Number 719, February 10, 2005
by Phil Schewe and 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.
(Johnsonet al., Physical Review Letters,11 February 2005; lab website
at http://www.physics.berkeley.edu/research/falcone/ )
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