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.
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,email@example.com,
812-855-7359; lab website, http://biocomplexity.indiana.edu/research/info/beggs.php