Saccades and LED lights.
http://en.wikipedia.org/wiki/Saccade

Saccades are the name for the way human eye movements work in jerks. The brain blanks the visual system while the eyes move and fills in the gaps with a remembered version of the scene. The quickest way to verify this is to look in a mirror and switch your gaze from eye to eye. Despite the lack of blinking, you can't see your eyes move. This has all kinds of side effects, not least the suggestion that it is responsible for at least some SMIDSY accidents. It's also an entry point into the discovery that a lot of what you think is objective reality out there is actually a constructed reality in here.

Here's the thing though. A lot of LED lights are driven from either mains frequency or a created alternating voltage in the bulb and go completely dark between cycles so they flash at about 50 or 100 Hz (UK. 60-120 in the US). Normally you never see the trails of flashes as your eyes move. But it is possible to deliberately slide your eyes across the scene slowly instead of using the Saccade mechanism and see the after images as the lights flash on and off. With the early evenings I'm finding this increasingly irritating on walks round the town as more and more lights are replaced by LEDs. There's these damn after images flickering in the corner of your vision.