Thread drift... I met a guy who was a really good competitive trap/skeet shooter. Like Olympic team good. To help train his eyes and maintain the ability to quickly find and track moving objects, he would take a golf ball, color in one dimple on the ball, and sit around tossing the ball in the air, looking for the color dot and tracking that dot. On long drives, especially as a passenger, he would look not at road signs, but try to look at the bolt heads on the road signs and track them as they pass the sign. Sounds like the same concept as the string-thing jamis posted about.
