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{at}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{at}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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