Q: Why do I see a streak from LED tail lights when I scan the road at night? Is this dangerous?

A: You are seeing positive afterimages, says James Ver Hoeve, a vision scientist at UW-Madison. "If you stare at a bright red line on a white background and look away, you'll see a green line; that's a negative afterimage. If you wave a sparkler around, you'll see a streak of light that is the same color as the red-hot part; that's a positive afterimage."

The high-intensity light-emitting diodes in some modern taillights flicker quickly, so when you move your eyes across them, you may see multiple positive afterimages with a stroboscopic effect.

In some circumstances, the effect can be disturbing, Ver Hoeve continues. "When you see something in the center of your vision, your brain thinks, 'The object is located at a heading aligned with my eye position.' If your eyes move during a positive afterimage, your brain can misinterpret the eye movement as motion of the image rather than the eye. When motion appears to come from an object that should not be moving, the result can be disorienting."

While negative afterimages are produced in the retina, positive afterimages are mostly formed in the brain. Past use of LSD, lesions in the brain and drug reactions have been associated with prolonged positive afterimages, Ver Hoeve says.

The technical name for abnormal persistence of positive afterimages is palinopsia. "In Greek, 'Palin' means 'again' and 'opsia' means 'seeing.' Palinopsia doesn't mean seeing lots of Sarah Palins. It means seeing a trail of positive afterimages."