To observe the brain in action, scientists and physicians use imaging techniques, among which functional magnetic resonance imaging (fMRI) is the best known. These techniques are not based on direct observations of electric impulses from activated neurons, but on one of their consequences. Indeed, this stimulation triggers physiological modifications in the activated cerebral region, changes that become visible by imaging. 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 have now demonstrated that, contrary to what was thought, another physiological variation is involved: the activated neurons swell due to the massive entry of water.