Hermann grid illusion / nobody knows anything
The illusion above: In your peripheral vision, you may see dark spots where the lines intersect. If you look at an intersection directly, you can see that there is actually no dark spot.
The textbook explanation for this effect has to do with the ganglion cells in our eyes. In the center area of our ganglion cells, there is a positive response to luminance. In the surrounding area, any light has an inhibiting response. The ganglion cell fires less and this results in lower perceived brightness. When the ganglion cell is directly positioned over an intersection, the surrounding inhibitory area is looking at 4 lines. When the ganglion cell is positioned over a line but not an intersection, it is looking at 2 lines. So when it is looking at the intersection, the ganglion cell is being inhibited more and this corresponds to the lower luminance.
However, there is strong evidence against the textbook theory being taught in universities. In the image below, the classic explanation would predict that the illusion would be stronger. To my eyes however, the illusion is either very weak or has no effect.
For another refutation of the classic explanation, check out Michael Bach’s visual illusions website. It references many academic sources with other possible explanations of this illusion. However, I haven’t seen an explanation that explains the following variation:
In the Flash demonstration above (you will need Flash to see it), you can drag the opacity slider to change the luminance of the squares in the center. Drag it to 0 to see a version of the classic Hermann grid effect. As you drag the luminance higher, you should see that the effect gets weaker and weaker. And then you hit a point where it starts becoming stronger again! I have no idea why this effect occurs. It could be that there are two different illusions/effects going on, but I wouldn’t know.
One issue that this raises is whether or not we really know anything about how the human visual system works. Perhaps it is an overstatement to say that “nobody knows anything”, but I certainly do feel that we have a lot more to learn about color science.