The first map of the brain of a fly will help to better understand the human mind

Fruit fly. / Archive

Science | biology

The team of researchers that has achieved this milestone is led by the Spanish Albert Cardona

It has 3,016 neurons and 548,000 synaptic connections between them. It is the brain of the Drosophila larva, or fruit fly, which has been fully mapped in the laboratory for the first time. This milestone, published in an article in the magazine
Science and led by the Spanish biologist Albert Cardona, represents a valuable resource for future studies of neural circuits and the function of the human mind.

The brain is made up of complex networks of interconnected neurons that communicate through synapses. Understanding the architecture of the brain network is fundamental to understanding how this organ works. The newly published map is the first ever map of the connectome, or synaptic wiring diagram, of a larger and more complex brain.

The first attempt to map a brain began in the 1970s and resulted in a partial map and a Nobel Prize. Since then, only three small species with a few thousand neurons in their bodies had been able to generate complete connectomes from the brains: a roundworm, a larval sea squirt, and a larval marine annelid worm. Likewise, fly, mouse and even human systems have been mapped, but these reconstructions only represent a small fraction of the brain.

Target: the human brain

The lack of more detailed models is mainly due to technological limitations, which make it difficult to image whole brains with electron microscopy (EM) and to reconstruct neural architecture on a cell-by-cell basis. Getting a complete picture requires cutting the brain into hundreds or thousands of individual tissue samples, all of which have to be photographed with electron microscopes before all those pieces are reconstructed, neuron by neuron, into a complete and accurate portrait of a brain.

It took 12 years to do that with the brain of the baby fruit fly. The brain of a mouse is estimated to be a million times larger than that of this insect, which means that the possibility of mapping anything close to a human brain is not likely in the near future, perhaps ever.

The team in this new research purposely chose the fruit fly larva because the species shares much of its fundamental biology with humans, including a comparable genetic basis. It also has rich learning and decision-making behaviors, making it a useful model organism in neuroscience. Furthermore, its relatively compact brain can be photographed and its circuitry reconstructed in a reasonable time frame. All this has allowed the authors to characterize different types of neurons, connections and structural characteristics.

In their final step, the entire team mapped out every neuron and every connection, and classified each neuron by the role it plays in the brain. They discovered that the busiest circuits in the brain were the ones that led to and away from neurons in the learning center. As they have highlighted, some of the identified characteristics resemble prominent features of current artificial intelligence machine learning networks.

This milestone opens a new door to the study of the human mind and could help to understand and develop better treatments for diseases such as Parkinson's or Alzheimer's, as well as other disorders (autism, epilepsy, schizophrenia...). Likewise, it is a step forward towards obtaining a complete map of the human brain, which is the ultimate goal.

The methods developed in this study are applicable to any brain-wiring project, and its code is available to anyone trying to map an even larger animal brain. For now, the brain of the adult Drosophilade fly is being mapped and the results are expected to come out next year. "If we want to know who we are and how we think, we need to understand the mechanism of thought," said lead author Joshua T. Vogelstein, a biomedical engineer at Johns Hopkins University. "The key to that is knowing how neurons connect to each other."