Francis Crick suggested in 1999 that light could be a suitable method for achieving cell-type specificity in neurostimulation due to its speed, relative non-invasiveness, scattering properties, and the fact that most neurons do not respond to light at baseline.

“in 1999 he'd even suggested that not only did we need a way in neuroscience to control individual cells individual cell types but he said maybe light would be a good way of doing it he didn't have an idea of how to do it but he said you know light would have some good properties it would be fast it would be relatively non-invasive photons could scatter through tissue and most neurons don't respond to light at baseline unlike electricity and so it would be a way of getting great specificity”