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thin layer (6 millimeters or 0.25 inches thick) covering the brain (cor- texcomes from the Latin word for tree bark). It is the area where, according to traditional views of the brain, much of our thinking goes on; hence, the association of gray matterwith intellectual activity. The cortex consists of layers of neurons, along with other cells that give them physical and physiological support.The neurons are arranged in orderly fashion,lying in vertical columns that are further grouped into functional clusters such as the visual and auditory cortices.

Much of the power of the brain comes from the sheer processing power of its 100 billion neurons.The cortex contains more of these than you might think because its wrinkles disguise its large surface area. Smoothed and spread out, the cortex would cover a square yard. Also significant is the number of interconnections among the neu- rons. Each neuron is connected to a thousand or more others, made possible partly by the fact that they link through three dimensions, vertically as well as horizontally.

Artificial brainsthat is, microprocessor and memory chips that manipulate and store dataare also a form of gray matter, of a shinier sort, the color of pure silicon, polished to a reflective gunmetal sheen. Like neurons, the interconnected transistors etched into a chip trans- fer signals among themselves and come in huge numbers. Compared to a computer chip from the late 1970s, a modern Pentium processor has a thousand times more transistors and manipulates bits several thousand times faster, typically executing about 2,000 million instruc- tions per second (MIPS).Todays memory chips typically hold hun- dreds of megabytes of data.

But even the millions of transistors in a computer chip hardly compare to the billions of neurons in the brain. Further, the transis- tors are interconnected less abundantly than are neurons, partly be- cause the chip is flat and not three-dimensional, and partly because it works serially,by executing one instruction after another.That scheme is less powerful than the massively parallel multiple processing carried out by an organic brain. In this respect, chips are inferior to natural brains, but they are also faster, transferring electronic signals in nano- seconds, a million times faster than organic systems. This enormous

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