Text 334, 73 rader
Skriven 2005-02-10 17:54:40 av Herman Trivilino (1:106/2000.7)
Ärende: PNU 719
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PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 719 February 10, 2005
by Phillip F. Schewe, Ben Stein

MEMORY AND CRITICAL AVALANCHES IN THE BRAIN.  Physicists at Indiana University
are extending their study of the relation between observed patterns of neuron
activity and memory storage in the brain.  First came experimental work with
slices of rat brain. Later the researchers performed simulations to try to
emulate the data.  Activity in the actual samples displayed two fascinating
features: (1) the ensemble of neurons firing varies in size very much like
"avalanche" phenomena such as occur in sandpiles and forest fires; and (2)
there are stable activity patterns that resemble memory sequences measured in
lab studies of rats in a maze.  Every time a rat runs a particular route the
same sequence of neural firings occur.  At night the same sequence might be
replayed as a rat "dream."  If the rat's dream is interrupted, his ability to
run the same route the next day might be compromised.  This has added evidence
to the notion that sleeping and dreaming help to consolidate memories from the
previous day's activities.  Stable activity patterns also appear in artificial
neural networks as a way of storing information. The Indiana physicists take a
fundamental look at those patterns. They used a 60-electrode array to look at
firings in a thin slice of rat brain tissue.  The cells in the slice, supplied
with oxygen and nutrients, go on behaving as if they were part of a living
brain. The general ensemble firing of cells is classified as subcritical (one
cell firing leads, on the average, to less one additional cell firing),
critical (one firing leads to another firing), or supercritical (a firing leads
to two or more cells firing).  In this regard, neural cells triggering each
other are somewhat like chain reactions among uranium-235 atoms in a nuclear
reactor.  The subcritical case is uninteresting.  The supercritical situation
often leads to the case in which all the cells in the sample end up firing,
which is also uninteresting.  The critical case has the most to offer: neural
ensembles of all sizes ensue.  If you plot (with logarithmic rulings) the
number of firing events versus the size of the firing ensemble, you get a
straight line, indicative of classic "power law spectrum" behavior.  In other
words, the likelihood of an event (earthquake, sand avalanche, hurricane) of
size E drops off according to E raised to a negative exponent. Now, in the
simulation work, the notion that the most interesting outcomes occur when the
brain system is maintained right at criticality is reinforced.  The
simulations, which do roughly match the observed behavior, are surprising and
even counterintuitive. This is because precisely amid conditions which favor
the greatest number of avalanches the largest number of stable neural activity
patterns also occurs.  One of the researchers, John M. Beggs, says that the
work is meant to explore how avalanches in brain cells might be used to store
information.  (Haldeman and Beggs, Physical Review Letters, 11 February
2005,jmbeggs@indiana.edu, 812-855-7359; lab website,
http://biocomplexity.indiana.edu/research/info/beggs.php )

LIQUID CARBON CHEMISTRY.  The chemistry of carbon atoms, with their gregarious
ability to bond to four other atoms, is a major determinant of life on Earth. 
But what happens when carbon is heated up to its melting temperature of 5000 K
at pressures greater than 100 bars?   Although liquid carbon may exist inside
the planets
Neptune and Uranus, the main interest in studying liquid carbon here on Earth
might be in the indirect information provided about bonding in ordinary solid
carbon or in hypothetical novel forms of solid carbon   A new experiment
creates liquid carbon by blasting a solid sheet of C with an intense laser
beam.  Before the liquid can vaporize, its structure is quickly probed by an
x-ray beam.  At low carbon density, two bonds seem to be the preferential way
of hooking up, while at higher density, three and four bonds are typical.  This
is not to say that complex organic molecules (carbon bonded to other atoms such
as hydrogen or oxygen) could survive at 5000 K, but carbon bonds are tougher
and can persist.  The experiment was performed by physicists from UC Berkeley,
the Paul Scherrer Institute (PSI) in Switzerland, Lawrence Berkeley National
Lab, Kansas State, and Lawrence Livermore National Lab.   A team member,
Steve Johnson (steve.johnson@mailaps.org), says that one next step will be to
study carbon, as well as other materials, at even higher temperatures in order
to look at "warm dense matter," a realm of matter too hot to be considered by
conventional solid-state theory but too dense to be considered by conventional
plasma theory. (Johnson et al., Physical Review Letters,11 February 2005 lab
website at http://www.physics.berkeley.edu/research/falcone/ )

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 * Origin: Big Bang (1:106/2000.7)