When we try to understand how the brain works, we cannot be satisfied with a functional understanding, we must also explore the underlying structure, that is to say the neuronal “wiring” in a way. For example, if we launch a hypothesis on the influence of a function compared to another, it is necessary that there exist neuronal afferences being able to justify it “physiologically”. However, even in recent years with the dazzling progress of medical imaging, mapping thousands of neurons and their connections remains a particularly difficult task. For more than ten years, a team of researchers have been working on the subject within a project called “human connectome”.
A connectome is the most exhaustive map possible of the neuronal connections of a brain and appears to be an essential tool for being able to make the link between the function and the structure of the brain. To date, the first (and only) complete connectome has been made on an ascaris worm Caenorhabditis elegans in 1992 by the American team of Theodore B. Achacoso and William S. Yamamoto using electron micrographs. Since then, other researchers have focused on different targets such as Drosophila (a fly that has nearly 135,000 neurons), or even on the primary visual cortex of mice.
But, back to the monumental efforts underway for the human connectome. Initially, the program was supposed to last five years, which, with hindsight, appears to be very (even too) ambitious. Launched in 2009 by the National Institutes of Health (United States) with funding of 21 million dollars, it has expanded within two consortia bringing together, on the one hand, Washington University in Saint-Louis and that of Minnesota and on the other side Harvard University, Massachusetts General Hospital and the University of California at Los Angeles. The latter having added 8.5 million dollars to the common pot, which gave the project a record budget of around 30 million dollars!
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The human connectome project continues
To meet this challenge, the teams first started with the hardware. Thus, improvements have been made to magnetic resonance imaging (MRI) machines, just as they have developed new ways of processing information and analyzing images in order to map our brains. In total, more than a thousand patients were recruited, including twins and all of their siblings. Quickly, structural MRI showed its limits, so much so that it was necessary to use another technology for this journey between our neurons, namely diffusion MRI initially developed to follow the movements of water over long distances. and the crossroads of white matter in our brain.
Today, the “human connectome” project continues with rare continuity. It has been enriched by new teams and is divided into three fields of research: the connectome of the “healthy” young adult, the connectome lifespan which is interested in the evolution during life (people of different ages) and finally the connectome linked to brain diseases, focusing mainly on Alzheimer’s disease and dementia, anxiety and depressive disorders, psychoses and finally seizure disorders.
Scientists are far from understanding all the connections of the human brain, but the parts that have already been mapped are immediately available to the whole community but also to the public. Result, in February 2021 version 2.0 of the connectome lifespan has been leaked and over 27 petabytes of data have already been shared which has since resulted in over 1500 published articles! Above all, in parallel with the progress of the project, imaging technologies are always progressing towards greater resolution, which opens the door to “personalized” neuroscience thanks to the ability to be able to examine the brain functioning of a particular individual. and compare it to the connectomes already listed.
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On the occasion of Brain Week, which ends on March 20, it is important to remember that the human connectome remains one of the most ambitious scientific projects of the 21st century. And above all, no one knows what he could reveal to us in the future.
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