The spotlight shines on the magician’s assistant. The woman in the tiny white dress is a luminous beacon of beauty radiating from the stage to the audience. The Great Tomsoni announces he will change her dress from white to red. On the edge of their seats, the spectators strain to focus on the woman, burning her image deep into their retinas. Tomsoni claps his hands, and the spotlight dims ever so briefly before reflaring in a blaze of red. The woman is awash in a flood of redness.
Whoa, just a moment there! Switching color with the spotlight is not exactly what the audience had in mind. The magician stands at the side of the stage, looking pleased at his little joke. Yes, he admits, it was a cheap trick; his favorite kind, he explains devilishly. But you have to agree, he did turn her dress red—along with the rest of her. Please, indulge him and direct your attention once more to his beautiful assistant as he switches the lights back on for the next trick. He claps his hands, and the lights dim again; then the stage explodes in a supernova of whiteness. But wait! Her dress really has turned red. The Great Tomsoni has done it again!
The trick and its explanation by John Thompson (aka the Great Tomsoni) reveal a deep intuitive understanding of the neural processes taking place in the spectators’ brains—the kind of understanding that we neuroscientists can appropriate for our own scientific benefit. Here’s how the trick works. As Thompson introduces his assistant, her skintight white dress wordlessly lures the spectators into assuming that nothing—certainly not another dress—could possibly be hiding under the white one. That reasonable assumption, of course, is wrong. The attractive woman in her tight dress also helps to focus people’s attention right where Thompson wants it—on the woman’s body. The more they stare at her, the less they notice the hidden devices in the floor, and the better adapted their retinal neurons become to the brightness of the light and the color they perceive.
All during Thompson’s patter after his little “joke,” each spectator’s visual system is undergoing a brain process called neural adaptation. The responsiveness of a neural system to a constant stimulus (as measured by the firing rate of the relevant neurons) decreases with time. It is as if neurons actively ignore a constant stimulus to save their strength for signaling that a stimulus is changing. When the constant stimulus is turned off, the adapted neurons fire a “rebound” response known as an afterdischarge.
In this case, the adapting stimulus is the red-lit dress, and Thompson knows that the spectators’ retinal neurons will rebound for a fraction of a second after the lights are dimmed. The audience will continue to see a red afterimage in the shape of the woman. During that split second, a trap door in the stage opens briefly, and the white dress, held only lightly in place with Velcro and attached to invisible cables leading under the stage, is ripped from her body. Then the lights come back up.
