Could this be double vision? Many seizure meds cause problems with eye alignment, and if you had a very small change it could feel like what you describe. Can you get relief from the sensation by closing one eye? If you wore a patch for a day, would the problem be completely gone during that period? If so, it's probably something affecting your eye alignment (of which keppra is one possibility, although it's uncommon).

edit: And if this is the case, you could potentially fix it completely with a very small prism added to your glasses rx. Very simple, but maybe so small the ophtho can't see it. Just make them aware, and get help!