Wednesday, 3 October 2007

Green Dots

If your eyes follow the movement of the rotating pink dot, the dots will remain only one color, pink.

However if you stare at the black " +" in the centre, the moving dot turns to green.

Now, concentrate on the black " + " in the centre of the picture. After a short period, all the pink dots will slowly disappear, and you will only see only a single green dot rotating.

This and many other illusions courtesy of Michael Bach of the Universit├Ąts-Augenklinik Freiburg.


KG said...

That's pretty cool, ZB. Any idea why and how the brain does that?

Zoe Brain said...

First, I'll have a guess.

It's to do with the different circuits we have for recognising movement, as opposed to a constant picture.

Constant pictures have LOTS of information in them, too much to handle. So our internal circuitry concentrates on changes, not the whole picture.

When we concentrate on the cross in the centre, that takes up all the processing, so the difference between pink and grey at the edges of the area get edited out, much as we can't read the words in our peripheral vision.

But the change-recognition circuitry is still in operation at the edges of our vision. Stuff that has been around since our mammal-like ancestors were dodging dinosaurs. Now things aren't actually moving, but due to the time-lag and "fatigue" in the colour receptors in the eye, it appears they are.

So we perceive a "greening" of what was there before. Actually from pink "greened" to grey, but as the other circuits have simplified pink to be the same as the grey majority, it comes out as grey "greened' to, well, green.

The initial effect, the "green dot" when the pink dots have not yet been "edited out" is caused by the hysteresis - the lag - in the rods and cones in our eyeballs. If you look at a solid object for a while of one colour, then look at a white wall, you will see the same shape in an inverted colour due to the time it takes for the photo-reactive chemicals to "reset".

That's my guess based upon what I know of the brain's workings. Now I'll go look it up...

OK, here's what has been said
"This illusion works on a principle involving a trait human eyes have: the color sensitive areas actually get “fatigued” if they stare at an area with the same color for too long. As a result, you see the complementary color once you take your eyes off that area. In this case, because the pink dot at one position is there for all but one time when the animation has it disappear, you see the green dot there because you’ve stared at the pink for too long. Simple. If you stare at the picture for a while, then look away, you should see a ring of green dots for a split second. Same principle."

A full explanation - with controls so you can change the dots' colour - is over at

I have been repeatedly asked to explain this in more detail, so here goes:

1. There is something called the “negative retinal afterimage”. It becomes visible when one given hue stays on the same retinal position for several seconds (usually we would move our eyes typically 3 times per second, so this is no disadvantage in normal viewing). The afterimage builds up as that retinal location adapts to this special hue, and when looking at a neutral background the complementary colour is seen.
2. This is a good thing, normally, because it helps “colour constancy”, that is we see colours somewhat independent of the ambient illumination (compare the bluish glacier noon sun with a reddish tint in the evening living room by the fireside).
3. Ok, so the afterimage is “burnt in”, meaning: that retinal location is adapted. Now the magenta patch is suddenly switched to gray. Because of the adaptation, the complementary colour is now seen, which would be green for magenta, or light gray for a dark gray.
4. The retinal afterimage typically fades away rapidly (over a few seconds under normal conditions). But here this fade-out does not reduce the perception of the afterimage, because a new one is uncovered right after at the next location.
5. In addition, a Gestalt effect, here the “phi phenomenon” comes into play: the afterimage from the successive retinal locations is integrated and perceived as one single moving object, namely the green disk.
6. In summary, the following factors make this illusion rather compelling:
* it is rather easy to steadily fixate on the centre
* most of the time the retinal locations are re-adapted and the afterimage is uncovered only briefly
* a Gestalt effect leads to the perception of a flying green disk.

Now being a CompSci person, I naturally put emphasis on the processing bits first, and it seems these are, well, peripheral. Most of the effect is due to what happens in the eyeballs. So I was sorta right in my guess, but with the completely wrong emphasis on what was important.

SnoopyTheGoon said...

Looking at a single point fixedly with a running spot: I wonder how much time is needed before a person starts biting his/her keyboard?

Reminds me one of Stanislaw Lem's short stories...