Sunday, 30 January 2011
Which way is the head spinning?
Look at the face from a distance of a few metres for a minimum of one full rotation. Also try looking at it while covering one eye, as well as changing viewing position and distance.
The head is actually spinning in the same direction all the time. However, as the convex side changes to the concave side, the direction of the spinning head appears to change.
We do not observe the world in a passive manner, but rather, our brain makes interpretations of the information sent by our senses. These interpretations also draw upon our experiences, and we try to change surprising or odd observations into something more usual. The real human faces we see are convex. As we look at the spinning head, we ignore the observational cues which tell us that the other side is concave. Instead, the direction of the spinning appears to change when light and shadow change their locations on this face that we interpret as being convex.
The illusionary effect is further affected by light and depth perception. By covering one eye, we miss the concave features of the face, since the ability to perceive depth is based on the use of both eyes. The significance of light is revealed by viewing the face from different directions.
Depth perception is the ability to perceive the world in three-dimensional form and to assess distances. Depth perception is based on slightly different superimposed images that our eyes provide of the same object. Test this principle by lining up your finger with, for example, the vertical frame of a door. Now open and close each eye.
More about optical illusions