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By Shankar Vedantam
Scientists for the first time have linked
multiple brain cells with silicon chips to create a part-mechanical,
part-living electronic circuit.
To construct the partially living electronic
circuit, scientists at the Max Planck Institute for Biochemistry
in Germany managed to affix multiple snail neurons onto tiny
transistor chips and demonstrated that the cells communicated
with each other and with the chips.
The advance is an important step toward
a goal that is still more science fiction than science: to
develop artificial retinas or prosthetic limbs that are extensions
of the human nervous system. The idea is to combine the mechanical
abilities of electronic circuits with the extraordinary complexity
and intelligence of the human brain.
Such combinations of biology and technology
may not only one day help the blind to see and the paralyzed
to move objects with their thoughts, but also help to build
computers that are as inventive and adaptable as our own nervous
systems and a generation of robots that might truly deserve
to be called intelligent.
Meshing nerve cells with electronics has
become a hot new field in science -- and has long been a staple
of science fiction. But what "Star Trek" accomplished
in a stroke of the pen has proved harder to achieve in real
life.
"The nervous system is quite different
than a computer," said Eve Marder, a professor of neuroscience
at Brandeis University who studies how the brain adapts to
change. "Many functions
that are physically separate in a computer are carried out
by the same piece of tissue" in the brain
and nervous system.
The greatest challenge has been in building
the interface between biology
and technology. Nerve cells in the brain find each
other, strengthen connections and build patterns through complex
chemical signaling that is driven in part by the environment.
Slice away some neurons, for example,
and others will leap in to replace their function. No one
understands how the brain learns to adapt to change, but it
is a process that is as sophisticated as it is messy.
Silicon chips, on the other hand, can
perform specific functions with great reliability and speed,
but have limited responsiveness to the environment and almost
no ability to alter themselves according to need.
"Things are constantly changing .
. . processes are growing, there are substances called neuromodulators
that change the properties of nerve cells and the strength
of connections," said Marder. "That's the challenge
of making a silicon-brain interface -- the rules of computation
are not the same."
The German researchers used micropipettes
to lift individual cells
from the snail brain and then puff them out onto silicon chips
that were layered with a kind of glue. The snail neurons,
according to biophysicist Peter Fromherz, are a little larger
than human or rat neurons and were therefore easier to work
with.
"They suck them out and then blow
them onto the structure," said Astrid Prinz, a post-doctoral
researcher at Brandeis University, who used to work with the
German group. "It's a matter of practice to learn to
handle individual cells. You have them in a little pipette
with fluid. You blow them out and you can maneuver them. One
guy in the lab made a little movie on how to blow cells."
Each cell was positioned over a Field
Effect Transistor, a device that is capable of amplifying
tiny voltages, and a stimulator to prod the cell into activity.
The process was repeated with some 20
cells over multiple transistors and stimulators. By using
polymers, the German scientists built tiny picket fences around
the neurons to keep them in place over the transistors --
one of the great difficulties in building such circuits is
that nerve cells tend to wander around, as they do in the
brain.
Neurons on this silicon base developed
a connection between each other known as a synapse.
When researchers stimulated one neuron, it released an electrical
signal.
That signal was detected by the transistor
that the neuron sat on as well as the transistor beneath a
second neuron -- showing that the electrical signal had passed
from the chip to the first neuron, through a synapse to the
second neuron and then converted back into electricity and
the second transistor.
"It's very primitive, but it's the
first time that a neural network was directly interfaced with
a silicon chip," said Fromherz, who published the results
in today's issue of the Proceedings of the National Academy
of Science. "It's a proof of principle experiment."
The group, he said, was already working
on linking greater numbers of neurons with more transistors.
The real challenge, he said, lay in figuring out where exactly
the neuron's synapse was relative to the transistor, and in
developing techniques that could reliably construct larger
circuits.
Fromherz said plans were underway to build
a system with 15,000 neuron-transistor sites.
When the number gets large enough, researchers
hope they will begin to see the early glimmers of what actually
happens in the brain: neurons forming complex connections
that transmute electrical activity into computation and thoughts.
Edited from
Washington Post August 28, 2001; Page A03
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