[MURG] [>Htech] trnmag: model of inferior olive replicates chaotic firing (fwd from alito@organicrobot.com)

Thu May 20 12:18:52 EST 2004

----- Forwarded message from Alejandro Dubrovsky <alito at organicrobot.com> -----

From: Alejandro Dubrovsky <alito at organicrobot.com>
Date: Thu, 20 May 2004 21:30:08 +1000
To: transhumantech <transhumantech at yahoogroups.com>
Subject: [>Htech] trnmag: model of inferior olive replicates chaotic firing
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(
http://www.trnmag.com/Stories/2004/050504/Chaos_seems_to_aid_learning_050504.html
)

Chaos seems to aid learning
May 5/12, 2004
By Kimberly Patch, Technology Research News

Although it's clear that the cerebellum is the part of the human brain
involved in coordinating movements in ways that allow people to learn
skills like riding a bike, there are mysteries about how the learning
process works. 

Researchers from Core Research for Evolutional Science and Technology
(CREST) in Japan have built a computer simulation of the inferior olive,
a portion of the brain that probably relays errors in movement to the
cerebellum. It has been difficult to explain the mechanics of this
relationship because inferior olive cells that connect to the cerebellum
fire slowly, and this does not fit well with the common hypothesis that
high-fidelity error signals are needed for efficient learning. 

The researchers got the idea for the simulation after initial research
showed that if neurons were electrically coupled, or linked, a certain
type of chaotic signal could emerge. 

The researchers' simulation shows that moderate electrical coupling
between nerve cells in the inferior olive could produce a type of
chaotic firing that effectively recodes the high-frequency information
into slower signals by imparting information within the rhythm rather
than just the frequency of nerve firing. "The chaotic firing was more
robust than we expected," said Nicholas Schweighofer, a researcher at
Core Research for Evolutional Science and Technology. The model shows
that "chaos can be useful in the brain," he said. 

In addition to allowing researchers to better understand the mechanics
of the brain, the researchers' theory of chaotic resonance could speed
electronic communications and improve robotics. "In communications, our
work [could] maximize the information transmitted in networks," he said.
"In robotics, chaos could be used to explore the environment to optimize
learning," he said. 

Electrical signals carry information from one end of a nerve cell to the
other, while a chemical reaction is responsible for passing signals from
one cell to another through their interconnected dendrites, or nerve
cell fibers. 

The researchers' results explain some unusual properties of the inferior
olive cell input to the Purkinje cells of the cerebral cortex, according
to Schweighofer. Each Purkinje cell contains two types of nerve synapse
inputs, or connections to other nerve cells. The cells connect to about
100,000 other nerve cells via parallel connections, but have only a
single connection to an inferior olive neuron. 

The parallel connections generate simple nerve spikes, or on signals,
but the inferior olive connection generates a more complicated signal.
Experiments have also uncovered apparently random firing, and chaotic
subthreshold activity, or signals that are not strong enough to trip the
chemical reaction that ordinarily passes a signal to neighboring cells.
It is also known that the inferior olive neurons are electrically
coupled. 

It was a challenge to make a realistic model of the inferior olive, said
Schweighofer. "Finally showing the existence of chaos... necessitated
very lengthy computations," he said. 

The researchers' inferior olive cell models included the known location
of the ionic currents that carry signals between nerve cells, the gap
junctions between the cells and the synaptic inputs. 

The researchers modeled two types of networks of a few simulated
inferior olive cells: chain networks, and grid networks. In chain
networks, each neuron is electrically coupled to its one or two
neighboring cells depending on its position in the chain. In grid
networks of 2 by 2, 3 by 3, and 9 by 3 cells, cells are connected to
two, three or four neighbors depending on their grid positions. 

When the researchers removed to the connections between cells, each cell
generated plain periodic spikes, or signals at an average rate of 3.1
spikes per second. When the researchers connected cells within a network
using just an intermediate coupling strength, the firing pattern of
individual cells appeared chaotic and the average firing rate was
reduced to 1.8 spikes per second. When the researchers used a strong
coupling strength, the cells generated regular, synchronized spikes at a
firing rate of 3.5 spikes per second. These results are consistent with
experiments on actual nerve cells. 

The simulation showed that moderate electrical coupling speeds
information transfer, according to Schweighofer. The mutual information
per spike for a single cell at the center of the 3 by 3 networks was 48
percent greater than the same network without coupling and 37 percent
greater for the coupled network as a whole despite the lower
spike-per-second rate. The researchers found similar results for the
other types of networks. 

Now that they have proved computationally that chaotic signals are
capable of carrying extra information, the researchers are aiming to
show empirically that this is what happens. The next step is "doing in
vivo work showing that chaos actually exists in the inferior olive,"
said Schweighofer. 

Schweighofer's research colleagues were Kenji Doya, Hidekazu Fukai, Jean
Vianney Chiron, Tetsuya Furukawa and Mitsuo Kawato. The work appeared in
the March 30, 2004 issue of Proceedings of the National Academy of
Sciences. The research was funded by the Telecommunications Advancement
Organization and the Human Frontier Science Program. 

Timeline:   5 years 
Funding:   Private 
TRN Categories:  Chaotic Systems, Fuzzy Logic and Probabilistic
Reasoning; Logic; Artificial Intelligence; Robotics; Computer and
Machine Learning
Story Type:   News 
Related Elements:  Technical paper, "Chaos May Enhance Information
Transmission in the Inferior Olive," Proceedings of the National Academy
Of Sciences, March 30, 2004 

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