Oh, those puppy eyes. Just by gazing at their owners, dogs can trigger a response in their masters' brains that helps them bond, a study says. And owners can do a similar trick in return, researchers found. This two-way street evidently began when dogs were domesticated long ago, because it helped the two species connect, the Japanese researchers say. As canine psychology experts Evan MacLean and Brian Hare of Duke University wrote in a commentary on the work, "When your dog is staring at you, she may not just be after your sandwich." The new work is the first to present a biological mechanism for bonding across species, said researcher Larry Young of Emory University. Neither he nor the Duke scientists were involved in the study, which is reported in a paper from Japan released Thursday by the journal Science. The brain response is an increase in levels of a hormone called oxytocin (ahk-see-TOH'-sin).