Discovery reveals brain chemicals at work influencing social behaviour

We are always intrigued by the human behaviour. Why do people behave in the way they do? Why don't everybody behave in the same way? Why don't they behave  in a culturally-accepted good manner?

Scientists are trying to find out answers to these questions and several others with regard to human nature.

In a new study published in Nature Human Behaviour, scientists delve into the world of chemical neuromodulators in the human brain, specifically dopamine and serotonin, to reveal their role in social behaviour.

The research, conducted in Parkinson's disease patients undergoing brain surgery while awake, homed in on the brain's substantia nigra, a crucial area associated with motor control and reward processing.

 The international team of scientists revealed a previously unknown neurochemical mechanism for a well-known human tendency to make decisions based on social context—people are more likely to accept offers from computers while rejecting identical offers from human players.

The idea that people make decisions based on social context is not a new one in neural economic games. But now, for the first time, researchers show the impact of the social context may spring from the dynamic interactions of dopamine and serotonin.

When people make decisions, dopamine seems to closely follow and react to whether the current offer is better or worse than the previous one, as if it were a continuous tracking system. Serotonin, meanwhile, appears to focus only on the current value of the specific offer at hand, suggesting a more case-by-case evaluation.

This fast dance happens against a slower backdrop, where dopamine is overall higher when people play other human beings—in other words when fairness comes into play. Together, these signals contribute to our brain's overall assessment of value during social interactions.

Dopamine levels are higher when people interact with another human as opposed to a computer. And here it was important that the scientists also measured serotonin to give them confidence that the overall response to social context is specific to dopamine.
Scientists have seen these signaling molecules before, but this is the first time they have seen them dance. No one has ever seen this dance of dopamine and serotonin in a social context before.

Teasing out the meaning of the electrochemical signals recorded from patients in surgery was a major challenge that took years to solve. The raw data that they're collecting from patients isn't specific to dopamine, serotonin, or norepinephrine—it's a mixture of those. They 're essentially using machine-learning type tools to separate what's in the raw data, understand the signature, and decode what's going on with dopamine and serotonin.
In the new Nature Human Behavior study, researchers showed how the rise and fall of dopamine and serotonin are intertwined with human cognition and behaviour.
In Parkinson's disease, a significant loss of dopamine-producing neurons in the brainstem is a key characteristic that usually coincides with the onset of symptoms.

This loss impacts the striatum, a brain region heavily influenced by dopamine. As dopamine diminishes, serotonin terminals begin to sprout, revealing a complex interaction, as observed in rodent models.

Psychiatry is an example of a medical field that could benefit from this approach, according to the researchers.

Enormous number of people in the world suffer from a variety of psychiatric conditions, and, in many cases, the pharmacological solutions do not work very well.
Dopamine, serotonin, and other neurotransmitters are in some ways intimately involved with those disorders. This effort adds real precision and quantitation to understand those problems. The one thing scientists can be sure of is this work is going to be extremely important in the future for developing treatments.

In first-of-their-kind observations in the human brain the scientists published in Neuron in 2020, researchers revealed dopamine and serotonin are at work at sub-second speeds to shape how people perceive the world and take action based on their perception.

More recently, in a study published in October in the journal Current Biology, the researchers used their method of recording chemical changes in awake humans to gain insight into the brain's noradrenaline system, which has been a longtime target for medications to treat psychiatric disorders.

And, in December, in the journal Science Advances, the team revealed that fast changes in dopamine levels reflect a specific computation related to how humans learn from rewards and punishments.

Scientists made active measurements of neurotransmitters multiple times in different brain regions, and they have now reached the point where they are touching on crucial elements of what makes us human beings.

Dopamine and serotonin in human substantia nigra track social context and value signals during economic exchange, Nature Human Behaviour (2024). DOI: 10.1038/s41562-024-01831-w. www.nature.com/articles/s41562-024-01831-w