Afterimages

Stare at the black plus sign in the middle of the image for 20-30 sec, then look at the plus to the right of it.  What do you see?

Stare at the bottom left corner of the yellow square for 20-30 secs, then look at the plus sign to the right.

Stare at the black plus on the birst for 20-30 seconds, then stare at the black plus in the cage.