How Can it Shrink?? :Optical Illusion-8

Concentrate on the black dot at the middle.




After a while the Gray area will appear to shrink.

Source
Illusion World

Related Posts:

Moving or Stationary?:Optical Illusion-1
Anything Rotating??:Optical Illusion-2
Coil or Circle?:Optical Illusion-3
Which One is Bigger? : Optical Illusion-4