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layers of neurons in the visual cortex, and to implement the thou- sandfold connectivity among neurons that characterizes the brain.

Another new design with similarities to the brain is the adaptive or reconfigurable chip. In standard chip technology, once the transis- tors and other electronic elements are etched into the silicon, the circuitry they form is physically fixed, which also fixes what the chip can do. Normally the only way to change the circuit is to make a new chip. An adaptive chip, however, contains huge numbers of funda- mental data-processing elements. Like myriad Lego blocks connected to form any one of an infinite number of structures, these elements can be connected in different configurations to perform different functions. Remarkably, this can be done on the fly with software commands rather than by the laborious process of changing hardware. Paul Master of QuickSilver Technology, a firm that along with Intel, IBM, and others is investigating adaptive chips, says: Until now, the hardware had to match the problem. Now we can change that.

According to proponents of the technology, adaptive chips offer speed and efficient use of power, in themselves important factors for artificial brains. But the truly novel aspect of the adaptive chip is its ability to change itself in real time, a crucial feature of the human brain not available in conventional chips: plasticity, the ability to change neural pathways according to new experience. Plasticity plays a large role in our development when we are young, and in learning and memory throughout our lives. It also gives the brain a way to compensate for functions that have been lost or reduced due to injury.

At this point in the development of adaptive chips, potential ap- plications are limited to specialized devices such as cellphones, and there are questions about the long-term impact of the technology. But if it proves viable, then as is usually the case in nanoelectronics, improvement will come at a furious pace. We might expect to see second-generation silicon brains with the built-in ability to change and learn just as the human brain doesor perhaps solve for them- selves the essential problems, such as finding the right mixture of rational and emotional thought.

There is, however, a different direction to consider in making

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