Shade

More information: Otten, Pinto, Paffen, Seth & Kanai (in press). The Uniformity Illusion: Central stimuli can determine peripheral perception. Psychological Science

Two examples of the Peripheral Overflow illusion with shades of blue as an example. 

• Open the video fullscreen.
• Keep your eyes fixated on the centre of the screen for a good amount of time (tens of seconds to minutes).
• The first example contain no fixation spot, the second is identical to the first, but it does contain a fixation spot. Some people find that the illusion works better without a fixation spot, but you can try out which works best for yourself. 

For more information, see the text below the examples.

More info:  The central patch is darker blue than the periphery. If you keep your eyes focussed on the center of the screen, the entire screen will become dark blue at moments. At other moments, only parts of the periphery will become dark blue, or the entire periphery will revert to its actual lighter blue shade. Why exactly the PO illusion wanes in and out of existence is unclear, but it could be due to blinks, or small eye movements.