The Janus Illusion

Abstract

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.

Get full access to this article

View all access options for this article.

References

Gibson

J. J.

(1966). The senses considered as perceptual systems. Boston: Houghton-Mifflin.

Gibson

J. J.

(1979). The ecological approach to visual perception. Boston: Houghton-Mifflin.

Gogel

W. C.

(1990). A theory of phenomenal geometry and its applications. Perception and Psychophysics, 48, 105–123.

Gregory

R. L.

(1970). The intelligent eye. New York: McGraw-Hill.

Gregory

R. L.

(1980). Perceptions as hypotheses. Philosophical Transactions of the Royal Society of London, B 290, 181–197.

Hochberg

(1986). Visual perception. In Atkinson

R. C.

Herrnstein

R. J.

Lindsey

Luce

R. D.

(Eds.), Stevens' handbook of experimental psychology (Vol. 1, pp. 195–276). New York: Wiley.

Klopfer

(1991). Apparent reversals of a rotating mask: A new demonstration of cognition in perception. Perception and Psycho-physics, 49, 522–530.

Ramachandran

V. S.

(1988). Perceiving shape from shading. Scientific American, 259(2), 76–83.

Rock

(1983). The logic of perception. Cambridge, MA: MIT Press.