MacGregor Campbell, consultant Here's a freaky twist on a
classic illusion. Thomas Papathomas, a vision researcher at
Rutgers University in New Brunswick, New Jersey, has created a
new variation on the hollow-mask illusion - where viewers find
it difficult to distinguish whether they are seeing a mask's
interior or its exterior. Papathomas's version uses both a
hollow mask and a hollow torso, each inverted from the other so
that when the head is concave, the torso is convex, and vice-
versa. As the torso and head rotate, in the same direction, they
appear to rotate oppositely to one another, resulting in the
head appearing to twist unnaturally. Papathomas calls this the
"Exorcist Illusion," referring to a famous scene in the 1973
horror movie, The Exorcist, in which a girl possessed by the
devil twists her head around a full 360 degrees. This illusion
requires no satanic involvement however. A constantly rotating
hollow mask - or torso - appears to rotate in one direction when
the exterior is facing the viewer