When we learned about this in biology class they told us it is linked to "visual persistence" which is those after images you sometimes get if you stare at something too hard.
They say that's where the idea of "auras" comes from. If you stare at someone for a while, you'll start to get a visual persistence. If your eyes aren't completely still (which they hardly ever will be) you'll kind of smear the "aura" around the person you're looking at, so it will seem like they have a light around them.
I've seen it happen myself when I was having to stare at someone speaking in a meeting for too long.