The visual cortex is attentive to sudden changes in the environment, both when something new appears and when something disappears, Dr. Martinez-Conde said. A sudden disappearance causes what neuroscientists call an after-discharge: a ghostly image of the object lingers for a moment.

This illusion is behind a spectacular trick by the Great Tomsoni. The magician has an assistant appear on stage in a white dress and tells the audience he will magically change the color of her dress to red. He first does this by shining a red light on her, an obvious ploy that he turns into a joke. Then the red light flicks off, the house lights go on and the now the woman is unmistakably dressed in red. The secret: In the split-second after the red light goes off, the red image lingers in the audience’s brains for about 100 milliseconds, covering the image of the woman. It’s just enough time for the woman’s white dress to be stripped away, revealing a red one underneath.

In a conference last summer, hosted by Dr. Martinez and Dr. Macknik, a Las Vegas pickpocket performer and co-author named Apollo Robbins took advantage of a similar effect on the sensory nerves on the wrist. He had a man in the audience come up on stage and, while bantering with him, swiped the man’s wallet, watch and several other things. Just before slipping off the timepiece, Mr. Robbins clutched the man’s wrist while doing a coin trick  thereby lowering the sensory threshold on the wrist. The paper, with links to video of Mr. Robbins’ performance, is at http://www.nature.com/nrn/journal/vaop/ncurrent/full/nrn2473.html.

“That was really neat, and new to me,” said Dr. Bach, who was in the audience. The grasp, he said, left “a sort of somatosensory afterimage, so that the loss of the watch stays subthreshold” in the victim. The visual cortex resolves clearly only what is at the center of vision; the periphery is blurred, and this is likely one reason that the eyes are always in motion, to gather snapshots to construct a wider, coherent picture.

A similar process holds for cognition. The brain focuses conscious attention on one thing at a time, at the expense of others, regardless of where the eyes are pointing. In imaging studies, neuroscientists have found evidence that the brain suppresses activity in surrounding visual areas when concentrating on a specific task. Thus preoccupied, the brain may not consciously register actions witnessed by the eyes.

Magicians exploit this property in a variety of ways. Jokes, stagecraft and drama can hold and direct thoughts and attention away from sleights of hand and other moves, performers say.

But small, apparently trivial movements can also mask maneuvers that produce breathtaking effects. In a telephone interview, Teller explained how a magician might get rid of a card palmed in his right hand, by quickly searching his pockets for a pencil. “I pat both pockets, find a pencil, reach out and hand it to someone, and the whole act becomes incidental; if the audience is made to read intention  getting the pencil, in this case  then that action disappears, and no one remembers you put your hand in your pocket,” the magician said. “You don’t really see it, because it’s not a figure anymore, it has become part of the background.”