The Allen Institute for Brain Science has just released a massive data set showing how the brain of a mouse is all wired up, the first comprehensive neuronal connectivity map for a mammal. Each one of those dots in the rendering above is an injection site for one of 80 viral tracers. The tracers "infected" neurons in those areas, and then traveled down the neuronal pathways that they were connected to. Researchers were able to image these virus trails, creating a sort of circuit diagram showing which parts of the brain are connected to each other. By repeating this procedure with 1,772 mouse brains, and then slicing each one of them into 140 sections for imaging at resolutions smaller than a micrometer, the researchers collected nearly two petabytes of data, which they synthesized into a detailed mouse brain wiring diagram.
Were we to ask a real scientist about this, which maybe we did, they would almost certainly point out that the above image should not be construed to represent an entire mouse brain. An entire mouse brain has something on the order of 75 million interconnected neurons, whereas this picture is only showing 80, which is far fewer than even an incredibly simple organism like a C. elegans worm, which sports 302. In order to see all of those connections, it takes a lot more than a static image, but for the curious, there's an online browser (and a downloadable application) that lets you explore this ludicrously large data set in its entirety.
It's also important to understand that not all mouse brains are the same, and so it's not really possible to make a definitive and complete structural map for all mice. In its current incarnation, the mouse brain atlas is more of a high-level street map, which includes major connections that all mice probably share. As technology improves, the atlas will be updated to include more connections that are currently too small for us to resolve, and will probably reflect more variation between individuals.
This is not to say that what we have now isn't useful: for example, it makes for a baseline comparison point to see if certain neurological diseases change the way neurons connect with each other. The most interesting part, however, will come when we're able to correlate these structures with the electrical impulses that travel along them as thought, as the researchers describe: "this information will provide a framework for what we ultimately want to understand: ‘traffic patterns’ of information flow in the brain during various activities such as decision-making, mapping of the physical environment, learning and remembering, and other cognitive or emotional processes."
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