Déjà vu? Subjects were shown dozens of pictures like these and tested on whether they remembered them the next day.

U. Rimmele et al., J. Neuroscience, 7 January 2009

Haven't I Seen You Before?

The next time you spot an old friend from across the room, thank oxytocin. Researchers have shown that the brain hormone helps us sense whether a face is familiar.

Oxytocin is a powerful social chemical. In voles, for example, the hormone is key to attachment behavior: Males with higher levels of oxytocin are more likely to be faithful to their mates. Humans also make use of the hormone. Oxytocin helps us maintain our trust in others, even when they have done us wrong (ScienceNOW, 21 May 2008)

Curious about other effects of oxytocin in people, psychologist Ulrike Rimmele of the University of Zurich in Switzerland and colleagues tested the hormone's role in social memory. The team recruited 44 heterosexual men and gave half of them a nasal spray spiked with oxytocin. The researchers then asked the men to look at 168 pictures--half of them of people and half of objects, such as houses and paintings--for a few seconds. The men then had to rate how much they would like to approach each person or object on a scale of one to seven. The next day, the researchers tested the men's memories by presenting them with the pictures they had already seen, along with 72 new photos.

Oxytocin did not appear to affect subjects' approachability ratings, for either faces or objects, the researchers will report tomorrow in The Journal of Neuroscience. However, it did influence whether they recognized familiar faces. The oxytocin-treated subjects picked out 46% of faces they had seen before compared with 36% for the control group. And men who'd received oxytocin were more accurate than the controls in determining which faces they had never seen. There was no difference between the two groups in object recognition.

The results are "striking," says psychologist Thomas Insel, director of the National Institute of Mental Health in Bethesda, Maryland. Insel and colleagues have shown that oxytocin improves the ability of mice to recognize other mice (ScienceNOW, 3 July 2000), but he notes that this is the first time such a specific effect has been seen in humans. The research "supports the notion that social memory is a unique form of memory, biologically distinct from general object memory," he says.

Study co-author Markus Heinrichs says he and his colleagues next want to probe the role of oxytocin in mental disorders that affect social functioning. Already, he says, they have evidence that treatment with oxytocin is beneficial for people with social phobias and autism.