But don't get too excited about having holodeck-style flashing headsets anytime soon – there are two big catches between monkey and human applications:
(1) Gene therapy is involved, using a virus to deliver a gene that produces a light-responsive protein to trigger a neuron to fire. Given that gene therapy is not approved in many countries, any sort of technology or treatments based on this technology are still years away.
(2) Light source required: you need to be able to fire light signals into the nerve cell somehow. I'm not sure if there are many possibilities for producing light intrinsically, so a fibre-optic tether or an LED implant may be the only possibilities for the forseeable future.
Scientists Control Monkeys’ Brains with Light – Technology Review
The new study, the first to demonstrate optogenetics in primate behavior, inches the technology closer to the clinic.