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Scientists broke down the complex mating behaviors into subcategories such as partner searching, copulation and rest. Then they mapped the neuronal activities onto the worm connectome to identify the brain mechanisms that process information from the environment during mating. The resulting map of brain activity was so pronounced and consistent between the eight worms used in the study that they could use it to predict the behaviors of a ninth worm, explained Vladislav Susoy, a neuroscientist and first author of the study.

So the scientists decided to put their understanding to the test experimentally. Taking a new worm, they precisely eliminated one of the five neurons involved in a movement called “turning,” in which the worm wraps around its mate just prior to copulation. Without that neuron, the worm lost its ability to turn. “It was really striking how clear the link was,” Susoy said.

Florian Engert, a Harvard neuroscientist working on a brain map of the zebra fish, another widely used laboratory species, called the C. elegans paper “a milestone” in the field for its use of the connectome to generate insights into a complex behavior. The connectome is “becoming a critical resource,” he said.

“The way that the entire field wants to use connectomics now is as a tool and a database for investigating how neuronal circuits operate,” said Gregor Schuhknecht, a neuroscientist and a postdoctoral fellow in Engert’s lab.

But in addition to explaining the underpinnings of behaviors, connectomics studies can also reveal subtle details about how those behaviors are wired into brains.

For example, it has been known for some time that in C. elegans, the connections between neurons dramatically reorganize themselves between birth and adulthood. To understand how the brain changes throughout development, in a recent paper in Nature, the labs of Lichtman, Samuel and Mei Zhen at the Lunenfeld-Tanenbaum Research Institute of Mount Sinai Hospital in Toronto compared the connectomes of eight genetically identical roundworms ranging between larval and adult stages.

The most interesting finding of the study, according to first author Daniel Witvliet, was that even though the worms were genetically identical, as much as 40% of the connections between nerve cells in their brains differed. Moreover, the connections that varied between individuals were weaker than those that were similar. Stronger connections that contained 100 synapses or more were consistent across multiple organisms.

For Witvliet and Lichtman, this finding points to the power of brain-map comparisons in bulk. “You can’t just say, ‘We’ve mapped the worm connectome,’ because each connectome is slightly different,” Witvliet said.

Lichtman says the finding points to the existence of two classes of connections: variable ones and consistent ones. If it turns out that animals make more consistent connections to support neural activity essential for survival, then he thinks that the level of variation in connections could become an important marker for significant features in the connectome.

“If you have multiple connectomes, the functionally important parts may pop out from that sea of randomness,” Lichtman said. He hopes that in the future, connectomics will regularly analyze the brains of multiple individuals, comparing healthy and unhealthy animals, young and old ones, and so on. “I think that’s where things will head once mapping the brain becomes routine.”

Neuroscience’s “n of 1” Problem

Getting large-scale connectomics to be routine, however, will be easier said than done. The field of connectomics still suffers from what Lichtman calls an “n of 1” problem: Even with all the recent improvements in neural mapping technology, diagramming the brain wiring of even one individual, especially in a species more complex than a worm, is not a task to be undertaken lightly.

His Harvard colleague Aravinthan D.T. Samuel, a co-author with Susoy of the worm mating paper, agrees. “I tend to think of connectomics in most contexts like Mount Everest expeditions,” Samuel said. “You do it once and say you’re done.”

That challenge represents a significant handicap for research, particularly on complex creatures. When Lichtman and his colleagues mapped the snippet of human brain, for example, they had no idea whether the strange things they saw were normal or one-offs due to the unique history and genetic makeup of the person. If they could map equivalent samples from 100 human brains, then they would get some clarity on these unknowns, but at 1.4 petabytes per brain, that is unlikely to happen anytime soon.

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