Humans use touch to maintain their social relationships, and the emotional qualities of touch depend on who touches whom. However, it is not known how affective and social dimensions of touch are processed in the brain. We measured haemodynamic brain activity with fMRI from 19 subjects (10 males), while they were touched on their upper thigh by either their romantic partner, or an unfamiliar female or male confederate or saw the hand of one of these individuals near their upper thigh but were not touched. We used multi-voxel pattern analysis on pre-defined regions of interest to reveal areas that encode social touch in a relationship-specific manner. The accuracy of a machine learning classifier to identify actor for both feeling touch and seeing hand exceeded the chance level in the primary somatosensory cortex, while in the insular cortex accuracy was above chance level only for the touch condition. Restricting the task to classify the relationship (partner or stranger), while keeping the toucher sex fixed, amygdala, orbitofrontal cortex, and primary and secondary somatosensory cortices were able to discriminate toucher significantly above chance level. These results suggest that information on the social relationship to the toucher is processed in a consistent manner in several regions. More complex information about toucher identity is processed in the primary somatosensory and insular cortices, both of which can be considered early sensory areas. More research is needed to understand the temporal order of these effects and the impact of contextual factors.
Suvilehto Juulia T, Renvall Ville, Nummenmaa Lauri
affective touch, fMRI, naturalistic touch, social network