aren't sure what it means. Some have suggested that this synchronized firing allows the brain to perform certain sophisticated computations, but there are a lot of holes in that idea. For starters, no one knew how neurons pick up on the synchrony and pass it along. Now researchers may have provided a partial answer to that question. On page 2295, they report results suggesting that networks of fast-spiking (FS) cells, a type of inhibitory neuron, could play a central role in detecting and fostering synchrony in the cortex, the large outer region of the brain that processes everything from complicated images to math problems.

    2. Precise And Reliable Brain Trains, Science Next Article Bookmark and Share

      Excerpts: The precision of action potential timing has become a central issue for the understanding of interneuron function in the cortex. Using paired recordings in brain slices, Galarreta and Hestrin (p. 2295; see the news story by Helmuth) studied the reliability of precise spike transmission between pyramidal cells and fast-spiking interneurons, a subset of cortical inhibitory cells. Neuronal firing at this connection occurred during a limited time window that lasted for about 1 millisecond and corresponded closely to the rising phase of the excitatory postynaptic event. This precise transmission may constitute an essential mechanism that enables the synchronous firing of fast-spiking GABAergic networks.

  10. Ionic Mechanism Of Hearing, Science Next Article Bookmark and Share

    Excerpts: The outer hair cells of the mammalian cochlea are specialized cells and function early on in the hearing process. Recently, prestin, the motor protein responsible for the electromotility of outer hair cells, has been identified and cloned. Oliver et al. (p. 2340) found that the unusual voltage sensitivity of this protein is not intrinsic but depends on the presence of intracellular anions, such as bicarbonate and chloride, which behave like extrinsic voltage sensors. In binding to the motor protein, they trigger the structural rearrangements in the molecule that underlie outer hair cell electromotility.

  11. Neural Details Of How Barn Owls Locate Sound Sources, Physics Today Next Article Bookmark and Share

    Excerpt: As you read this story, your gaze flutters about the page roughly three times a second. But the scene you perceive is fixed firmly in space. Your brain achieves this feat by multiplying the images on each retina by a function that describes where your eyes are pointing. Other examples of sensory multiplication have been found in the animal kingdom. Locusts use it to see and avoid other flying objects; humans and owls use it to localize sound. (...) Where and how does this multiplication take place?

  12. Neuroscience: Awareness Of Space, Nature Next Article Bookmark and Share

    Excerpts: Damage to particular parts of the brain can cause spatial confusion and even eliminate awareness of areas of space around the body. The brain regions responsible for spatial awareness, however, are still under debate.

    (…) Someone with hemispatial neglect might shave only half of his face, dress just half of his body, or, when copying a picture, draw only one side of it. (…)

    The patients with hemispatial neglect but no blindness had smaller lesions centred on the superior temporal lobe.


    1. Spatial Awareness Is A Function Of The Temporal Not The Posterior Parietal Lobe, Nature Next Article Bookmark and Share

      Excerpt: Unlike the monkey brain, spatial awareness in humans is a function largely confined to the right superior temporal cortex, a location topographically reminiscent of that for language on the left. Hence, the decisive phylogenetic transition from monkey to human brain seems to be a restriction of a formerly bilateral function to the right side, rather than a shift from the temporal to the parietal lobe. One may speculate that this lateralization of spatial awareness parallels the emergence of an elaborate representation for language on the left side.


  13. Single Neurons In Prefrontal Cortex Encode Abstract Rules, Nature Next Article Bookmark and Share

    Excerpt: The ability to abstract principles or rules from direct experience allows behaviour to extend beyond specific circumstances to general situations. For example, we learn the 'rules' for restaurant dining from specific experiences and can then apply them in new restaurants. The use of such rules is thought to depend on the prefrontal cortex (PFC) because its damage often results in difficulty in following rules1. Here we explore its neural basis by recording from single neurons in the PFC of monkeys trained to use two abstract rules.

  14. Parallel-distributed Processing In Olfactory Cortex, Chem. Senses Next Article Bookmark and Share

    Abstract: A working hypothesis is proposed for piriform cortex (PC) and other olfactory cortical areas that redefines the traditional functional roles as follows: the olfactory bulb serves as the primary olfactory cortex by virtue of encoding 'molecular features' (structural components common to many odorant molecules) as a patchy mosaic reminiscent of the representation of simple features in primary visual cortex. The anterior olfactory cortex (that has been inappropriately termed the anterior olfactory nucleus) detects and stores correlations between olfactory features, creating representations (gestalts) for particular odorants and odorant mixtures. This function places anterior olfactory cortex at the level of secondary visual cortex. PC carries out functions that have traditionally defined association cortex-it detects and learns correlations between olfactory gestalts formed in anterior olfactory cortex and a large repertoire of behavioral, cognitive and contextual information to which it has access through reciprocal connections with prefrontal, entorhinal, perirhinal and amygdaloid areas. Using principles derived from artificial networks with biologically plausible parallel-distributed architectures and Hebbian synaptic plasticity (i.e. adjustments in synaptic strength based on locally convergent activity), functional proposals are made for PC and related cortical areas. Architectural features incorporated include extensive recurrent connectivity in anterior PC, predominantly feedforward connectivity in posterior PC and backprojections that connect distal to proximal structures in the cascade of olfactory cortical areas. Capabilities of the 'reciprocal feedforward correlation' architecture that characterizes PC and adjoining higher-order areas are discussed in some detail. The working hypothesis is preceded by a review of relevant anatomy and physiology, and a non-quantitative account of parallel-distributed principles. To increase the accessibility of findings for PC and to advertise its substantial potential as a model for experimental and modeling analysis of associative processes, parallels are described between PC and the hippocampal formation, inferotemporal visual cortex and prefrontal cortex.

  15. Nanopore: High-Speed. Single-Molecule DNA Sequencing, Harvard University Website Next Article Bookmark and Share

    Excerpt: A revolutionary technology for probing, and eventually sequencing, individual DNA molecules using single-channel recording techniques has been conceived. This technology is based on a very high-speed method that: 1) Translates the characteristics of a polynucleotide into electronic signals; 2) Has the potential to sequence DNA at unprecedented speeds; 3) Can probe very long stretches of DNA or RNA; 4) Is a high throughput, single molecule technique compatible with high levels of nanofabrication. These design criteria are met by an instrument developed.

    Contributing Editor's Comment: A refined type of device is envisioned by the authors: a chip containing 500 operational pores reading at a rate of 1000 bases/sec each that could, assuming no redundancies, read an entire human genome equivalent (3000 Mbp) from one human cell in less than two hours, or the entire genome of a viral biological agent in seconds. Although current methods for sequencing have been greatly improved through automated protocols and advanced sequencing machinery and software, the demo presented provides the foundation for a radically different approach with greater potential, sensitivity, and speed. This new approach involves a direct read-out of single molecules and bypasses the truly rate-limiting biochemical steps of sequencing, such as labeling and assembly of contiguous elements.


  16. A Continuous Model Of Computation, arXiv Next Article Bookmark and Share

    Abstract: Although the Turing-machine model of computation is widely used in computer science it is fundamentally inadequate as a foundation for the theory of modern scientific computation. The real-number model is described as an alternative. Physicists often choose continuous mathematical models for problems ranging from the dynamical systems of classical physics to the operator equations and path integrals of quantum mechanics. These mathematical models use the real or complex number fields and we argue that the real-number model of computation should be used in the study of the computational complexity of continuous mathematical models. The study of continuous complexity is called information-based complexity. In this expository article we apply information-based complexity to topics such as breaking the curse of dimensionality, approximating the calculation of path integrals, and solving ill-posed problems. Precise formulations of these ideas may be found in J. F. Traub and A. G. Werschulz, "Complexity and Information", Cambridge University Press, 1998.

    Contributing Editor's Note: Even when, in theory, the Turing-machine model of computation can describe the real-number model of computation, and vice versa, the discussion is not centralized in the capabilities of each model, since both deal with computation. The difference lies in the abstractions that each model facilitate.


  17. Statistical Mechanics Of Complex Networks, arXiv Next Article Bookmark and Share

    Abstract: Complex networks describe a wide range of systems in nature and society, much quoted examples including the cell, a network of chemicals linked by chemical reactions, or the Internet, a network of routers and computers connected by physical links. While traditionally these systems were modeled as random graphs, it is increasingly recognized that the topology and evolution of real networks is governed by robust organizing principles. Here we review the recent advances in the field of complex networks, focusing on the statistical mechanics of network topology and dynamics. After reviewing the empirical data that motivated the recent interest in networks, we discuss the main models and analytical tools, covering random graphs, small-world and scale-free networks, as well as the interplay between topology and the network's robustness against failures and attacks.

  18. Acoustic Oscillations In Accord, Science Next Article Bookmark and Share

    Abstract: A standard model of the evolution of the universe assumes that an initial inflationary phase was followed by a Hot Big Bang. This process created a universe full of ionized plasma during its first 100,000 years, and the competing interactions between photon pressure and gravitational pressure in higher density clumps created acoustic oscillations in the plasma. These acoustic oscillations left features in the cosmic microwave background (CMB), which represents a radio-wavelength remnant of the early universe, that were recently measured. Miller et al. (p. 2302) have analyzed the acoustic oscillations imprinted on the matter-density distribution of clusters of galaxies and individual galaxies formed in the recent universe and found that the oscillations are consistent with the CMB results. Thus, oscillations recorded in young galaxies match with oscillations recorded in the very old plasma and provide further support for the Inflationary Hot Big Bang model of formation of the universe.

  19. Cosmology: Math Trick May Cause Tension Headache, Science Next Article Bookmark and Share

    Excerpt: By reanalyzing the basic equation of general relativity--which treats space and time as a stretchy membrane--a physicist has discovered that magnetic fields tend to flatten and stiffen the fabric of space-time. The discovery might force cosmologists and astronomers to reexamine how magnetic fields have shaped the evolution of the cosmos.

  20. Links & Snippets Next Article Bookmark and Share

    1. Announcements Bookmark and Share


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