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Foresight Update 6 (page 1)

A publication of the Foresight Institute

Table of Contents

BioArchive Update
Computational Power of the Human Brain
Chinese Scientist Visits FI
Student Builds STM
Upcoming Events
Military Implications of Nanotechnology
Online Nanotechnology Discussion
Hypermedia Journal
Journals to Watch
Agoric Software Advances
Hypertext Multievent
Protein Engineering:

Recent Progress: Steps Toward Nanotechnology

Books of Note
Media Coverage
Meeting News
Nanotechnology Conference
Errata
Letters to FI
Thanks
FI Wish List
Foresight Policy

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.

Table of Contents - Foresight Update 6

Energy Limits to the Computational Power of the Human Brain

by Ralph C. Merkle

[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.

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.

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

  1. Ionic Channels of Excitable Membranes, by Bertil Hille, Sinauer, 1984.
  2. Principles of Neural Science, by Eric R. Kandel and James H. Schwartz, 2nd edition, Elsevier, 1985.
  3. Tom Binford, private communication.
  4. Mind Children, by Hans Moravec, Harvard University Press, 1988.
  5. From Neuron to Brain, second edition, by Stephen W. Kuffler, John G. Nichols, and A. Robert Martin, Sinauer, 1984.
  6. "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.
  7. Myelin, by Pierre Morell, Plenum Press, 1977.
  8. "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

Table of Contents - Foresight Update 6

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.

Table of Contents - Foresight Update 6

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

Table of Contents - Foresight Update 6

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.

Table of Contents - Foresight Update 6

Foresight thanks Dave Kilbridge for converting Update 6 to html for this web page.

From Foresight Update 6, originally published 1 August 1989.
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