Okay, then you likely have a neurological problem affecting your eyesight. Your brain is not processing visual signals fast enough to see the entire field of scan lines when you are chewing on something. The vibrations the crunchy food are causing in your skull are limiting you vision processing, kind of like a strobe light effect where things appear to stand still if the timing is just right. Perhaps you should go see the optometrist and get one of those squiggly line tests to see if your brain and optic nerve connections are still working properly. If you can't see the random squiggly distortions in the test, then you have a neurological vision problem.