What The Shrunken-Finger Illusion Reveals About the Brain
There’s a popular and simple magic trick involving multiplying balls: The magician holds a ball, usually red, between her thumb and index finger. They flick their wrist and suddenly, a second ball appears between their index and middle fingers. They keep on flicking and balls keep on appearing..
The secret is simple: the first “ball” is really an empty hemispherical shell, with the second ball nesting inside it. When the magician flicks their wrist, they roll the second ball out of its shell. With a few flourishes, they can then load the third and fourth balls into the shell, before rolling them out in turn. Audience members automatically assume that the empty shell is a full sphere, because it’s indistinguishable from one when viewed from the front.
But there’s more to the trick than that, says Vebjørn Ekroll, a psychologist at the University of Leuven in Belgium. We deeply, strongly, resolutely perceive the shell as a sphere because our brains create a full representation of the ball. The hypothetical back half that we cannot see feels as real as the front half that we actually can. Read more..