Again, it's not easy or ethical to test, but any charged particle interaction with a nerve for instance will possibly trigger an action potential, inducing the "light" that is witnessed. However, this is unlikely given that the optic nerves are pretty well protected.

What is more likely to occur is that a sufficient amount of gamma rays or x-rays ionizing the nerve or nerve sheathe itself and those unstable particles decay into something that releases a charged particle triggering the nerve signal.