Recent functional magnetic resonance imaging (fMRI) studies have shown that interpersonal synchronization of brain activity can be measured between people sharing similar emotional, narrative, or attentional states. There is evidence that odors can modulate the activity of brain regions involved in memory, emotion and social cognition, suggesting a link between shared olfactory experiences and synchronized brain activity in social contexts.

We used fMRI to investigate the effects of a positively-valenced odor on inter-subject correlation (ISC) of brain activity in healthy volunteers watching movies. While being inside an MRI scanner, participants (N = 20) watched short movie clips to induce either positive (happiness, tenderness) or negative (sadness, fear) emotions. Two movie clips were presented for each emotional category. Participants were scanned in two separate randomized sessions, once while watching the movie clips in the presence of an odor, and once without.

When all emotional categories were combined, the odor condition showed significantly higher ISC compared to the control condition in bilateral superior temporal gyri (STG), right middle temporal gyrus, left calcarine, and lingual gyrus. When splitting the movies according to valence, odor-induced increases in ISC were stronger for the negative movies. For the negative movies, ISC in the supramarginal gyrus and STG was larger in the second compared to first movie clips, indicating a time-by odor interaction.

These findings show that odor increases ISC and that its effects depend on emotional valence. Our results further emphasize the critical role of the STG in odor-based social communication.