Baby zebrafish, tiny and transparent, are perfect for studying neuronal activity without having to perform invasive surgery. The larvae can be observed under a microscope, and, with a bit of tweaking, the firing of their neurons can be seen loud and clear.
That's what researchers at the Howard Hughes Medical Institute's Janelia Farm Research Campus have done. Using zebrafish larvae that have been genetically modified so that each neuron has a chemical indicator that glows fluorescent under a sheet of laser light a split second after the neuron fires.
This technique is called light-sheet functional imaging, and it has allowed the researchers for the first time to see the neuronal activity of the zebrafish's entire brain as it sees, thinks and moves. In the video below, you can see 80,000 neurons -- 80 percent of the fish's brain -- firing up.
To start with, the brain is pretty quiet as the fish just hangs out, but then the researchers showed it moving bars, creating the illusion that the fish was drifting backwards. With electrodes on its muscles gauging its intent to swim, the researchers were able to map which neurons are involved in any given action.
"There must be fundamental principles about how large populations of neurons represent information and guide behavior," study co-author Jeremy Freeman told Wired. "In this system where we record from the whole brain, we might start to understand what those rules are."
The research, published in Nature Methods, is part of an ongoing project by the Institute to monitor zebrafish neural responses across the entire nervous system at single-cell resolution, creating a map of the brain and which neurons correspond with which behaviour.