In a recent study, University of Pennsylvania and Princeton University researchers tried to determine how muscle-controlling neurons known as Purkinje cells could tell the difference between brain signals intended to tell them about unexpected events and the random ones that impinge on them at all times. In their study, the researchers had mice walk on treadmills while their heads were kept stationary. This allowed the researchers to blow random puffs of air at their faces, causing them to blink, and to use a non-invasive microscopy technique to look at how the relevant Purkinje cells respond. The technique, two-photon microscopy, uses an infrared laser and a reflective dye to look deep into living tissue, providing information on both structure and chemical composition. Neural signals are transmitted within neurons by changing calcium concentrations, so the researchers used this technique to measure the amount of calcium contained within the Purkinje cells in real time. They found that calcium levels were higher when the mice received the unexpected stimuli in the form of the puff of air.