Complexity Digest 2000.11

13-Mar-2000

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Content

  1. Attention Strengthens Neuronal Synchronization, Nature
  2. How Do Brains of Children Grow?, Nature
  3. Bio-Chemistry Of Learning and Unlearning, PNAS
  4. Salmons And Human Brain Aging, Science Daily
  5. Do Flies Have To Go To Sleep, Too?, Science
  6. Extreme Acid Loving Microbes, Science
  7. Pollution Stops Rain and Snow In Clouds, Science
  8. Ahead Of The Game, World Link
  9. Iridium Global Satellite System Terminated, Wall Street Journal
    1. The Sims And Agent Based Modeling, ComDig 2000.9.6
    2. Letters To The Editor:
    3. A Computer To Outsmart A Raging Fire, ComDig 2000.9.7

  1. Attention Strengthens Neuronal Synchronization, Nature Next Article Bookmark and Share

    The view of the brain as a computer is currently being superseded by the understanding of the brain as a complex adaptive system. Part of that transition is triggered by a new understanding of how information is encoded and modified in the brain. Individual neurons are not just on-off switches or biological transistors ("wet-ware").

    It has been known for a while that some aspects of the information is encoded in the rate at which a neuron fires. With the help of micro-electrodes connected to a loudspeaker researchers were basically poking around in the brain looking for neurons that would signal a response to a specific stimulus by a clearly audible increase in their firing rate. More recently Freeman, Braitenberg, and others introduced the concept that synchronous activity of "cell-assemblies" contribute to the signal encoding.

    Now Steinmetz et al. could demonstrate the contribution of synchronized neuronal firing to the process of paying attention to specific aspects of the environment. They recorded the firing of several neurons (separated between 0.5 and 4 millimeters) in the secondary somatosensory cortex of monkeys who were trained to shift attention between visual and tactile tasks. As one would expect from a cell-assembly perspective they found that during the tactile task not only the firing rate of the neurons increased but also their degree of synchrony increased significantly beyond the level that one would expect from the increased firing rate alone. Chaotic pattern formation in a neural network Jung/Mayer-Kress

    The authors did not discuss the mechanisms by which such a sharp synchronization of just a few milliseconds could take place. A simple explanation of the temporal coincidences of neuron firing is the formation of spatio-temporal excitation waves. Those waves show up universally in neuronal arrays even in the presence of strong noise perturbations. The figure shows an example of an artificial neural network where it displays spatio-temporal chaos. The amount of information that can be stored in those pattern sequences is huge. But even in strong spatio-temporal chaos, temporal synchronization between distant neurons would be observed with a high probability.

    After a "Decade of the Brain" one can now observe an exciting convergence between experiments and theoretical models.


  2. How Do Brains of Children Grow?, Nature Next Article Bookmark and Share

    How does a complex organ like a brain grow? That is a very difficult question since the growth is certainly not a uniform change in size but also involves many developmental changes. Thompson et al. used state of the art computational tools fro continuum mechanics that allowed them to track in considerable detail the dramatic changes that a human brain undergoes from the age of three to almost grown-up age of fifteen. The basic strategy of the researchers involved "tensor" mapping. A tensor is a mathematical object like a point and a vector. While a point only has a position, a vector additionally has a direction attached to a point. Analogously a tensor attaches each point in the developing brain independent directions of change and growth that will be tracked on a computer.

    The researchers obtained their data from repeated magnetic resonance imaging (MRI) that provides a three-dimensional, high-resolution image of the brain during different developmental stages. They found that one of the most striking features is a growth in the structure that connects the two brain hemispheres (corpus callosum). It does not grow uniformly but starts growing in the front and then the center of growth moves more towards the back of the head. Brain areas that are related to association and language slow down in their growth after puberty. During the same time the amount of gray matter deep inside the brain is reduced. At young ages (3-6years) fastest growth takes place in areas corresponding to planning and decision making.


  3. Bio-Chemistry Of Learning and Unlearning, PNAS Next Article Bookmark and Share

    It is now a widely accepted fact that learning takes place by changing synaptic connections between those neurons that are simultaneously active during the process that is to be learned. A common misunderstanding is that the change in the connection strength is always positive in the sense of long-term potentiation ("LTP"). But what is often forgotten is that synaptic modification can also take place in the form of long term depression ("LTD") i.e. neurons that were firing together in the past are less likely to trigger mutual firing in the future. Sometimes LTD is also referred to as "unlearning" a process that could be shown in theoretical models to greatly enhance the memory of a neural network for instance by unlearning all structures that are not relevant in the long term and therefore better forgotten.

    It is a fascinating indicator of the complexity of brain functioning that the same synapse of the same neuron can sometimes "learn" (LTP) and sometimes "forget" (LTD) depending on the presence or absence of neuromodulators. These are basically three substances (noradrenaline, acetylcholine, serotonin) that act as "enabling factors" that can speed up either LTP or LTD. These neuromodulators, are not simply "potentiators" or "depressors", they can play both roles depending on some conditions: Kirkwood reports that for the case of neurons in the cat visual cortex short, high frequency stimulations cause LTP whereas prolonged low frequency stimulation leads to LTD. At least that is the simple rule for two of the enabling factors. But for serotonin the story is a little more complex: it seemed to facilitate both LTP and LTD in an unpredictable fashion. A solution to this mystery was found with the discovery that receptors for serotonin are not uniformly distributed but come in patches. In some patches it acts with low frequency LTP in other patches with the opposite effect. One could speculate if this could implement some form of "firmware" frequency filter.

    The new insights into the complexity of the bio-chemical conditions under which learning /unlearning takes place might also shed light on the question why we sleep: Only during the awake and alert state are all three neuromodulators active and "(…) a given pattern of input activity might weaken synapses when only one neuromodulatory system is on, but strengthen them when the three systems are active simultaneously". Thus it might be possible that random events that we experience during the waking hours will be recalled during the random activation of cell assemblies during the dream (REM) state but this time they will be "unlearned". One might speculate that this is the reason why we don't remember our dreams except for the ones that we vividly remember. It looks like there are still a few puzzles left to figure out how our brain does what it does.


  4. Salmons And Human Brain Aging, Science Daily Next Article Bookmark and Share

    A University of Colorado at Boulder study of landlocked salmon indicates they possess a genetically programmed "aging clock" timed by reproduction, which may provide insight into human aging and Alzheimer's disease.

    Richard Jones, a professor emeritus in the environmental, population and organismic biology department, said the study is the first ever to identify deposits of a peptide known as beta amyloid in the brains of an aging, wild vertebrate population under natural conditions. Sticky deposits of beta amyloid, called plaques, are considered one of the hallmarks of aging and Alzheimer's disease in human brains.

    Many neurologists believe that beta amyloid -- produced when a brain protein known as amyloid precursor protein, or APP, is chopped into pieces by enzymes -- causes brain neurons to degenerate and die, said CU-Boulder doctoral student and lead study author Tammy Maldonado. But other researchers now believe beta-amyloid deposits in the brain are relatively harmless and may even be beneficial.

    The CU-Boulder biologists found that specific brain regions of spawning salmon exhibit neurodegeneration and amyloid plaques remarkably similar to those in humans. But other brain areas used for migration and spawning tasks continued to function, despite the presence of beta-amyloid plaques, Maldonado said.

    (…)

    Both salmon and humans exhibit remarkably similar aging symptoms, including brain decay, cardiovascular disease, muscle atrophy, skin lesions and the resorption of internal organs. Laboratory studies of APP and beta-amyloid molecules obtained from salmon brains and from a small piece of brain tissue from a human who died with Alzheimer's disease showed the molecules "to be very similar if not identical," said Maldonado.

    In the study, young, castrated kokanee salmon were shown to live to be 7 years to 9 years old, instead of dying at age 2 or 3 like normal, spawning kokanee salmon, suggesting that salmon have an "aging alarm" timed to go off at reproduction. Massive surges of a stress hormone known as cortisol occur in both reproducing and sterile salmon just prior to the onset of the rapid aging process and subsequent death, Norris said.

    "Cortisol surges may help these fish metabolize sugar and produce enough energy to locate their home streams and reproduce, but the surges also eventually may trigger brain aging and death," said Maldonado. The researchers plan to inject juvenile salmon with cortisol and several other hormones next fall to see if the experiment causes amyloid plaques to form in the brains of the study fish.

    Lab experiments have shown that high levels of cortisol spikes that occur in the blood of aging rats and humans, are present in higher quantities in Alzheimer victims, and are known to kill certain areas of neurons in the brain.

    "If we find that stress hormones cause amyloid plaques to form in salmon brains, that would be quite a breakthrough," said Norris.


  5. Do Flies Have To Go To Sleep, Too?, Science Next Article Bookmark and Share

    There is a saying in physics that you don't understand what you are doing unless you are able to explain it to a child. It seems that finally science has a halfway plausible answer to one of the 3-year-old classics: "Do flies have to go to sleep, too?" Even the standard scientist's disclaimer: "That depends on what you mean by 'sleep'." can be answered in a way that makes sense: Sleep can be described as a behavioral state of rest with an increased arousal threshold, in other words: if you sleep it is harder to wake you up. The second criterion is that if you didn't get enough sleep you have a desire to catch up on your sleep, something that experts call "homeostatic control".

    Shaw et al. applied these criteria in a number of careful experiments to the fruit fly. The first challenge is to quantify and measure the state of "rest" in flies. The researchers came up with a combination of ultra sound and infrared devices that would record the movements of the flies or the absence thereof during states of rest.

    Their experimental answer to the simple question about sleeping flies is a simple: "Yes, they do." When they sleep it takes about a hundred times the effort (in the form of vibrations) to startle them compared to their waking state. In the same way the researchers could show that sleep-deprived flies were really tired and needed more rest. An interesting side-result is that flies seem to respond to drugs just like people: coffee at night keeps them awake and sleeping pills put them to sleep.

    Because of the surprising similarities in sleep behavior of flies and mammals one might expect that sleep patterns are genetically controlled. Indeed Shaw et al. could confirm that several "waking" genes can be found in the fly that are similar to those found in rats. They are predominantly expressed during the first few hours of waking. These results make Drosophila a prime candidate to become a realistic model to study the genetic foundations of sleep functions.

    It is unlikely that these findings will silence a curious 3-year-old for very long. The next question probably is:" But do flies have dreams, too?"


  6. Extreme Acid Loving Microbes, Science Next Article Bookmark and Share

    In the past few years a number of reports were published about life that has adapted to extreme conditions. The first living cells are believed to have emerged under conditions without sunlight in the deep ocean that would be considered extreme today. Edwards et al. describe a new species of archaea bacteria that grows in hot places with acid concentrations that would burn human tissue instantly. These truly extreme-loving bacteria thrive at the highest naturally observed acid concentrations, at a pH level of zero, the strongest acid concentration that was ever reported to support life. These extreme creatures are found deep in a mineshaft inside of Iron Mountain, California at temperatures of 40 degrees Celsius. If they are put in regular tap water they disintegrate. Nevertheless they can be found all over the world. How these bacteria manage such a global distribution is still a mystery (Maybe in the acidic stomachs of animals?) It is also an open puzzle how these microbes manage to withstand these extreme acid concentrations. The researchers expect that it must have to do with special membrane characteristics since the bacteria found in Iron Mountain do not possess any cell walls. Maybe these microbes have found a some tricks that might be interesting for material science.

    Unquestioned, however, is the contribution of these acid-loving bacteria to the global environment: They play a major role in the global cycling of iron and sulfur.


  7. Pollution Stops Rain and Snow In Clouds, Science Next Article Bookmark and Share

    Clouds form one of the sensitive mechanisms in the control of weather and climate dynamics. Since they are associated with phase transitions of water between all its three states (ice, liquid, vapor) their formation and dissolution is associated with large conversions of thermal energy. Since phase transitions are critical phenomena, they can be triggered by changes in microscopic factors. In the case of cloud formation the size of Cloud Condensation Nuclei (CCN) plays that critical role. If CCNs are absent, clouds will not form even if the dew point is reached i.e. humidity is high enough and temperature is low enough that vapor would condense into water droplets. If CCNs are too small, the droplet will not grow big enough to cause precipitation in the form of rain or snow.

    In general CCNs consist of aerosols of different origins. It has been known -for instance from the satellite based Tropical Rainfall Measurement Mission (TRMM)- that smoke from burning vegetation produces plumes of small aerosols that are not very efficient in forming droplets of size large enough to produce precipitation.

    There has been a debate about the impacts of human activity on cloud dynamics. Reports about enhanced rainfall over cities have been associated with large CCNs but there are other factors that can lead to enhanced rainfall such as the heat-island effect and increased friction of urban areas. Observation of ship tracks in marine stratocumulus clouds, however, showed that CNNs from ship stacks redistribute water into small droplets and thus suppress precipitation.

    Rosenfeld could confirm these results with the help of satellite data that reveal that plumes of reduced cloud particle size and suppressed precipitation originate from major urban areas and industrial facilities.

    The impact of this observation on climate change can be significant: It is known that water vapor is one of the most potent greenhouse gases. Reduction of rainfall could therefore lead to a significant heating of the atmosphere. Since this effect is not uniform but concentrated around populated areas, one can also expect other impacts for instance on the formation of storms.


  8. Ahead Of The Game, World Link Next Article Bookmark and Share

    It is hard to get used to seeing kids, too young to rent a car starting e-companies and making so much money that they can to buy private jets instead. Meyer gives a theoretical analysis of this new world with a connected economy of increasing returns where rivals can "instantaneously clone competitors' killer innovations" (unless they are patented or otherwise protected by law). He explains that it is very difficult today to gain a " sustainable competitive advantage" in the sense of making higher than normal profits over on extended period of time.

    In the good old days of diminishing returns the system had a built in stabilizing factor or negative feedback in the form of natural economic limitations like the limited availability of farmland etc. In the information industry those natural limits don't exist and it is no problem at all for a single company to supply 100% of a certain software product for all computers in the world. The same is true for movies or video games especially if they are not tied to substances like celluloid or game computers (e.g. Sony's PS2): information in the form of software or data can be reproduced at basically no marginal cost. This clearly creates an incentive for mega-mergers and "winner-takes-all" markets with serial monopolies.

    In the context of non-linear dynamical systems this would be a positive feedback situation with one globally attracting mode or order parameter. If the fitness of the resulting system is reduced it becomes unstable against slowly changing parameters due to adaptation. Meyer mentions that subtle aspects of the rules of baseball have been continually changed to keep the balance of the game in such a way that it remains interesting for the audience. (Being from Germany I have always wondered why baseball bats are legal with which the balls can be hit outside of the playing field where they might injure a spectator; it must be one of the thrills of the game.)

    Just in the same way Meyer argues the government slowly changes the rules of the market place in updating antitrust regulations in the interest of the consumers. The ruling against Microsoft and the patenting of genetic information are just two examples of stabilizing changes in the boundary parameters of an economy with networked "eco-systems" of knowledge-based firms.


  9. Iridium Global Satellite System Terminated, Wall Street Journal Next Article Bookmark and Share

    The speed at which the Internet found global acceptance is has been seen as sign for global self-organization and a transition to a new phase for the global economy. It seemed to be obvious that technological developments that support this trend will be a good investment. The creation of a network of low earth orbiting satellites seemed to be one of those examples that would allow global access to information around the clock independent of where you are. From a cell-phone like simple hand-set one could directly uplink to one of a couple of dozens satellites and be connected to the global information system. The first set of satellites was launched just a few years ago under the brand name Iridium. Today it becomes obvious that a great idea for global communication with cutting edge technology is not necessarily economically sustainable, the Iridium project has been declared bankrupt. This is bad news for business and adventure travelers in areas with poor telephone service (like Antarctica).

    On the other hand it demonstrated the technical feasibility of global communication for rural communities in the third world to leapfrog past the industrial revolution and the migration into city slums and directly connect to the information economy. It shows that it is technically possible to use global satellite communication for the education of women in third world countries a factor that has been identified as the single most important leverage factor to limit over-population, hunger and a number of other urgent third world problems.

    It is sad that this technical opportunity to make global changes in the third world will be missed and, instead, 66 perfectly fine satellites are going to be "deorbited" to burn up in the atmosphere. But maybe this evolutionary dead-end will make place for even better and cheaper solutions: Motorola mentions "High Frequency Single Sideband Radio Systems" as an alternative to the Iridium system for long-range communications of thousands of kilometers for voice, data, e-mail and fax. (http://www.motorola.com/satellite/info/)


    1. The Sims And Agent Based Modeling, ComDig 2000.9.6 Next Article Bookmark and Share

      I read "The Sims And Agent Based Modeling" of the current Digest with great interest. Glad to see that also a more playful viewpoint to ALive/complexity is being accepted now. I wonder if a tool such as AgentSheets also would be of interest to the Complexity community? AgentSheets' is not a substitute for a tool like Swarm but on the other hand it is end-user programmable and has been used by thousands of non-programmers to build pretty sophisticated simulations (e.g.,. http://www.apple.com/education/LTReview/fall99/agentsheets/index.html). Here are some simulations built: http://www.agentsheets.com/showcase.html. There are extensions available to build more complex applications including a Sugarscape language kit.
      • Alexander Repenning

    2. Letters To The Editor: Next Article Bookmark and Share

    3. A Computer To Outsmart A Raging Fire, ComDig 2000.9.7 Bookmark and Share

      A NAME="10.2">
      Peter Jung, Physics Department, Ohio State University points out a sloppy use of the terms "meta-stable" and "excitable" in ComDig 2000.9.7 (A Computer To Outsmart A Raging Fire, NY Times). He writes: "A metastable system falls from its energetic higher state into the globally stable state and will stay there (like the buildings or the tanker). The activation energy can be small but the gain can be large.

      In contrast an excitable system will automatically travel back to its original rest state after being excited."

      That means the property of the system to restore its meta-stable state is part of the definition of "excitable system". For instance a forest could be considered but isolated buildings are only meta-stable. On the other hand a city could be considered "excitable system" if burned down buildings are eventually restored by new ones.

      • Erratum

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