A flash of light seen when the optic nerve is subjected to a strong changing magnetic field (such as in an MRI)2010 Xiaoping Hu and David G. Norris, "Advances in high-field magnetic resonance imaging", Annual Review of Biomedical Engineering:
A number of transient phenomena were reported for human subjects in the first 4 T whole-body systems, including vertigo, difficulty in balancing after leaving the magnet, nausea, headache, numbness or tingling, magnetophosphenes, and an unusual metallic taste in the mouth.
