The other comments have pretty much covered it. We can pretty clearly demonstrate that neurons in general are important to behavior (brain damage, etc) and we e

The other comments have pretty much covered it. We can pretty clearly demonstrate that neurons in general are important to behavior (brain damage, etc) and we even have some understanding about specific neurons or populations/circuits of neurons and their relation to specific behaviors (Grid cells are a cool example). And this work is all ongoing, but we're also starting to relate the connectivity of networks of neurons to their function and role in information processing. Recently the first full connectome of a larval fruit fly was published - stay tuned for the first full adult connectome from our lab ;) Again, IANA neuroscientist, but this is my understanding from the literature and conversations with the scientists I work with.