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Because of the brain's isolation, everything that forms my knowledge of the world reduces down to a sequence of electrical signals, like the dots and dashes of Morse code, reporting in from millions of nerve sensors. Think of the voice that comes to you over the telephone. Someone on the other end speaks, and electronic equipment changes those sound waves into electrical signals which pass through relay stations to be reassembled on your end as vibrating sound waves. If the caller uses a cell phone, the sound is translated into packets of digital code and broadcast through the air, like a radio transmission, before entering your telephone receiver. Yet you "hear" your mother's voice.

My isolated brain must trust signals much like those digital codes in order to perceive the world. The doorbell rings and I get up to answer it. Tom, the UPS driver, has a package for me. I greet him, sign for the package, and return to my desk. It would take a computer programmer to appreciate fully the miracle involved in my recognizing Tom. Sound receptor cells in my ear first detected the frequency of my doorbell, approximately an octave above the musical "middle C," and then interpreted the much more variable pitch of Tom's bass voice. Computer software now has the ability to recognize individual voiceprints, and even words spoken clearly by a speaker. No computer, however, has yet mastered the much more difficult task of recognizing a human face.

The human eye has 127,000,000 receptor cells, called rods and cones, that report on the shape, texture, and color of Tom's lips, eyes, eyebrows, nose, and hair. They do this effortlessly, comparing the results to a memory bank of all the faces I know, in a fraction of a second. I do not have to stand and consciously assemble all the data corresponding to Tom's face. (In a brain disorder called prosopagnosia, a person loses this ability; even while staring at the closest family member, a person afflicted with this malady cannot recognize the face.)

All the processing of data from the senses—touch, pain, hearing, sight, smell—takes place inside the bony box of skull. Less than one percent of nerve cells gather this information for the brain, and then carry out its orders: playing the drums, cooking a meal, speaking a language, typing this sentence. All other nerve cells remain locked inside my skull, communally processing the data and delegating instructions at lightning speed. The brain's total number of connections rivals the quantity of stars and galaxies in the universe.

Naturally errors, or "illusions," creep in, and in fact every person who has ever lived has a different perception of the world. A color-blind person does not notice Tom's blue eyes; a deaf person does not know the pitch of his voice. Yet so resourceful is the brain that a great composer, like Beethoven, can "hear" an entire symphony in his head even when totally deaf. Or, a deaf and blind person, such as Helen Keller, can assemble a "picture" of the world that gives striking new insight to those of us more sensorially gifted.

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