It's a well known fact that our brains don't always see what is actually there... certain tricks of shape and light confuse us and our brain fills in the gaps. This is actually an evolutionary function of our minds, allowing us to focus on primary threats and events rather than being distracted by things in the background.
Check out the bizarre effect caused by these simple flashing images and marvel at what your own brain invents for you.
Keep your eyes focussed on the cross!
Whoa! Suddenly all of the faces warp into weird caricatures as your brain can't process so many different face shapes in quick succession.
This creates a caricature-style distortion...
This is known as the Flashed Face Distortion Effect, "an optical illusion involving the fast-paced presentation of eye-aligned faces."