Suchow and Alvarez report a very compelling optical illusion in an upcoming edition of Current Biology. The illusion is very simple and I was quite surprised that it actually works. A cloud of colored dots is arranged in a circle around a central fixation point. In one condition, the dots gradually change color, but they don't move. As you would expect, it is very easy to spot the color changes in this condition. However, in another condition, the dots move around as well as change color. Surprisingly, in this condition the color changes are extremely difficult to detect! This is demonstrated quite nicely in the video below (provided by the authors).

How does the illusion work? An important clue is that retinal motion is required. If you match the movement of the dots with your eyes, thus eliminating the retinal motion, it becomes considerably easier to detect the color changes (not as easy as when the dots are static, but this is presumably because it is difficult to match the movement perfectly). Simply put, this suggests that we detect the color changes with neurons that “see” only a small part of our retina. If the dots move around on our retina, they are continuously “seen” by different neurons, and this compromises our ability to detect changes.

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