Comment by cajal

Lets put it this way: prior to optogenetics, you had absolutely no way to answer questions like: "What happens during this behavior if we block dopamine at the exact moment we think dopamine is important for this behavior?" without trying our best to put a tiny electrode in the exact spot where we think dopamine neurons are found in the brain (on the micron scale), and pray that your stimulation exactly only hits dopamine neurons locally and nothing else (a technical impossibility).

now replace dopamine with any cell type you can think of in the brain, and you can even be more clever and do genetic tricks like "only this type of cell in this part of the brain that only receive input from this other part of the brain, but not others"

it really truly unlocked the entire era of neuroscience we are in, with the focus on circuits.

I predict the next likely era after circuits is networks, and the cool thing about opto is the scalability and the way we can precisely control and shape light to ultimately control networks.