Did you notice the figure above? To see the spinning lady, click this link and return back. Is it a spinning lady? In which direction, you feel the lady is spinning? clockwise or counter-clockwise? One can feel the spinning of the lady in either clockwise or counterclockwise. But once you feel lady spin in one direction, your brain continues in that state and insist to be in that state. If you feel that the lady is spinning in clockwise, it is too difficult to see the motion in counter-clockwise direction. But sometimes our brain switches from one state to other state automatically, but changing the state of feeling of brain is difficult to achieve manually.
This is a beautiful eye illusion. Do you know how that beautiful lady, cheated your brains?
Answer is, the lady didn't cheat you, but the cheater is her shadow. Actually, the image shows the shadow of a spinning lady. In this, a 'shadow' plays in important role in cheating you. The pure shadow of an image will not provide the entire information about an object to the brain. From a shadow, we can't predict whether the front side or backside of the object we are viewing. If you didn't understand what I just said, please go through an example.
Look at the picture below.
The middle picture is photo-shot of the spinning lady. Concentrate on the picture and tell what you are viewing- the frontside or backside? It may be both. Look at the left picture. It is the modified version of middle one by adding eyes in her face. Now you feel that you are viewing the front side of the lady. At the same time, you see the backside of the lady in the right picture, but that also a modified version of middle picture with marking of a hair bud. This is important to be noted. While you are viewing the middle picture only, your brain fall in to the mess of finding whether the shadow is of the front side or backside. Then your brain 'randomly' choose one possibility (not actually). But in other pictures, your brain has powerful proof that indicates the state of the lady's body.Now the problem associated with the spin direction of the lady is not vague. If your brain 'assumed' a side of the lady's body you are viewing, this will determine which direction, she is spinning.
