Lateralized perception of static and dynamic social interactions in left and right visual cortex

The visual system has extensively been studied in relation to its function in object and action recognition. Latest research findings show that it also plays a specific role in representing social interactions (an agent acting towards another), hosting specialized neural structures for this task. In current studies, static representations of social interactions (two people facing towards vs. away from one another) and dynamic representations (video-clips of interacting/non-interacting people) yielded stronger activity for facing/interacting in left extrastriate body area (EBA) and right posterior superior temporal cortex (pSTS), respectively. We asked whether different localization and lateralization of the effect might depend on the stimuli being static or dynamic. We reanalyzed two fMRI datasets, where the same 15 female and male adults saw video-clips and static images of facing –seemingly interacting– and non-facing people. First, whole-brain analysis replicated higher activity for facing (vs. non-facing) bodies in visual cortex, which was overall stronger for dynamic stimuli. For both static and dynamic stimuli, the effect was stronger in left areas. Next, we individually localized the body-selective EBA, the motion-selective MT/V5, the biological motion-selective pSTS, the so-called social-interaction pSTS (SI-pSTS)¬ –and other (control) visual areas. Region-of-interest analysis showed that the facing>non-facing effect in EBA occurred for both static and dynamic stimuli, and was stronger in the left. MT/V5 and pSTS showed the same left-lateralized effect but only for dynamic stimuli. The SI-pSTS showed a third response pattern with a selective, bilateral effect for dynamic stimuli. Challenging the common view that allocates social stuff to right visual areas, these results support a prominent role of left regions in social-interaction processing. Moreover, they suggest that within the hub for social processing in pSTS, there are two different regions, biological-motion pSTS and SI-pSTS, with different response profiles, and presumably different functions in the representation of socially related/interacting agents.