If you stare at one point for too long, small details will disappear. It's known as the Troxler effect or Troxler fading. It seems to affect small details away from the center of your field of regard. Of course objects at the center of regard can also disappear but that is a different topic (related to how the optic nerve leaves the eye).
An interesting Flash demo can be found here. Try it with different options, as the dot size, background color and speed of rotating crosses affect how long it takes for the fixed dots to disappear The time to disappear will vary with each person. Even the smallest shift in gaze and blinking will bring the fixed dots back to view, at least for a few seconds. Changing the center of your gaze needs to be combined with fixing your gaze on a point, especially to findchanges on the ground. And not knowing what the feature looks like until after it has been found only increases the challenge. Here is a similar demo via You Tube:
David Martin , 1LT, CAP
Assistant Emergency Services Officer