Oxytocin helps mice recognize one another, and now researchers in Zurich have provided the first evidence that the hormone has the same effect on humans.
“The most basic class of social stimuli for humans is faces, so I compared faces to non-social stimuli,” said Ulrike Rimmele, a psychologist at the University of Zurich and the lead author of the report on the research, in the January 2009 Journal of Neuroscience. “We found that oxytocin changes recognition for faces but not for non-social stimuli, such as landscapes.”
Oxytocin, a hormone found in mammals, stimulates contractions during birth and promotes milk production in lactating females, and it appears to exert a powerful influence on the brain as well, promoting trust, bonding and recognition of emotional states in others.
A 2000 study published in Nature Genetics demonstrated that mice genetically engineered to lack oxytocin appear to have “social amnesia,” according to Larry Young of Emory University, a co-author. When placed together with mice they had encountered previously, the amnesiacs continued to sniff one another just as much as they would sniff new mice, indicating they had no memory of the other.
“They can’t remember mice they’ve been around before,” Young says, “but if you give them oxytocin, they can. That suggests that oxytocin focuses the brain’s attention on social cues around us.”
Rimmele’s study takes that observation and applies it to humans, substituting facial recognition for sniffing behavior. “That’s what makes this study so interesting,” Young says. “It shows that people, if you give them oxytocin just before they see a face, are more likely to remember they’ve seen that face before. It’s very analogous to what’s going on in mice. I’m actually surprised it’s so similar.”
Rimmele had 44 men look at 84 pictures of male and female faces, and 84 pictures of houses, landscapes and artificial objects such as sculptures. Half the men inhaled oxytocin before viewing the images; the other half inhaled a placebo. After looking at each image for a few seconds the men were asked to rate on a scale of 1 to 7 how interested they were in approaching the person or object.
The following day, the men returned for another session, viewing all 168 of the original images mixed with 36 new faces and 36 new nonhuman images.
The two groups did not differ in how they rated the novel images (approachable vs. nonapproachable), but the men who inhaled oxytocin remembered 46 percent of the faces they had viewed previously, compared with 36 percent for the men who did not receive oxytocin, and they were more accurate than the controls in identifying faces they had not seen before.
Rimmele thinks oxytocin produces this effect primarily by acting on the fusiform face area, a part of the visual system dedicated to recognizing faces. Oxytocin also may affect the superior temporal sulcus and the amygdala, which respond to variable aspects of faces such as emotional expression.
“Objects such as landscapes are processed in different brain areas,” she says. “So I think oxytocin probably modulates the face-processing network. It does not seem to modulate networks for nonsocial stimuli.”
In the future, oxytocin might be used to treat such problems as autism, Rimmele says. One study found that oxytocin levels are lower in the blood plasma of children with autism, while another found that such children improved their ability to understand the emotional tone of spoken words after inhaling oxytocin. Two of Rimmele’s colleagues at the University of Zurich, Markus Heinrichs, and Gregor Domes, are already investigating such clinical applications. “They’re looking at people with autism and social phobia to see if it helps them in a therapeutic setting,” she says.