I recently had an operation in one eye for cataracts. Both eyes have them equally, but of course only one at a time gets treated. 

Before the surgery, one of the more pronounced effects was vivid rainbow halos around bright light sources. This was really striking, especially apparent when looking at a full moon. It was exactly the same with both eyes. I know because I tested them many times over a long period. Closing either eye made no difference. The effect was the same.

After the operation I did the test again. As expected, the halo had disappeared with the operated eye. What was not expected, is that there is no longer any trace of it with either eye! I have tested this repeatedly. Not a glimmer of rainbow effect regardless of the eye.

The focus is what it should be, much sharper in the operated eye, still blurry in the other one, but no rainbow colours to be seen. How can this be? I assume it was the result of refraction in the cloudy lenses, but why should it also have vanished in the eye that hasn't been operated? Is this some trick of the brain? Can anyone enlighten me? I am intrigued.