What happens in the brain when we make a decision ? How hundreds of interconnected brain areas, processing information related to sensations and behaviors, lead to a specific action? ?
The first comprehensive map of brain activity has been unveiled by an international group of neuroscientists. On September 3, researchers from theInternational Brain Laboratory (IBL) published two articles in the journal Nature, revealing how decision-making takes place across the entire brain in mice, with cell-level resolution.
Brain Atlas
Other brain maps have recently been published. First, in March 2025, the first brain mapping of “power plants” (cell compartments called mitochondria) of the human brain, then two weeks later, for the first time, a cubic millimeter of brain mouse was reconstructed in 3D. The latter was a meticulous work on anatomical details down to the synapse (the connection between two neurons), focusing on the circuit of information in the brain, while the work of the IBL focuses more on the function of the different areas, in other words, finding which area codes for which mechanism.
95 % of mapped brain volume
“ Thanks to this large-scale collaboration, 21 laboratories worked together on the same experimental model to record the individual activity of neurons distributed in 279 areas of the brain, or 95 % of the mouse brain volume., explains Alexandre Pouget, full professor at the Department of Fundamental Neurosciences at the Faculty of Medicine of the University of Geneva and co-founder of IBL.
The activity of 621,733 neuronal units (neurons and groups of neurons) was recorded using 699 spike electrodes in 139 mice studied in twelve laboratories. In fine, the researchers identified 75,708 well-isolated neurons and retained them for analysis of the whole mouse brain.
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Each rodent had to perform a task requiring decision-making. In this task, a mouse is placed in front of a screen and a light appears to the left or right. The mouse then responds by moving a small wheel in the appropriate direction to align it with the light point, in order to receive a reward (water). If it fails, white noise (like that of televisions without a signal) plays for two seconds.
On the map, when he is deciding, the whole brain lights up
However, in some trials, the light is so dim that the animal has to guess which way to turn the wheel. The mouse uses the frequency with which the light has previously appeared on the left or right to make guesses.This allows us to study how prediction based on prior experience influences perception and decision making”, points out Alexandre Pouget. “And we found that when he's deciding, the whole brain lights up like a Christmas tree!”
These experiments resulted in an extremely detailed map of the brain in this situation, from the very beginning of the process to the achievement of the reward. Neuroscientists also observed that, during decision-making, the signals from neurons were distributed throughout the brain, a sign of constant communication between its different areas.
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For the authors of the study, this demonstrates the importance of considering the organ as a whole in the future when analyzing complex behaviors. This mapping tool could be adopted to gain a more comprehensive view of brain function.Traditionally, neuroscience has studied brain regions in isolation. Recording the whole brain now gives us the opportunity to understand how all the pieces fit together.”, underlines Kenneth Harris, professor of quantitative neuroscience at University College London (UCL) who participated in the study.
For Tatiana Engel, professor at Princeton University (USA) and member of the IBL, the creation of such a map demonstrates the importance of broad scientific collaborations in neurosciences, as was the project Human Genome for genetics or is it the Cern for nuclear physics. The publication of this map is “impressive, but not an end in itself”. "Now that we have developed this model, and the platform is accessible, we hope that it will be taken up by other teams to study other behaviors.", adds Alexandre Pouget. The researcher expects that neuroscientific tools of this type will be increasingly shared, " as are the advanced telescopes in astronomy“.
In the future, the neuroscientist hopes to be able to extend the experiments to subjects who can move freely in the experimental space. For now, the mice have their heads fixed to hold the device. But the technology is developing rapidly! The electrodes are becoming increasingly miniaturized, so they can be placed on the mice without being too heavy., he rejoices.
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