Brain cells linked to silicon chips

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Brain cells linked to silicon chips

Researchers create part-mechanical, part-living circuit

By Shankar Vedantam THE WASHINGTON POST

Aug. 28 — 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.

NOT JUST SCIENCE FICTION 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.”

PAINSTAKING RESEARCH 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 Fromherz, are a little larger than human or rat

neurons and were therefore easier to work with. Advertisement

“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.”

CHALLENGES AHEAD 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, thoughts and maybe consciousness

itself.

© 2001 The Washington Post Company