During the Optical Art Movement of the 1960s, artists would create all sort of puzzling effects with color. For instance, this "flashing squares" drawing seems to wobble and flash when you concentrate on one particular area of the image. How many squares can you see in this diagram? Can you feel the "motion" of the image?
When sets of disks with tangential greylevel gradients are arranged in concentric circles (see image above, most observers perceive these disks moving around the centre, similar to Kitaoka’s ‘snake illusion’. This motion illusion is enhanced for large-scale and bright images and depends to a large extent to dynamic changes in the stimulus such as elicited by involuntary eye movements or blinks – fixating the centre of the pattern does abolish the illusion, whereas scanning the picture the motion sensation. A reliably effective version of this illusion, which does not require eye movements (i.e. persists when observers fixate the target in the centre of the image), can be generated by modulating the background luminance of the array of disks . This stimulus offers the opportunity of studying this motion illusion – the percept of spinning disks in the absence of any physical displacement – in a highly controlled manner in psychophysical and physiological experiments, because it is not depending on involuntary eye movements or eye blinks. Work in preparation (Zanker 2005) will demonstrate how this illusion can be explained in terms of a two-dimensional motion detector network
Just like you did for the other Illusions before, stare intently at the red dot in the center of the left block for about a couple of minutes. Once you've done this, look over to the center of the right block. It should act weird - by fading out and 'flickering'. If you move your head closer and further from the monitor (while focussing in the center), you should see some interesting interference effects too.
Do you remember the story of Dr. Jekyll and Mr. Hyde? Dr Jekyll, the famous scientist, is not all he seems, and the other side of his character - Mr Hyde - is, you will recall, a distinctly nasty piece of work.
Continuing the idea of the two sides of the human character, have a look at the picture above. On the left is Dr Angry, and on the right is Mr Calm.
Now, back away from your computer screen. This effect will work at different distances for different people, but you should see the two characters swap over...! As you come closer to the screen again, they revert to their original characters. Spooky or what!
As optical illusions go, we think this is pretty impressive. A short explanation goes something like this. When we look at an object, we can normally see both fine detail and coarse detail. However when we are close, the fine detail will dominate, and when we are further away, we lose the fine detail, and see more of the coarse detail.
Both of the faces you see above are hybrids - each face is actually a combination of two faces. The left hand face shows an angry man in fine detail, but within the picture there is also coarse detail of the calm face. Move away, and you lose the fine (angry) detail, and just see the coarse (calm) detail.
The right hand face shows the calm face in fine detail, and the angry face in coarse detail.
