Fascinating physiological phenomenon: a handful of times during the past few years I've seen bright glowing patches of afterimage-like blobs in my visual field while running. It happened in the 2002 Marathon in the Parks around mile 16, for instance, and again a month ago when I was jogging at night around the track at an old neighborhood high school. The effect is rather like looking through a few big phosphorescent clouds, randomly placed, like one might see after a sudden flash of light. It lasts maybe 10 minutes or so and then fades away.
The blobs don't change shape, or grow slowly, or have zig-zaggy edges --- unlike the classic visual hallucinations associated with migraine headaches, which I've witnessed a few times ~25 years ago. The blobs just interfere a bit with vision, especially in the shade or after dark. I don't know whether they're in the brain or in the eyes or somewhere in between. (They do seem to invert in hue when one blinks; is that a clue?) They're not associated with great physical effort or excessive speed (as anybody who knows my wimpiness can testify!).
I asked about this topic in the Montgomery County Road Runners online discussion group, and got a variety of helpful responses: some thought that I was in big trouble and should see an ophthalmologist immediately ... some explained it as veins, always present in the eye but usually unnoticed ... some proposed an oxygen deficit or brain dehydration or ... etc.
The most plausible hypothesis, though, came from a person who had experienced similar visions and whose eye doctor diagnosed them as "ocular migraine" illusions. Such neural network disturbances could indeed be correlated with dehydration, stress, and other factors. And best of all, this theory doesn't mean that I have to stop running!