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The first wiring map of an insect's brain hints at incredible complexity

LEILA FADEL, HOST:

Scientists hoping to map the trillions of connections in the human brain are starting a lot smaller with fruit flies. NPR's Jon Hamilton reports on an effort to trace every connection in the brain of a fruit fly larva.

JON HAMILTON, BYLINE: A brain is more than just a collection of cells. Joshua Vogelstein of Johns Hopkins University puts it this way.

JOSHUA VOGELSTEIN: The brain is the physical object that makes us who we are.

HAMILTON: To understand that object, you need to know how it's wired, what scientists call its connectome. But Vogelstein says that's hard in a human brain, which has more than 80 billion nerve cells.

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VOGELSTEIN: And each one has about 10,000 connections.

HAMILTON: So in the 1970s, scientists began mapping the connections in worms and tadpoles with just a few hundred neurons. Now, Vogelstein and an international team have mapped a brain with more than 3,000 neurons and more than 500,000 connections. It's from the larva of a fruit fly or drosophila.

VOGELSTEIN: The larval drosophila is closer in many regards to a human brain than the other ones. There's regions that correspond to decision-making. There's regions that correspond to learning. There's regions that correspond to navigation.

HAMILTON: Vogelstein says that, like a human, this insect has a brain with a left side and a right side.

VOGELSTEIN: One surprise that led to actually a follow-up paper that we've already written is how similar the left and the right sides are.

HAMILTON: In human brains, the right and left sides can have very different wiring. For example, the circuits involved in speech and language tend to be on the left, while circuits that recognize faces tend to be on the right. Vogelstein says the larval fruit fly connectome will help scientists study things like learning and memory.

VOGELSTEIN: Or look at differences across gender or differences across species or differences across developmental stages. This is the landmark first reference that we can use to compare everything else.

HAMILTON: The finding, which appears in the journal Science, shows how hard it is to map an entire brain, even in an insect. Vogelstein says the team began by slicing a single tiny brain into thousands of very thin sections.

VOGELSTEIN: You don't screw it up at all because if you make one mistake, you have to basically throw out the entire brain and start over again.

HAMILTON: The team used an electron microscope to capture an image of each slice. Vogelstein says tracing the connections took powerful computers and specialized analytical tools.

VOGELSTEIN: They'll work on thousands or millions or maybe 100 million connections but not 10,000 trillion connections.

HAMILTON: Roughly the number of connections in a human brain. Nuno Macarico da Costa of the Allen Institute in Seattle is part of a team working on a mouse connectome.

NUNO MACARICO DA COSTA: We started by trying to map the connectivity of a millimeter cube of mouse cortex, which is kind of a grain of sand but has 1 billion connections, 100,000 neurons and 4 kilometers of cable.

HAMILTON: Da Costa says it took 12 days just to slice up that one tiny cube, which represents only about 1/500 of a complete mouse brain. Even so, da Costa says mapping more complex brains is worth the effort. Eventually, he says, it should help scientists understand how a human brain can be affected by disorders like schizophrenia.

DA COSTA: If your radio breaks, if someone has a wiring diagram of your radio, they'll be in a better position to fix it.

HAMILTON: Da Costa says mapping the human connectome will also help scientists answer basic questions like how we learn and why we behave the way we do.

DA COSTA: Every idea, every memory, every movement, every decision you ever made comes from the activity of neurons in your brain. And this activity is an expression of the structure.

HAMILTON: In other words, the wiring diagram that makes us who we are.

Jon Hamilton, NPR News.

(SOUNDBITE OF FRAMEWORK AND MURGE'S "CUSP") Transcript provided by NPR, Copyright NPR.

Jon Hamilton is a correspondent for NPR's Science Desk. Currently he focuses on neuroscience and health risks.
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