Exciting Brain Circuitry News!
- Wednesday June 21, 3:04 pm Eastern Time
Company Press Release
MIT and Bell Labs researchers create electronic circuit that mimics
Device may help create computers that work more like the brain
MURRAY HILL, N.J.--(BUSINESS WIRE)--June 21, 2000--Researchers at the
Massachusetts Institute of Technology and Lucent Technologies' Bell
Labs report in
the June 22 issue of Nature that they have created an electronic
circuit that mimics the biological circuitry of the cerebral cortex,
the brain's center of intelligence.
This latest advance in ``neuromorphic'' engineering -- creating
devices that behave like neural systems -- was achieved by a team
that included MIT researchers Richard
Hahnloser, a postdoctoral fellow in the Department of Brain and
Cognitive Sciences; Rahul Sarpeshkar, assistant professor of
electrical engineering and computer
science; and H. Sebastian Seung, assistant professor of computational
``Like electronic circuits, the neural circuits of the cortex contain
many feedback loops,'' Seung said. ``But neuroscientists have found
that cortical feedback seems to
operate in a way that is unfamiliar to today's electronic designers.
We set out to mimic this novel mode of operation in an unconventional
The circuit was designed in collaboration with Rodney Douglas and the
late Misha Mahowald from the Institute of Neuroinformatics in
Switzerland. Much of the
research was carried out at Lucent Technologies' Bell Labs in Murray
Hill, N.J., where Sarpeshkar and Seung are consultants.
In the future, general principles illustrated by this circuit, could
lead to hardware that efficiently accomplishes complex perceptual
tasks, such as recognizing objects by
Cooperation and competition among neurons
The circuit is composed of artificial neurons that communicate with
each other via artificial synapses. All of these elements are made
from transistors fabricated on a
silicon integrated circuit.
Like neurons in the cortex, nearby artificial neurons affect each
other. There also is an inhibitory neuron that receives input from
the 16 excitatory neurons and returns
inhibition to them. This inhibitory feedback keeps in check
excitatory feedback that can lead to explosive instability.
In the brain, synaptic feedback connections are thought to mediate
neurons' cooperative and competitive interactions. Such interactions
are expressed most strongly in
the circuit when multiple stimuli are presented at the same time.
When simultaneous electrical currents are applied to two artificial
neurons, the circuit responds to only one stimulus and suppresses its
response to the other, much like a
frog choosing which of two flies to strike at.
Like the brain, there is no single element in the circuit that
decides which stimulus to suppress. The decision is the outcome of an
emergent, collective property of all the
A combination of digital and analog
A typical neuron in the brain might be connected to 10,000 other
neurons. Because there are billions of neurons, this makes the brain
a vast and intricate network.
``Biologists like to focus on simple linear pathways through this
network, ignoring the tangled web of feedback loops, which seem too
complex to even contemplate,''
Seung said. ``But it seems unlikely that we could ever understand
intelligence or consciousness without understanding the role of
feedback in the neural networks of the
Because electrical engineers rely heavily on feedback in their
designs, researchers have been tempted to draw analogies between
electronic and neural circuits. But
recent neurophysiological experiments suggest that the brain does not
use feedback in the same way as conventional electronics, which is
distinctly analog or digital.
Perception, the authors write, combines digital and analog aspects.
When we see an object such as an approaching car, we also receive a
continuous stream of
information about its color, its changing size in relation to its
distance from us, its spatial relations to other objects and so on.
Nevertheless, the digital component is still
there because regardless of how the object appears, our brains make
an either-or decision: Is it a car or not?
The hybrid analog-digital nature of the brain may be very important
for its computational efficiency. ``The electronic world is evolving
more and more towards mixed
analog-digital computation as the brain has already done,''
Sarpeshkar said. ``However, the brain's mixed-signal circuits combine
analog and digital functions in a much
more intimate way than is done in the electronic world.''
``Philosophers and psychologists have long been struck by the duality
between analog and digital in perception,'' Seung said. ``They have
further speculated about
whether the computational operations underlying perception in the
brain are analog or digital.
``Our research suggests that the two sides of this duality are not
mutually exclusive: the brain's neural circuitry is actually a hybrid
in which analog and digital coexist.''
Sarpeshkar, who does research in hybrid electronic circuits, said
that hybrid electronics has the potential to revolutionize computing
in the future because it combines the
digital advantages of programmability, noise immunity and divide-and-
conquer processing with the analog advantages of efficiency.
``The most immediate applications of such biologically inspired
circuits are likely to be in sensory data processing, where the input
is analog, and in prosthetic applications
for the deaf and blind, where mimicing the biology is important,''
This work was supported by the Swiss National Science Foundation SPP
Program, Lucent Technologies and MIT.