Visual Perception questions

  1. I’ve made some simple pictures with a single-coloured shape on a single-coloured background. Or sometimes I’ve used a grid of dots. If the background is lighter than the shapes, I can see ghost-like versions of them that are the same colour as the background, except lighter - even if the background colour is white. When I move my eyes around, these ghost images move around. When the background colour is darker than the shapes then the ghost shapes are even darker than the background colour - even if the background colour is black.
    I was wondering what that effect is called so that I can do more research on it. I don’t understand why the “ghost” image (what is it called?) is even lighter or darker than the rest of the picture.

  2. I’ve seen pictures which show the path that our eye takes when it is viewing a picture - a jagged line which shows where it has focused on. I was wondering what equipment they use to track eye movement.


I don’t know the answers to your question but I’ve experienced this too. I especially notice it with red on a blue background. The image seems to become 3-D with the dots seemingly floating above the background and the seem to jiggle if the paper is moved.

Another of this type of illusion can be had from a disk of white with black markings on it. If the disk is spun you stop seeing black-and-white and start seeing colors.

I wouldn’t be surprised if persistence of vision had something to do with this but I’m just guessing.

As for the equipment used to track where the eye is looking I don’t know what it’s called but I’ve see it too. I think they use a low powered laser but I’m not sure on that either. Doesn’t the military use this technology for sight guided tracking systems?

Here’s some examples of what I’m talking about:

Here, you can see light green x’s - you might need to stare a while at the dark blue x’s first… then move your eyes around.

This one makes dark blue x’s - that have a yellow halo.

It works with any colour combination really, and all that it does is lighten or darken the background - depending if the shapes are darker or lighter than the background. I looked up “persistence of vision” a bit and there were some things about images staying in your eyes for a while… I think it said that the retina (rods or cones?) gets fatigued and over-compensates.

You may also want to look up the term “afterimage.”

Some good sites with related information: – go to “Illusions” and “Fun Stuff”
The Joy of Visual Perception
Afterimage, from the Exploratorium

Thanks Earthling…
“afterimage” looks like the word I was looking for.

Those links talked about the afterimages being colour negatives:

I see now that the afterimage can be in inverted colours though it takes quite a while for this to happen. I think afterimages form faster in the light/dark realm.

Well now I know what to expect with the inverted colours I’ve become quite sensitive to it… it’s pretty weird.
This illusion is actually the one that I’m interested in. I wanted to work out how it works. This effect was only discovered in about a decade ago and it’s called the “scintillating grid”. My theory is that it involves both rods and cones
The white dots you look at directly are seen properly, and I think that the rods only see a grey and black grid. To see that I pointed at a white dot and fixed my eyes on it and tried to be aware of the rest of my field of view without moving my eyes. I think the spots flash black because they are originally seen white and then they go out of the view of the cones and for some reason it goes darker in the centre of the dot. (it is like opponent afterimages except that the afterimage is properly aligned - maybe the grid helps with the alignment)
Well anyway, I think the scintillating grid has a similar explanation to the Hermann’s grid explanation - lateral inhibition… I’m not sure what that is really but I’ll find out eventually…