Question

Gervase is staring at the blades of his ceiling fan, searching for inspiration for a paper that he is writing. when he turns off the fan, he is very surprised that the blades appear to be rotating in the opposite direction. to reassure gervase that he is not hallucinating, you could tell him about:

Answer 1

I believe the answer is: motion aftereffects
motion aftereffects refers to a visual illusions that make a non-moving stimulus appear to no move in opposite direction.
Motion aftereffects is commonly caused when we fixated on a moving objects with a stationary eyes for a long period of time.