In the picture below, there's an orange dot to the right of the middle — focus on it! The longer you stare at it, the more of the surrounding image should disappear.
This is a more extreme example of the Troxler effect in action. Remarkable how the image appears to vanish, huh? The brain has a knack for tuning out visual stimuli that don’t change, thus rendering them almost non-existent to our eyes. Our environments are constantly shifting so we're never really made aware of this phenomenon.