ananyo writes "Researchers have imaged an entire vertebrate brain at the level of single neurons for the first time. A team of scientists based at the Janelia Farm Research Campus in Ashburn, Virginia, were able to record activity across the whole brain of a fish embryo almost every second, detecting 80% of its 100,000 neurons. The work is a first step towards mapping the activity of a whole human brain — which contains about 85,000 times more neurons than the zebrafish brain. The imaging system relies on a genetically engineered zebrafish (Danio rerio). The fish's neurons make a protein that fluoresces in response to fluctuations in the concentration of calcium ions, which occur when nerve cells fire. A microscope sends sheets of light rather than a conventional beam through the fish's brain, and a detector captures the signals like a viewer watching a cinema screen. The system records activity from the full brain every 1.3 seconds."