What to see?
This illusion is called “lilac chaser” or Pac-Man illusion. It’s represented by 12 lilac or 12 blurred discs arranged in circle around a black small cross. Every disc tends to disappear for a short period of time, reappear, then the next one disappears and so on. There are two things you have to notice:
- A gap that is moving around the lilac discs;
- A green circle filling in the disappearing lilac disc. The green color appears because it’s a complementary for lilac.
What to do?
For you to feel the illusion stronger you have to look directly at the central cross. After fixating the point you will easily see that around the cross there are a bunch of dots appearing and disappearing one at a time. The gap created by their disappearance is filled in by a green disc.
If instead of fixating the black cross you chose to follow the movement of the lilac discs, you will rather see their moving and the gap than the green disc. For the aftereffect to happen it’s necessary for the eye to remain steady, thing that will not happen in this case.
If you look at the image for a minute or so and then you turn your look for a few moments away from the picture, you will see that when you look at the picture again they will be 12 stationary green discs instead of the lilac ones.
How it works?
If a movement is perceived at one place, in the visual field and then a similar event occurs at an adjacent place in the same visual field, we perceive movement from the first place to the second and we call it apparent movement because no move has actually occurred. In our case the apparent movement is represented by the disappearing and appearing of the lilac discs.
After their disappearance a small green spot appears and stay there for just a few moments, until the lilac appears again. This afterimage is actually the adaptation from the retina, of the rods and cons. Their adaptation begins right after they have been stimulated. . In the lilac chaser, we keep our eyes still, so the afterimage grow and is revealed when the stimulus disappears. Why does this happen? The phenomena occur because when a blurry stimulus is presented in a part of the visual field where we don’t fixate our look, the stimulus will still disappear. So even if the eyes move a little when we are fixating a point the movement is not big enough not to perceive the aftereffect.
This illusion has been created by Jeremy Hinton.