Nice explanation! In section The Basic Steps you wrote that ion movement across the membrane causes membrane charge to change, while you probably meant membrane potential.

I have few questions:

1. Are there any efforts to improve spatial resolution of optical stimulation and would that simply mean using proteins that are activated by lower wavelength electromagnetic waves?

2. You wrote that driver gene coding for Cre recombinase is expressed only in certain cells. Can localization of gene expression for this enzyme be controlled?

3. What is the relation of this method to neuromodulation techniques like TMS in regards to probing and therapy? I saw papers in which the researchers described how they used optogenetics to establish how certain neural pathway responded to stimulation, and TMS for therapy.