In the world of Harry Potter the one thing you don't want to be is a "muggle". Muggles are the regular folk lacking magical powers, and discrimination and prejudice against them is rampant. Muggles are not merely unable to attend Hogwarts School of Witchcraft and Wizardry, but are kept ignorant of the school's very existence. In fact, muggles are kept in the dark about the entire cryptic world of magic altogether. The sorcerers struggle for the heart and soul of humankind's freedom as the muggle folk blindly graze. Muggles are mutton.

In our world we are all muggles, but at least we can be content knowing we're not missing out on all the cool stuff.

...because in the real world there's no magic.

Or is there?

There may not be "true" magic in our world, but we have illusions. Gobs of them. And who's to say illusions are not magic? Perhaps in Harry Potter's world the spells only seem like magic because the natural principles underlying them are not well understood. And maybe sorcerers like Harry Potter have an inborn knack for employing these natural principles.

Illusions, I submit, are examples of real world magic. And the purveyors of illusions - artists and cognitive scientists - are our world's sorcerers.

But not all real-world sorcery is made alike. Just as the spells taught at Hogwarts vary in potency, the illusions of our world vary in potency.

The three tiers of magic.

The three tiers of illusion potency.

And Chris Chabris and Dan Simons, the authors of the new book The Invisible Gorilla, are among the most powerful of the real-world sorcerers. Their book is an engaging romp through a variety of cognitive illusions, with the theme that our intuitions often fail us. The book is written so well it would make Gladwell envious (and maybe a wee bit angry), and yet we must remember that these are the scientists themselves discussing their own discoveries and experiments. They have somehow mastered both the science and its communication.

But it's their master sorcery I wish to focus on here. Their illusions are at the top tier of real-world magic.

To appreciate just how powerful is their sorcery, let's start at the lowest level of real world sorcery, and build up to "the invisible gorilla."

The first level of visual sorcery includes illusions such as the Ames Room, the rotating mask illusion, and the uphill marbles illusion. Such illusions are exciting, but they are conceptually simple parlor tricks. These entry-level visual illusions rely upon ambiguity, or the fact that the light coming toward the eye does not uniquely determine what the world is like out there. The strategy is to devise an unusual scene that happens to look like something more common, and then let the hilarity ensue. For the Ames Room the true scene is a radically geometrically distorted room that happens to look like a normal square room from the right viewpoint; and the hilarity ensues when people stand in the two corners, in which case one person appears to be a giant relative to the other. Other examples of this first level are illusions like the Necker cube, rabbit/duck, vase/face and old-hag/young-maiden, where the strategy is to devise an image that could equally be due to two different scenes.

Why are these ambiguity illusions mere muggle magic? For starters, it is entirely unmysterious why the illusions occur. Magic needs to be enigmatic! And there's a deeper reason why these ambiguity illusions don't count as high sorcery: in a sense they are not illusions at all. One's brain elicits a perception of its best guess about what is out there given the light the eye received. It just happens that the light sent to the eye was rigged so that the best guess would fail.

The more potent tiers of magic are not of this "ambiguity" kind. In "real" illusions the brain creates a perception that is not even consistent with the light that was sent to the eye! That is, for the higher tiers of sorcery, of all the scenes that could potentially have sent that light to your eye, your brain guesses a scene not among those! Now that's an illusion!

The second level of visual sorcery includes the geometrical ones like the Hering illusion, where the two vertical lines project nearly parallel to one another in your eye, but you perceive them as bowing away from one another at the center. That's a perception that is not consistent with the stimulus, and that makes it a real illusion, and a real mystery. These stimuli have the power of making people perceive something that couldn't possibly exist out there. In my research I have argued that illusions of this kind are explained by your brain attempting to anticipate what the scene will look like when the perception is finally constructed a tenth of a second later, in order to overcome neural delays. In short, I have argued that the brain has mechanisms for anticipating the near future so as to thereby perceive the present. Here's a short, four-slide introduction to the idea here.

And now we get to level-3 sorcery, the dark and gorilla-ey stuff. Whereas the level-2 illusions involve perceiving angles a few degrees off, level-3 illusions can be radically stronger. invisible gorillas. In the real stimulus, as everyone knows, Chabris and Simons have a movie of basketball players, and during the video a person dressed up as a gorilla walks in, bangs on her chest, and walks off. For observers asked to count the number of ball passes, about half fail to see the gorilla. Their visual system has "gorilla all over it," but they don't see it because they don't attend to it.

Now that's magic. And not only is the invisible-gorilla illusion a radically stronger illusion than level-2 illusions, but it is also as or more enigmatic: although it is somewhat plausible that a lack of attention will lead to missed stuff, why shouldn't the brain have bottom-up mechanisms that shift attention to large dangerous beasts that approach you menacingly?!

The sorcery of Chabris and Simons is so potent one suspects they've been dabbling with the darker forces of he who shall not be named. It can make those of us involved with level-2 optical illusions cower with respect at these superior sorcerers. It certainly made a muggle out of me.

Mark Changizi is the author of The Vision Revolution and a professor at Rensselaer Polytechnic Institute.

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