You really can't believe your eyes and here's the proof
The human eyesight is an interesting and complex thing. Part of your overall field of vision is made up of your peripheral vision, which can sometimes make you see funny things. Here's the proof. Focus on the cross in the picture below and try not to look away, not even for a second. You'll soon notice some funny faces in the pictures.
It's true, you really can't believe your eyes. Watch the video below, while focusing on the white cross in the middle. Don't look away, not even for a second, and observe the funny faces you'll see on the sides. We assure you, there are no monsters in the pictures, it's just your brain playing tricks on you.
As you watched the video, the faces became more and more distorted. Here's why.
There is an area in your retina called "the yellow spot". It has the larges number of receptors in your eye, that make you see a given area clearly. This is the main field of vision, it covers about 10% of the entire range of your sight and it shows you the area your eyes are focusing on.
In this case, the main field is empty, because all you've got is a dark area and a white cross. The brain is trying to gather more info from other sources - the peripheral vision - combining everything into one image. And since the pictures are alternating quickly, it has to do this in a short period of time. As a result, you end up seeing various features joined together in some funny faces.