"It seems that the hormone oxytocin influences what the dog sees and how it experiences the thing it sees," said Sanni Somppi, doctoral student at the University of Helsinki in Finland.
For the study, published in the journal Frontiers in Psychology, the team showed 43 dogs images of smiling and angry faces on a computer screen.
Each dog was tested twice: once under the influence of oxytocin which was administered as part of the test, and once without oxytocin.
The dogs were found to typically focus on the most remarkable aspect of each situation, such as threatening stimuli in a frightening situation.
Dogs under the influence of oxytocin were more interested in smiling faces than in angry ones.
Without oxytocin, the dogs' pupils were at their largest when they looked at angry faces, indicating that the angry faces caused the most powerful emotional reaction in them.
However, once under the influence of oxytocin, the images of smiling faces enhanced the dogs' emotional state more than angry ones, suggesting that oxytocin probably made the angry faces seem less threatening and the smiling faces more appealing.
In addition, oxytocin also influenced the dogs' emotional state, which was evident in their pupil size.
"We were among the first researchers in the world to use pupil measurements in the evaluation of dogs' emotional state. This method had previously only been used on humans and apes," added Outi Vainio, Professor from the university.