One can literally watch which neurons are active in particular behaviors by labeling the neurons with fluorescent calcium indicators. Calcium flows into electrically active cells, and when it does, it binds to the indicator and the nerve cells get brighter. By eliciting a behavior and looking into the fish, we can see which neurons likely contribute to that behavior. This can be done anywhere in the brain or spinal cord of the intact living fish.
Watching active neurons light-up in the intact fish. Neurons in the brain labeled genetically with a fluorescent sensor that gets brighter when calcium enters, so the neurons light up when they are electrically active.