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ternate approach is to replace standard digital chips with neuro- morphic chipsalso made of silicon, but with a different mode of operation, one that mimics the workings of neurons and organic brains, which do not operate digitally.The transistors in a digital chip function as on-off switches that represent the binary digits 1 and 0, whereas the currents that flow among neurons do more than turn on and off. In neurons, the magnitude of the currents and how the cur- rents change with time are essential information. Data transmitted in this way are said to be analog rather than digital in nature.

The differences between electronic digital processing and bio- logical analog processing motivated two founders of the so-called neuromorphic approach:Carver Mead, Professor Emeritus at Caltech, who coined the term in the late 1980s, and Eric Vittoz of the Swiss Federal Institute of Technology. Compared to digital technology, writes Mead,Biological information-processing systems operate on completely different principles,and adds:

For many problems . . . biological solutions are many orders of magnitude more effective than those . . . using digital methods.This advantage can be attributed principally to . . . the representation of information by the rela- tive values of analog signals . . .

For Vittoz, an important advantage is that,The collective computa- tion carried out by the brain in its massively parallel architecture can be emulated on silicon.

Kwabena Boahen, at the University of Pennsylvania, uses neuromorphic principles in the synthetic retina mentioned in the previous chapter, and emphasizes the efficiency of the method for the construction of an artificial brain.Transistors in a digital mode operate faster than neurons, but they also use more power. Boahen estimates that a digital chip as capable as the human brain would consume a billion watts of electrical power.If his estimate is anywhere near accu- rate, it is highly unlikely that we could ever build a humanlike digital brain in a form suitable for an artificial being.To explore the possibili- ties of using neuromorphic chips that copy how the brain operates, researchers in Boahens group have projects under way to develop a chip that follows the special pattern of connectivity among the six

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