The neurotransmitter dopamine is commonly known for its role in brain networks regulating pleasure and reward.
But many people with disorders that affect the dopamine system, such as Parkinson’s disease or schizophrenia, also struggle with social abilities. These include recognising or understanding the emotions and mental states of others, something scientists call the “theory of mind”.
These social difficulties can significantly impact the social relationships and quality of life of those affected. However, it has remained unclear whether these challenges are directly caused by dopamine imbalances or by other factors that co-occur with these disorders.
Now our new research, published in PLOS Biology, shows that a lack of dopamine seems to directly cause social difficulties. This suggests that dopamine-based drugs could one day also help treat the social difficulties in disorders with dopamine imbalances.
People with Parkinson’s disease often display fewer facial expressions than healthy individuals. Others may respond to this in negative ways – eventually leading sufferers to withdraw from social situations altogether. And people who are socially withdrawn for a long time can struggle more with social interactions as they don’t get a chance to practice (remember how difficult it was to return to the social world after lockdown?).
This is one possible explanation for why people with Parkinson’s might experience difficulties in understanding others.
However, it is not the only explanation. Parkinson’s is linked to the death of dopamine neurons in the brain. So some scientists have suspected that dopamine plays a key role in our ability to understand others.
Experimental design
Our latest study tested this latter explanation in 33 healthy individuals. We did this by giving people a drug intended to lower dopamine levels in parts of the brain by blocking the receptors that the neurotransmitter binds to.
We then assessed their theory of mind abilities by using an adaptation of a classical scientific experiment. In the original study introducing this experiment, the authors observed that when people are shown short videos of interacting triangles, they readily attribute social roles and mental states to the moving objects.
We used similar videos. Some were “mental state interactions”, which involved one triangle acting upon or causing a mental state in the other triangle, such as “surprising” it. We also had control videos, involving simpler, action-based interactions, such as one triangle following the other. This is a great way to investigate people’s ability to understand others because you exclude other factors that may influence participants’ performance, such as how they process language or faces.
All participants completed the test twice, once after receiving the dopamine drug, and once after receiving a placebo pill. The order in which they received the interventions was randomised.
We found that taking the drug, in comparison to the placebo, reduced participants’ ability to accurately label both types of triangle videos. This suggests that dopamine is crucial for interpreting social interactions, whether they involve understanding mental states (such as surprise) or not (following).
Interestingly, those participants who showed reduced ability to accurately attribute mental states to triangles were also more likely to struggle with recognising emotions (as measured by another test showing a human form walking in angry, sad and happy emotional states) after receiving the drug.
This indicates that there might be a shared mechanism underlying both emotion recognition and theory of mind abilities in the brain. It is possible that dopamine regulates these abilities via its reward network, encouraging us to seek out those things that make us feel good, and repeating those behaviours. In the social context, dopamine might help us to pay specific attention to the subtle social cues communicated by other people.
Our results suggest that people who suffer from dopamine imbalances might find it hard to adequately recognise the social signals communicated by others. But we also discovered a second possibility. Beside its role in processing rewards, dopamine is well known to play a crucial role in movement. This is why the main symptoms of Parkinson’s disease include difficulties in this domain, such as problems with initiating movements.
Previous research suggests that we use (representations of) our own movements to understand others’ movements. The way people move is often a telltale sign of their emotions and mental states. We typically move slower, and slouch, if we’re sad and walk faster if we’re happy.
We showed that people were better at interpreting triangle movements when the triangles moved around in a way that was similar to their own movements. This suggests that people who struggle with movements may find it harder to interpret the movements of others, and therefore their mental states.
That said, the drug we used in the study made them move more slowly. But this did not have any effect on participants’ ability to interpret the triangle videos.
This is likely because, over a lifetime, we associate our own movements with certain mental states and use these patterns to understand others’ movements. This means that if someone gives you a drug that temporarily changes your movements, you still continue to use your long-held patterns to interpret other people’s actions.
But it is possible that in the later stages of Parkinson’s, long-term dopamine depletion might change their stored movement patterns – making it harder to interpret them in others.
Possible treatments
We believe these new insights represent a significant step forward in our understanding of the neurochemical bases of social cognition.
Our results can also help us better understand the social effects of dopaminergic drugs, which are prescribed to millions of people every day, including to people with Parkinson’s and schizophrenia. People with schizophrenia, for example, have too much dopamine in some parts of their brain. And research suggests this may also lead to problems with understanding others – with balanced levels being optimal. What we previously didn’t know is that the dopamine blockers aimed at treating hallucinations and delusions may also help with their mentalising abilities.
Our study will hopefully pave the way for future studies aiming at the development of new therapeutic interventions for people who find it hard to understand others. Besides people with Parkinson’s and schizophrenia, this might be beneficial for a range of other conditions affected by dopamine imbalances, such as Huntington’s disease or Tourette’s syndrome.