My friend Gerry Pareja forwarded a link to this story from ‘Improbable Research’ about the first prize winner in the Neural Correlate Society’s 2007 Illusion of the Year contest.
The image certainly is pretty cool. But to test the explanation I tried covering each image in turn and the effect was still there! I wondered if there was a perceptual delay effect in that our memory of one picture affects our interpretation of the other but then I also noticed that the effect changes depending on where the picture is in our field of view. If I position myself facing the right hand edge of the monitor, then the right hand tower seems more vertical and the left one almost seems to lean left or backwards. So the illusion may be more (or at least partly) due to the fact that in the absense of other visual cues in the picture we tend to interpret as if viewed from the direction at which we are looking at the picture – ie we interpret the picture edge as a window frame.