Imagine this scene from the future. You are staring at a screen flickering with snow. Scientists have hidden one of two patterns in the dots, and eventually you spot one. But you don't have to tell the scientists what you are seeing; they already know. They are looking at the electrical signals from one of the billions of cells in your brain. When the cell fires, you see one pattern; when it stops, you see another—your awareness can be read from a single neuron. Now, in an even more unsettling trick, they send an electrical current to the neurons in that part of your brain and, with a push of a button, make you see one pattern or the other.

These feats of pinpoint mind study are not fantasies. They have already been performed by the Stanford neuroscientist William Newsome. Not with people, of course, but with monkeys. Yet few scientists doubt the trick would work with us.

This is just one example of how much we have learned about the workings of the brain in the past 10 years—a period of intense research proclaimed by the U.S. Congress and the President as the Decade of the Brain. Every facet of mind, from mental images to the moral sense, from mundane memories to acts of genius, have now been tied to tracts of neural real estate. Using fmri, a new scanning technique that measures blood flow, scientists can tell whether the owner of the brain is imagining a face or a place. They can knock out a gene and prevent a mouse from learning, or insert extra copies and make it learn better. They can see the shrunken wrinkles that let a murderer kill without conscience, and the overgrown folds that let an Einstein deduce the secrets of the universe. How far will this revolution go? Will we ever understand the brain as well as we understand the heart, say, or the kidney? Will mad scientists or dictators have the means to control our thoughts? Will neurologists scan our brains down to the last synapse and duplicate the wiring in a silicon chip, giving our minds eternal life?

No one can say. The human brain is the most complex object in the known universe, with billions of chattering neurons connected by trillions of synapses. No scientific problem compares with it. (The Human Genome Project, which is trying to read a long molecular string composed of four letters, is a snap by comparison.) Cognitive neuroscience is attracting so many brilliant minds that it would be foolish to predict that we will never understand how the brain gives rise to the mind. But the problem is so hard that it would be just as foolish to predict that we will.

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