Another example of how the brain just fills in the missing blanks . Unless you are one of the 0.7% of people who suffer from schizophrenia, you are unable to instruct your brain to see the hollow side of the rotating mask.
Remember the lazy visual brain when designing slides. The brain tends to follow lines in the reading direction, and sometimes finds it hard to spot the word "not" in a sentence, just to name a few examples.
I can recommend the book "Brain Rules" if you are interested in learning more about how the brain absorbs (and does not absorb) information.

Thank you Orli Naschitz and Dep for pointing me to this.