To observe the brain in action, scientists and physicians use imaging techniques such as functional magnetic resonance imaging (fMRI) to observe physiological modifications to the cerebral region of activated neurons. Until now, it was believed that these differences were only due to modifications of the blood influx towards the cells. By using intrinsic optical signals (IOS) imaging, researchers from the University of Geneva (UNIGE), Switzerland, demonstrated that, contrary to what was thought, another physiological variation is involved: the activated neurons swell due to the massive entry of water. This discovery provides evidence that a much finer analysis of the functioning of the brain - and of its dysfunctions - is possible. These results are published in the journal Cell Reports.