Jen Pearly:
It is a very common migraine aura. I see these all the time, just as they are described here, and that is what they are. Meditation can induce them. Taking a calcium channel blocker prevents them, and prevention is good because migraine aura is a huge risk factor for stoke.
This is not the best advice if this has come up as a result of meditation.
Here's my take...
My own vision is constantly flickering. All day long, eyes open or closed. The frequency of the flickering corresponds exactly with the frequency of vibrations I can feel in the body. So for example, as I go through the Dark Night stages in sitting practice the flickering will often slow down, and when in the A&P or Equanimity it can be extremely fast/fine in line with the vibrations in the body. My own conclusion has been that this is simply a result of the increased resolution at which I experience sensations.
Often during sitting practice I will use the visual field as object along with the body and sounds. It can be very powerful to see all of these sensations as impermanent at the same time. To try to see them as "the same". Flickering, strobing, vibrating, changing...
Sounds to me like you're making good progress