The optical illusion is merely a grid of black squares on a white background. But this simple pattern has a powerful effect on our perception that can be quite dazzling.
When you look at the image, you'll probably see dark circles appear sporadically at the intersections of the white grid lines between the black squares. At least, I hope you do because that's what I see!
The best theory so far is that the effect has to do with the response of our eyes' retinal ganglion cells. Light excites the ganglion cells. The theory is that the intersections in Hermann's grid look dark because they produce less net excitatory stimulation in comparison to the lines.
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