How brains differ from computers
You often hear people comparing the human brain and the electronic computer and, on the face of it, they do have things in common.
A typical brain contains something like 100 billion minuscule cells called neurons (no-one knows exactly how many there are and estimates go from about 50 billion to as many as 500 billion). [1]
Each neuron is made up of a cell body (the central mass of the cell) with a number of connections coming off it: numerous dendrites (the cell's inputs—carrying information toward the cell body) and a single axon (the cell's output—carrying information away). Neurons are so tiny that you could pack about 100 of their cell bodies into a single millimeter. (It's also worth noting, briefly in passing, that neurons make up only 10–50 percent of all the cells in the brain; the rest are glial cells, also called neuroglia, that support and protect the neurons and feed them with energy that allows them to work and grow.) [1] Inside a computer, the equivalent to a brain cell is a nanoscopically tiny switching device called a transistor. The latest, cutting-edge microprocessors (single-chip computers) contain over 50 billion transistors; even a basic Pentium microprocessor from about 20 years ago had about 50 million transistors, all packed onto an integrated circuit just 25mm square (smaller than a postage stamp)! [2]
That's where the comparison between computers and brains begins and ends, because the two things are completely different. It's not just that computers are cold metal boxes stuffed full of binary numbers, while brains are warm, living, things packed with thoughts, feelings, and memories. The real difference is that computers and brains "think" in completely different ways. The transistors in a computer are wired in relatively simple, serial chains (each one is connected to maybe two or three others in basic arrangements known as logic gates), whereas the neurons in a brain are densely interconnected in complex, parallel ways (each one is connected to perhaps 10,000 of its neighbors). [3]
This essential structural difference between computers (with maybe a few hundred million transistors connected in a relatively simple way) and brains (perhaps 10–100 times more brain cells connected in richer and more complex ways) is what makes them "think" so very differently. Computers are perfectly designed for storing vast amounts of meaningless (to them) information and rearranging it in any number of ways according to precise instructions (programs) we feed into them in advance. Brains, on the other hand, learn slowly, by a more roundabout method, often taking months or years to make complete sense of something really complex. But, unlike computers, they can spontaneously put information together in astounding new ways—that's where the human creativity of a Beethoven or a Shakespeare comes from—recognizing original patterns, forging connections, and seeing the things they've learned in a completely different light.
Wouldn't it be great if computers were more like brains? That's where neural networks come in!