Dr. Sandra Barker, a professor of psychiatry and director of the  School of Medicine Center for Human-Animal Interaction at Virginia Commonwealth University , studies what happens psychologically to humans when they interact with dogs.

In an  interview with EarthSky , Barker revealed her findings in her 2009 study, in which people's brain waves were tested before and after spending time with a dog.

She reported that she saw an increase in certain frequencies that correlate with a state of relaxation, in addition to a decrease in cortisol - the human stress hormone - after her subjects interacted with a dog.