Mingzheng Wu, a graduate student at Northwestern University, plopped two male mice into a cage and watched as they explored their modest new digs: sniffing, digging, fighting a little. With a few clicks on a nearby computer, Mr. Wu then switched on a blue light implanted in the front of each animal's brain. That light activated a tiny piece of cortex, spurring neurons there to fire. Mr. Wu zapped the two mice at the same time and at the same rapid frequency putting that portion of their brains quite literally in sync. Within a minute or two, any animus between the two creatures seemed to disappear, and they clung to each other like long-lost friends. "After a few minutes, we saw that those animals actually stayed together, and one animal was grooming the other," said Mr. Wu. [He] and his colleagues then repeated the experiment, but zapped each animal's cortex at frequencies different from the other's. This time, the mice displayed far less of an urge to bond. The experiment, published this month in Nature Neuroscience, was made possible thanks to an impressive new wireless technology that allows scientists to observe " and manipulate " the brains of multiple animals as they interact with one another. Their tool ... uses a tiny LED light, implanted into an animal's brain, to activate discrete groups of neurons. (A gene that encodes a light-sensitive protein ... is first inserted into the neurons of interest, to make them responsive.)