The Janus Illusion

A perceptual illusion, especially suitable for introductory psychology demonstrations, is described. The essence of the illusion is that an ordinary dime-store plastic mask, when facing 180° away from an observer, will nonetheless appear to face that observer. Moreover, as one moves left and right in the frontal plane, thereby changing the angle from which the mask is viewed, the mask generally appears to follow the moving observer. When the observer is stationary but the mask is rotated slowly, it will appear to oscillate. In addition to being fascinating in its own right, the illusion can be used to illustrate several important points, including that perception is still, as enunciated in the 19th century by Helmholtz, usefully conceptualized as hypothesis testing.