Stare at the '+' and notice the green dot that circles around .... which is actually the purple dots turning off in turn. Stare for a little longer and notice in your peripheral vision the purple dots completely vanish! You can expidite the proccess by using only one eye.
This is an example of how the mind doesn't really 'see' in the peripheral but rather 'remembers" what was there or creates the image from slight eye movements. Actually, one last test is to very slightly move your eye on the '+' and notice all of the purple dots reappear.