Can you count the black dots between the squares below?
It’s an impossible task. Each time you move your eyes and focus, you see that the dots in the center of your receptive field are all actually white. The surrounding ones just appear to be black.
The Hermann grid illusion is an optical illusion characterized by “ghostlike” grey blobs perceived at the intersections of a white (or light-colored) grid on a black background. The grey blobs disappear when looking directly at an intersection.
So why do you see black dots? The visual system processes edges of objects so they are enhanced. Seeing edges is very important for the brain’s ability to understand and define an image. This sometimes leads to visual “artifacts” away from the center of the receptive field, such as the black dots in the Hermann grid.