Capacity for vicarious experiences is a fundamental aspect of human social behavior. For example, seeing others experiencing pain can activate brain circuits that are known to support actual first-hand experience of pain. 

A new study has revealed how the human brain’s opioid system modulates responses to other people’s pain. The less opioid receptors the participants had in their brain, the stronger were their emotion and pain circuits’ response to seeing others in distress. Similar association was not found for the dopamine system despite its known importance in pain management.

The study was conducted by using positron emission tomography (PET) and functional magnetic resonance imaging (fMRI). The participants were injected with radioactive compounds that bind to their brain’s opioid and dopamine receptors. Radioactivity in the brain was measured twice with the PET camera to map the distribution of opioid and dopamine receptors. Subsequently, the participants’ brain activity was measured with fMRI while they viewed videos depicting humans in various painful and painless situations.

"Our results demonstrate the importance of the endogenous opioid system in helping us to relate with others’ feelings. Interindividual differences in the opioid system could explain why some individuals react more strongly than others to someone else’s distress," says Tomi Karjalainen from the University of Turku PET Centre.

Citation: Tomi Karjalainen, Henry K. Karlsson, Juha M. Lahnakoski, Enrico Glerean, Pirjo Nuutila, Iiro P. Jääskeläinen, Riitta Hari, Mikko Smas, Lauri Nummenmaa, 'Dissociable Roles of Cerebral μ-Opioid and Type 2 Dopamine Receptors in Vicarious Pain: A Combined PET–fMRI Study', Cerebral Cortex, 24 May 2017 url:  DOI: