The retina is a transparent, paper-thin layer of nerve tissue at the back of the eyeball on which the eye's lens projects an image of the world. It is connected by the optic nerve, a million-fiber cable, to regions deep in the brain. It is a part of the brain convenient for study, even in living animals because of its peripheral location and because its function is straightforward compared with the brain's other mysteries. A human retina is less than a centimeter square and a half-millimeter thick. It has about 100 million neurons, of five distinct kinds. Light-sensitive cells feed wide spanning horizontal cells and narrower bipolar cells, which are interconnected by amacrine cells, and finally ganglion cells, whose outgoing fibers bundle to form the optic nerve. Each of the million ganglion-cell axons carries signals from a particular patch of image, indicating light intensity differences over space or time: a million edge and motion detections. Overall, the retina seems to process about ten one-million-point images per second.
It takes robot vision programs about 100 computer instructions to derive single edge or motion detections from comparable video images. 100 million instructions are needed to do a million detections, and 1,000 MIPS to repeat them ten times per second to match the retina.
The 1,500 cubic centimeter human brain is about 100,000 times as large as the retina, suggesting that matching overall human behavior will take about 100 million MIPS of computer power.
The most powerful experimental supercomputers in 1998, composed of thousands or tens of thousands of the fastest microprocessors and costing tens of millions of dollars, can do a few million MIPS. They are within striking distance of being powerful enough to match human brainpower, but are unlikely to be applied to that end. Why tie up a rare $20 million dollar asset to develop one ersatz-human, when millions of inexpensive original-model humans are available? Such machines are needed for high-value scientific calculations, mostly physical simulations, having no cheaper substitutes. AI research must wait for the power to become more affordable.
If 100 million MIPS could do the job of the human brain's 100 billion neurons, then one neuron is worth about 1/1,000 MIPS, i.e. 1,000 instructions per second. That's probably not enough to simulate an actual neuron, which can produce 1,000 finely timed pulses per second. Our estimate is for very efficient programs that imitate the aggregate function of thousand-neuron assemblies. Almost all nervous systems contain subassemblies that big.
The small nervous systems of insects and other invertebrates seem to be hardwired from birth, each neuron having its own special predetermined links and function. The few hundred million bit insect genome is enough to specify connections of each of their hundred thousand neurons. Humans, on the other hand, have 100 billion neurons, but only a few billion bits of genome. The human brain seems to consist largely of regular structures whose neurons are trimmed away as skills are learned, like featureless marble blocks chiseled into individual scupltures. Analogously, robot programs were precisely hand-coded when they occupied only a few hundred thousand bytes of memory. Now that they've grown to tens of millions of bytes, most of their content is learned from example. But there is a big practical difference between animal and robot learning. Animals learn individually, but robot learning can be copied from one machine to another. For instance, today's text and speech understanding programs were painstakingly trained over months or years, but each customer's copy of the software is "born" fully educated. Decoupling training from use will allow robots to do more with less. Big computers at the factory - maybe supercomputers with 1,000 times the power of machines that can reasonably be placed in a robot - will process large training sets under careful human supervision, and distill the results into efficient programs and arrays of settings that are then copied into myriads of individual robots with more modest processors.
Programs need memory as well as processing speed to do their work. The ratio of memory to speed has remained constant during computing history. The earliest electronic computers had a few thousand bytes of memory and could do a few thousand calculations per second. Medium computers of 1980 had a million bytes of memory and did a million calculations per second. Supercomputers in 1990 did a billion calculations per second and had a billion bytes of memory. The latest, greatest supercomputers can do a trillion calculations per second and can have a trillion bytes of memory. Dividing memory by speed defines a "time constant," rougly how long it takes the computer to run once through its memory. One megabyte per MIPS gives one second, a nice human interval. Machines with less memory for their speed, typically new models, seem fast, but unnecessarily limited to small programs. Models with more memory for their speed, often ones reaching the end of their run, can handle larger programs, but unpleasantly slowly. For instance, the original Macintosh was introduced in 1984 with 1/2 MIPS and 1/8 megabyte, and was then considered a very fast machine. The equally fast "fat Mac" with 1/2 megabyte ran larger programs at tolerable speed, but the 1 megabyte "Mac plus" was verged on slow. The four megabyte "Mac classic," the last 1/2 MIPS machine in the line, was intolerably slow, and was soon supplanted by ten times faster processors in the same enclosure. Customers maintain the ratio by asking "would the next dollar be better spent on more speed or more memory"?
The best evidence about nervous system memory puts most of it in the synapses connecting the neurons. Molecular adjustments allow synapses to be in a number of distinguishable states, lets say one byte's worth. Then the 100 trillion synapse brain would hold the equivalent of 100 million megabytes. This agrees with our earlier estimate that it would take 100 million MIPS to mimic the brain's function. The megabyte/MIPS ratio seems to hold for nervous systems too! The contingency is the other way around: computers are configured to interact at human time scales, and robots interacting with humans seem also to be best at that ratio. On the other hand, faster machines, for instance audio and video processors and controllers of high performance aircraft, have many MIPS for each megabyte. Very slow machines, for instance time-lapse security cameras and automatic data libraries, store many megabytes for each of their MIPS. One day we may find that plants remember a lot, but process it slowly, so have many megabytes/MIPS. Flying insects seem to be a few times faster than humans, so may have more MIPS than megabytes.