TUCoPS :: Crypto
:: merkle2.txtExplanation of above by John Gilmore |

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,