A group of researchers have completed the most advanced brain map yet, that of an insect, a historic achievement in neuroscience that brings scientists closer to a true understanding of the mechanism of thought.
The international team of scientists, led by Johns Hopkins (United States) and Cambridge (United Kingdom) universities, have produced an impressively detailed diagram that traces every neural connection in the brain of a fruit fly larva, a scientific model archetypal with brains comparable to humans.
The work, published Thursday in the journal Science, will likely underpin future brain research and inspire new machine learning architectures.
“If we want to understand who we are and how we think, part of that is understanding the mechanism of thought,” says Joshua T. Vogelstein, a biomedical engineer at Johns Hopkins, who adds: “The key to that is knowing how neurons connect to each other. ”.
The first attempt to map an animal brain was a 14-year roundworm study begun in the 1970s, which resulted in a partial map and a Nobel prize for John Sulston. Partial connectomes have since been mapped in many systems, including flies, mice, and even humans, but these reconstructions typically only represent a small fraction of the total brain. Integral connectomes have only been generated for several small species with a few hundred to a few thousand neurons in their bodies: a roundworm, a sea squirt larva, and a marine annelid worm larva.
The connectome of a fruit fly (‘Drosophila melanogaster’) larva is the most complete and extensive map of a complete insect brain ever completed. It includes 3,016 neurons and 540,000 synapses that connect them.
a laborious job
Mapping entire brains is difficult and time consuming, even with the best modern technology. Obtaining a complete picture at the cellular level of a brain requires cutting the brain into hundreds or thousands of individual tissue samples, all of which have to be captured with electron microscopes before the painstaking process of reconstructing all those pieces, neuron by neuron, into a complete picture of a brain. It took researchers more than a decade to do that with a fruit fly larva. The brain of a mouse is estimated to be a million times larger than that of a baby fruit fly, which means that the possibility of mapping anything resembling a human brain is unlikely in the near future.
The team deliberately chose the fruit fly larva because, for an insect, the species shares much of its fundamental biology with humans, including a comparable genetic base. It also has rich learning and decision-making behaviors, making it a useful model organism in neuroscience. And for practical purposes, its relatively compact brain can be imaged and its circuitry reconstructed in a reasonable time frame.
Even so, the researchers devoted 12 years to this work. The Cambridge researchers created the high-resolution images of the brain and manually studied them to find individual neurons, rigorously tracing each one and linking their synaptic connections. Cambridge turned the data over to Johns Hopkins, where his team spent more than three years using the original code they created to analyze brain connectivity. This team developed techniques to find groups of neurons based on shared connectivity patterns, and then looked at how information might propagate through the brain.
In the end, the entire team recorded every neuron and every connection, and classified each neuron based on the role it plays in the brain. They found that the most active circuits in the brain were those going to and from the neurons in the learning center.
The methods developed by Johns Hopkins are applicable to any brain-wiring project, and their code is available to anyone trying to map an even larger animal brain, according to Vogelstein.