Foresight Update 6
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A publication of the Foresight Institute
 BioArchive
Update
by Julia Tracy
The following is a report on FI's project to investigate
and perhaps coordinate the storage of cell samples from
endangered species for potential later restoration using
nanotechnology:
The BioArchive Project is in the initial stages of development.
We are compiling a list of all organizations, internationally,
who are engaged in seed and gene bank research and development.
An introductory letter has been sent to organizations on this
list, explaining who we are (a non-profit, educational
organization that looks at future technologies and is highly
supportive of seed and gene banks) and requesting printed
material on their projects. The plan is to gather information
from these groups, see how their goals and Foresight's goals
might mesh, then contact them with further information about
ourselves and discuss how we might work with them in the future.
Several people have sent us their names as potential volunteers
to help with this project. The challenge now is to figure out
what volunteers might do to further this work. If you have ideas
about the direction this project could take, please contact me at
1526-16th Avenue East, Seattle, WA 98112, (206) 325-8888 or (206)
632-4326.
Julia Tracy is an environmental activist with Greenpeace.
Energy
Limits to the Computational Power
 of the Human Brain
[Note: A related article on the memory limits of
the human brain is available in Update 4.]
The Brain as a Computer
The view that the brain can be seen as a type of computer has
gained general acceptance in the philosophical and computer
science community. Just as we ask how many mips or megaflops an
IBM PC or a Cray can perform, we can ask how many operations the
human brain can perform. Neither the mip nor the megaflop seems
quite appropriate, though; we need something new. One possibility
is the number of synapse operations per second.
A second possible "basic operation" is inspired by the
observation that signal propagation is a major limit. As gates
become faster, smaller, and cheaper, simply getting a signal from
one gate to another becomes a major issue. The brain couldn't
compute if nerve impulses didn't carry information from one
synapse to the next, and propagating a nerve impulse using the
electrochemical technology of the brain requires a measurable
amount of energy. Thus, instead of measuring synapse operations
per second, we might measure the total distance that all nerve
impulses combined can travel per second, e.g., total
nerve-impulse-distance per second.
Other Estimates
There are other ways to estimate the brain's computational
power. We might count the number of synapses, guess their speed
of operation, and determine synapse operations per second. There
are roughly 1015 synapses operating at about 10
impulses/second [2], giving
roughly 1016 synapse operations per second.
A second approach is to estimate the computational power of the
retina, and then multiply this estimate by the ratio of brain
size to retinal size. The retina is relatively well understood so
we can make a reasonable estimate of its computational power. The
output of the retina--carried by the optic nerve--is primarily
from retinal ganglion cells that perform "center
surround" computations (or related computations of roughly
similar complexity). If we assume that a typical center surround
computation requires about 100 analog adds and is done about 100
times per second [3], then
computation of the axonal output of each ganglion cell requires
about 10,000 analog adds per second. There are about 1,000,000
axons in the optic nerve [5,
page 21], so the retina as a whole performs about 1010
analog adds per second. There are about 108 nerve
cells in the retina [5, page
26], and between 1010 and 1012 nerve cells
in the brain [5, page 7], so
the brain is roughly 100 to 10,000 times larger than the retina.
By this logic, the brain should be able to do about 1012
to 1014 operations per second (in good agreement with
the estimate of Moravec, who considers this approach in more
detail [4, page 57 and 163]).
The Brain Uses Energy
A third approach is to measure the total energy used by the
brain each second, and then determine the energy used for each
"basic operation." Dividing the former by the latter
gives the maximum number of basic operations per second. We need
two pieces of information: the total energy consumed by the brain
each second, and the energy used by a "basic
operation."
The total energy consumption of the brain is about 25 watts [2]. Inasmuch as a significant
fraction of this energy will not be used for "useful
computation," we can reasonably round this to 10 watts.
Nerve Impulses Use Energy
Nerve impulses are carried by either myelinated or
un-myelinated axons. Myelinated axons are wrapped in a fatty
insulating myelin sheath, interrupted at intervals of about 1
millimeter to expose the axon. These interruptions are called
"nodes of Ranvier." Propagation of a nerve impulse in a
myelinated axon is from one node of Ranvier to the next--jumping
over the insulated portion.
A nerve cell has a "resting potential"--the outside of
the nerve cell is 0 volts (by definition), while the inside is
about -60 millivolts. There is more Na+ outside a nerve cell than
inside, and this chemical concentration gradient effectively adds
about 50 extra millivolts to the voltage acting on the Na+ ions,
for a total of about 110 millivolts [1, page 15]. When a nerve impulse
passes by, the internal voltage briefly rises above 0 volts
because of an inrush of Na+ ions.
The Energy of a Nerve Impulse
Nerve cell membranes have a capacitance of 1 microfarad per
square centimeter, so the capacitance of a relatively small 30
square micron node of Ranvier is 3 x 10-13 farads
(assuming small nodes tends to overestimate the computational
power of the brain). The internodal region is about 1,000 microns
in length, 500 times longer than the 2 micron node, but because
of the myelin sheath its capacitance is about 250 times lower per
square micron [5, page 180; 7, page 126] or only twice that
of the node. The total capacitance of a single node and
internodal gap is thus about 9 x 10-13 farads. The
total energy in joules held by such a capacitor at 0.11 volts is
1/2 V2 x C, or 1/2 x 0.112 x 9 x 10-13,
or 5 x 10-15 joules. This capacitor is discharged and
then recharged whenever a nerve impulse passes, dissipating 5 x
10-15 joules. A 10 watt brain can therefore do at most
2 x 1015 such "Ranvier ops" per second. Both
larger myelinated fibers and unmyelinated fibers dissipate more
energy. Various other factors not considered here increase the
total energy per nerve impulse [8],
causing us to somewhat overestimate the number of "Ranvier
ops" the brain can perform. It still provides a useful upper
bound and is unlikely to be in error by more than an order of
magnitude.
To translate "Ranvier ops" (1-millimeter jumps) into
synapse operations we must know the average distance between
synapses, which is not normally given in neuroscience texts. We
can estimate it: a human can recognize an image in about 100
milliseconds, which can take at most 100 one-millisecond synapse
delays. A single signal probably travels 100 millimeters in that
time (from the eye to the back of the brain, and then some). If
it passes 100 synapses in 100 millimeters then it passes one
synapse every millimeter--which means one "synapse
operation" is about one "Ranvier operation."
Discussion
Both "synapse ops" and "Ranvier ops" are
quite low-level. The higher level "analog addition ops"
seem intuitively more powerful, so it is perhaps not surprising
that the brain can perform fewer of them.
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A single computer with abilities
equivalent to a billion to a
trillion human beings will be a reality ... No field will
be left
unchanged by this staggering increase in our abilities.
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While the software remains a major challenge, we will soon be
able to build hardware powerful enough to perform more such
operations per second than can the human brain. There is already
a massively parallel multi-processor being built at IBM Yorktown
with a raw computational power of 1012 floating point
operations per second: the TF-1. It should be working in 1991 [6]. When we can build a desktop
computer able to deliver 1025 gate operations per
second and more (as we will surely be able to do with a mature
nanotechnology) and when we can write software to take advantage
of that hardware (as we will also eventually be able to do), a
single computer with abilities equivalent to a billion to a
trillion human beings will be a reality. If a problem might today
be solved by freeing all humanity from all mundane cares and
concerns, and focusing all their combined intellectual energies
upon it, then that problem can be solved in the future by a
personal computer. No field will be left unchanged by this
staggering increase in our abilities.
Conclusion
The total computational power of the brain is limited by
several factors, including the ability to propagate nerve
impulses from one place in the brain to another. Propagating a
nerve impulse a distance of 1 millimeter requires about 5 x 10-15
joules. Because the total energy dissipated by the brain is about
10 watts, this means nerve impulses can collectively travel at
most 2 x 1015 millimeters per second. By estimating
the distance between synapses we can in turn estimate how many
synapse operations per second the brain can do. This estimate is
only slightly smaller than one based on multiplying the estimated
number of synapses by the average firing rate, and two orders of
magnitude greater than one based on functional estimates of
retinal computational power. It seems reasonable to conclude that
the human brain has a "raw" computational power between
1013 and 1016 "operations" per
second.
References
- Ionic Channels of Excitable
Membranes, by Bertil Hille, Sinauer, 1984.
- Principles of Neural Science,
by Eric R. Kandel and James H. Schwartz, 2nd edition,
Elsevier, 1985.
- Tom Binford, private communication.
- Mind Children, by Hans
Moravec, Harvard University Press, 1988.
- From Neuron to Brain,
second edition, by Stephen W. Kuffler, John G. Nichols,
and A. Robert Martin, Sinauer, 1984.
- "The switching network of the
TF-1 Parallel Supercomputer" by Monty M. Denneau,
Peter H. Hochschild, and Gideon Shichman, Supercomputing,
winter 1988 pages 7-10.
- Myelin, by Pierre Morell,
Plenum Press, 1977.
- "The production and absorption of
heat associated with electrical activity in nerve and
electric organ" by J. M. Ritchie and R. D. Keynes, Quarterly
Review of Biophysics 18, 4 (1985), pp.
451-476.
Acknowledgements
The author would like to thank Richard Aldritch, Tom Binford,
Eric Drexler, Hans Moravec, and Irwin Sobel for their comments
and their patience in answering questions.
This
article is also available on Dr. Merkle's Web site.
Dr. Merkle can
be reached at Xerox PARC, 3333 Coyote Hill Road, Palo Alto, CA
94304 or merkle@xerox.com
Wei
Visits FI
On April 6 we were visited by Professor Wei Yu, president of
Southeast University, Nanjing, China. Her trip to the U.S. as a
visiting scientist was arranged by Sigma Xi, a scientific
research society, in cooperation with the National Academy of
Sciences. She was particularly interested in molecular
electronics and biosensors. Dr. Wei selected four researchers to
visit: K. Eric Drexler (Stanford), Harden M. McConnell
(Stanford), Richard Potember (Johns Hopkins), and H. Ti Tien
(Michigan State).
During her visit with Drexler she reported that her institution
is beginning work on molecular electronics, with eleven staff
members plus graduate students in the field. She asked about the
rod logic design for nanocomputers, explained why her group has
not attempted to implement it, and was assured that this was a
wise decision (such devices almost surely require assembler-based
construction).
Her view on scanning probe microscopes is that while they may be
useful for small numbers of molecules, self-assembly is the key
to making many complex molecular structures in parallel. She said
"I think biosensors may be the first molecular devices in
the market." She made clear her opinion that molecular and
biomolecular electronics is the technology of the future, and
could be useful in building artificial intelligence. Her comment
on the exploratory engineering approach: "You are very
careful in calculating what happens in the next century; this is
a good thing to do." She left us a gift of 'rain flower
stones' from Rain Flower Mountain in Nanjing, known as the Stone
City, former capital of China.
Student
STM
Todd Gustavson, 18, of Homestead High School in the San Jose
area has constructed a simplified scanning tunneling microscope.
It won a grand prize at a nearby science and engineering fair and
then went on to the International Science and Engineering Fair in
Pittsburgh; we haven't heard how it did there. Todd was assisted
on the design by his father David Gustavson of the Stanford
Linear Accelerator Center, but he built the device himself
Upcoming
Events
World Congress on Health, Aug. 5-10,
sponsored by AMA and similar organizations, Beijing. All events
(including a talk on nanotechnology) cancelled due to situation
in China.
2nd Carolina Conference on Protein Engineering,
Sept. 8-11, Southern Pines, NC, invitational meeting sponsored by
UNC Program in Molecular Biology & Biotechnology. Includes
"Engineering Macromolecular Objects" by Eric Drexler.
Contact 919-962-6820.
First Italian Multievent on Hypertext, Sept.
19-22, Milan. Sponsored by Eurogroup Marcopolo. Includes
tutorials, workshop, conference, exhibition. Contact P. Paolini,
Eurogroup Marcopolo, C.so Venezia 49, 20121 Milano, Italy.
First
Foresight Conference on Nanotechnology, fall 1989,
see writeup elsewhere in
this issue.
Symposium on Electroresponsive Molecular and Polymeric
Systems, Oct. 25-27, Brookhaven National Laboratory,
Upton, NY. Sponsored by US Dept. of Energy, $100. Includes
molecular electronics. Contact Betty Ivero, 516-282-2208.
Hypertext '89, Nov. 5-8, Pittsburgh Hilton.
Sponsored by Association for Computing Machinery. Contact Elise
Yoder, 412-327-8181 or ht89@zog.cs.cmu.edu.
Second Technology, Entertainment, and Design
Communications Conference, Feb. 22-25, 1990, Monterey
(Calif.) Conference Center. Speakers include Negroponte (MIT
Media Lab), Sculley, Kay, and Atkinson (Apple), Nelson
(hypertext), Lanier (VPL, the DataGlove company). $695. Contact
213-831-4225.
From Foresight Update 6, originally
published 1 August 1989.
Foresight thanks Dave Kilbridge for converting Update 6 to
html for this web page.
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