How is this done in living brains? Presumably you have to vivisect sufficiently be able to shine light on neurons, or have some sort of probe that does it. At that point how is it easier than electrically stimulating the nerves to generate action potentials?

(To be clear, since it's probably necessary, I'm asking a curiosity-driven question about the mechanism here, not throwing shade on the work. Given it's won the researchers a Nobel prize it's obviously highly significant.)