How many animals do you see in these images below?
You probably see only one at first.
Frog versus Horse
|Swan versus Squirrel|| Rabbit versus Duck
||Rabbit versus Duck
You'll find that you have a hard time if you try to see both animals at the same time. Your brain can only perceive one or the other at a given moment. This is attributed to the Gestalt effect—the idea that the brain perceives whole objects rather than individual attributes or parts. More specifically, this illusion illustrates a multi-stable perception effect. Ambiguous objects or drawings, like these, switch unstably between one interpretation and the other. Unfortunately, Gestalt theory only notes that this occurs, but does not provide neurological explanation for why it happens.