Emotions synchronize brain activity across individuals

Emotions are highly contagious and it is often noted that emotional states can spread rapidly through a large crowd such as when a peaceful demonstration turns into a riot. In their recent study Nummenmaa et al. (2012) shed light on the underlying neural mechanisms. The authors showed short (on average ~90 s) movie clips depicting humans experiencing either strong positive or negative emotions (or in control clips a neutral state) to 16 healthy volunteers during 3-Tesla functional magnetic resonance imaging.  Immediately after the scanning session, the subjects were asked to continuously rate their emotional state while watching the movie clips on two scales: emotional valence (varying from negative to positive emotional state) and emotional arousal (varying from drowsy to highly aroused).

The authors calculated 17-s moving average of inter-subject correlations of brain hemodynamic responses. The resulting inter-subject brain synchrony time courses were then predicted with the emotional valence and arousal ratings to probe which brain areas are synchronized by emotional states. While high arousal predicted across-subjects synchronous brain activity in somatosensory and visual cortices and dorsal attention networks (comprising intraparietal sulci and frontal eye fields), negative valence predicted increased inter-subject synchrony in emotion-processing networks and in the so-called default-mode network (i.e., precuneous, temporoparietal junction, medial prefrontal cortex and posterior superior temporal sulcus). In contrast to negative valence, positive valence was not observed to enhance inter-subject synchrony of brain activity.

Based on these results, the authors proposed that high arousal directs attention of individuals to similar features in their environment, and that negative valence (i.e., unpleasant emotional states) synchronizes the brain areas of individuals that give rise to emotional states and support understanding of the actions of others. The authors further suggested that positive emotions did not result in increased inter-subject synchrony of brain activity due to positive emotions triggering planning of novel and exploratory thoughts and behaviors, whilst negative emotions can be surmised to trigger for example fight-or-flight responses that results in narrowing of mental/behavioral repertoires. The authors conclude that by synchronizing brain activity across individuals, emotions may promote social interaction and facilitate interpersonal understanding.

Reference:

Nummenmaa L, Glerean E, Viinikainen M, Jaaskelainen IP, Hari R, Sams M. Emotions promote social interaction by synchronizing brain activity across individuals. PNAS (2012) epublication ahead of print.  http://dx.doi.org/10.1073/pnas.1206095109