Path: utzoo!attcan!uunet!cs.utexas.edu!rutgers!aramis.rutgers.edu!xerox.com!merkle.pa From: merkle.pa@XEROX.COM Newsgroups: sci.nanotech Subject: Energy Limits to the Computational Power of the Human Brain Message-ID: <8906150843.AA06312@athos.rutgers.edu> Date: 14 Jun 89 18:04:29 GMT Sender: nanotech@aramis.rutgers.edu Lines: 79 Approved: nanotech@aramis.rutgers.edu Energy Limits to the Computational Power of the Human Brain by Ralph C. Merkle Xerox PARC 3333 Coyote Hill Road Palo Alto, CA 94304 merkle@xerox.com This article will appear in Foresight Update #6 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 10**15 synapses operating at about 10 impulses/second [2], giving roughly 10**16 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 10**10 analog adds per second. There are about 10**8 nerve cells in the retina [5, page 26], and between 10**10 and 10**12 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 10**12 to 10**14 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