All that processing is dictated by which neurons have connections to others. For example, the visual system does some basic recognition of its own before passing the results to the brain’s visual processing centers. If those centers detect something like text, they can use connections to the language centers to interpret it, and so on.

To understand how a brain works, then, we need a catalog of the connections in the brain, since those dictate how information flows through its various systems. That catalog is a connectome.

In practical terms, a connectome is the list of every neuron in a brain, including its location in three-dimensional space, and the connections (termed synapses) it forms with other neurons. That’s more complicated than it may sound. Each neuron can form multiple, branched processes called axons, allowing it to form hundreds of connections to other neurons. So while the nervous system of the fruit fly consists of only roughly 150,000 neurons, and the brain contains only a fraction of those, the new work discovered over 300 million synaptic connections in the fly brain.

So how do you go about mapping something like that? Gerry Rubin, a senior group leader at the Janelia Research Campus and one of the senior authors on the new paper, described how things have changed considerably based on the complexity of the system. “I was a graduate student at the [UK’s Laboratory of Molecular Biology]… and when I got there in 71, they already bought this giant computer, and they had the idea that they were going to use machine vision and computers to assemble the C. elegans connectome,” Rubin said.

C. elegans is a small, transparent worm with just over 300 neurons and would seem to be a tractable system. “It took them about two years to realize that the computers were nowhere near powerful enough,” Rubin said, “and so they went with printing everything out on photographic prints and colored magic markers and circling neurons and tracing it by hand.”