What to see?
Easy question easy answer. If you ask yourself: “is it good that I see all these dots?” Then the answer is “yes”. If you look around the image you can see black dots appearing and disappearing at the intersection of the white lines.
What to do?
The illusion has black squares and a lot of white streets which make the “miracle” happen. To make the illusion happen it’s not very difficult, all you have to do is to look around the image and try to see it completely. That is not possible because all of the dots appear and disappear at the intersection of the streets.
In case you think you are being fooled, try taking two pieces of paper and cover all but two vertical or horizontal rows of black squares. What do you see now? No more spots, no more illusion, right? If you will increase the distance between you and the image, the illusion will not be so strong because the perception is different.
How it works?
Now here’s what you have to do: pick an intersection, pay attention to that dot and you may see some gray dots nearby and some black dots a little further away. More black dots seem to appear as the eye scans the image from top to bottom or from left to right. The effect seems to be reduced when the head is a little slanted, but it’s not eliminated. The effect seems to exist only at intermediate distances; if the eye is moved closer and closer to the image or very far away from it, the black dots do not appear. That happens because we appreciate objects in a different way when they are at certain distances from our eyes.
The illusion is possible because the contrast and the human eye or brain can be fooled with the color game. For example if we try to focus on a spot the rest of the field which is not part of our attention anymore stars to fade and to mix colors.
This illusion can be explained through the neuronal process called lateral inhibition. In an easier way the visual system is the result of a group of receptors which respond to the environment, to the stimuli… that is what we call receptive field. So when we focus on a certain object, stimuli, we tend to see the objects around the main one, as part of the receptive field. These object are losing lots of their details and start to look a little different because they are not part of the main picture.