Light-sensitive opsins activated by specific light can stimulate specific neuronal populations.

“We've worked on making very, very light-sensitive opsins. One challenge, and Botond would be the first to state this in fact, [chuckles] in solving this problem for the patient, he had to build goggles that created much brighter light than the normal ambient light delivery, because as I mentioned earlier, you have to pack a lot of these channelrhodopsins in, they don't have much current. You have to really make sure that you've got a tense enough light to activate enough of them to cause a stimulation-”