What's a gifted electrical engineer whose career has been
devoted to such Claude Shannon-centric topics as video compression,
signature verification, and coherent signal processing doing deep
inside the brain?
Delivering the sixth Viterbi centennial lecture, which was also the
fourth annual Viterbi Lecture, Thursday March 9, Toby Berger reported on the
results of seven years of investigation (and speculation) on how
neurons communicate with themselves and the outside world, bringing an
IT focus to an old set of enigmas.
The result was an intriguing mix of insight and mystery, of truly
remarkable facts (Every human brain continuously carries message
traffic as intense as the entire Internet.) and elegant inference that
left a feeling of awe at how enormously much remains to be discovered.
Berger recently moved to the University of Virginia after a long and
remarkably successful career at Cornell, which won him, among many
other honors election to membership in the National Academy of
Engineering this year.
His brain work focuses on communication, his specialty, but in an
"intraorganism" context, cells talking to cells. The key element of
this, in the context of Shannon's work, is communication without coding.
Coding is clearly present in some biological systems, Berger noted. DNA
is obviously a code -- but, as Berger pointed out, it serves primarily
as an interorganism code, a way to transmit characteristics from on
organism to another.
But for the brain's intraorganism communication, coding is absent. The
channels evolved to become completely specific to messages. The system,
from another perspective, becomes the code.
Shannon optimal performance, he noted (and proved, in equations using
basic Shannon understandings) is possible without coding under these
circumstances, "optimally matched over a wide range of power
consumption levels."
The matching establishes channel capacity in a unique way. The cost of
in joules per bit of transmitting a message at low rates is low, but
goes up rapidly as the volume of bits increase: the faster you talk,
the more each word costs, setting the limits.
But what are the messages, and where are the channels? Berger
enthusiastically plunged into basic neuroanatomy. A human brain
contains about 10
11 neurons. Each neuron is connected to about 1000 other neurons, creating the staggering total of 10
15 active interconnections.
 |
Viterbi lecturer and Andrew Viterbi |
The
neurons cycle at a rate of about 2.5 microseconds, with a non-firing
being as a much of a message as an energization. If you add up the
activity, Berger noted, "the brain is distributing impulses to its
cells at a rate greater than the Internet."
But the information processing patterns are vastly different than
electronic computers. There is no central clock, no sharply
synchronized dance of crisp ones and zeros. Each neuron fires according
to its own clock, and the pulse and its transmission along the nerve
axons stretch out over time, so that the signals' leading edge doesn't
get to the farthest neuron, among the 1000 or so that are receiving the
signal, until after the tailing edge has reached the nearest.
According to Berger, signal processing considerations dictate some
conclusions about what is going on in this intense bustle of activity:
- The message carried in an individual nerve impulse is independent
of the amplitude of the impulse. Rather, the message comes in the
temporal pattern of successive pulses.
- This pattern in turn is determined by dynamic adjustment of the
threshold of the firing neurons -- that is, the minimum input impulse
it takes to fire a neuron. If that threshold is determined and
inflexible, according to Berger, the system can't function
- But some messages (e.g., reflexes) have to be acted on too quickly for temporal coding to work.
- Messages aren't just repeated in the succession of energizations of
corresponding neurons, spreading to a wider area of the brain. Rather,
they change as they pass through successive neurons.
- Neurons need a certain minimum input of excitation to remain alive and functional,
The central mystery remains how this immense collection of simple,
standard individuals by elusive laws becomes a network that can process
information. Berger noted its development for some clues. Almost all
the brain cells are formed between the second trimester of pregnancy
and age one. All the synapses are in place by age two, and their
arrangement doesn't change much subsequently, though some rewiring
takes place.
And, according to Berger, it's clear that the brain is a Markov chain
-- a system in which the route that took it to a given state is
irrelevant to determining where it's going: all that counts is the
current state.
 |
Dean Yannis Yortsos congratulates Berger post-lecture |
Berger emphasized one crucial aspect of the system: its longevity and
adaptiveness. The elements had come together over millions of years
tuning themselves to each other. Almost wistfully, he recalled
asking his collaborator on much of his brain research, W, B, Chip Levy
of the University of Virginia medical school, "why organisms haven't
evolved using silicon for information." Levy told him, he
related, that a child falling on its head (not very hard) "might
stretch its nerve cells by 10 percent" from the impact.
In closing, he offered a Markov epigraph from Claude Shannon: "we have
knowledge of the past, but we can't control it. We can control the
future, but we have no knowledge of it."
