The messenger substances dopamine and serotonin in our brain play an important role in our social behavior. Researchers have now directly observed their interaction for the first time. This was made possible by four Parkinson's patients who took part in an experiment during brain surgery. The results reveal a neurochemical mechanism for our tendency to make decisions based on social context. In the long term, the technology may also help to better understand changes in brain chemistry in Parkinson's disease.
It's a classic experiment in behavioral psychology: ask a person to divide $20 between themselves and another person. If the other person accepts the proposed offer, both will receive the agreed amount. If the other person refuses, both of them come away empty-handed. From a purely economic perspective, it would be most worthwhile for the second person in this ultimatum game to accept any offer - even if the first person suggests only giving up a dollar. After all, a dollar is better than nothing. In practice, however, it has been shown that most people tend to turn down offers that they perceive as unfair. In order to punish the other person for the outrageous offer, they give up their own profit.
Research during brain surgery
But what happens in the brain during such decision-making processes? A team led by Seth Batten from Virginia Tech has now gotten to the bottom of this question. Since the processes in the brain cannot usually be observed in sufficient detail, the researchers chose an unusual approach: They asked four Parkinson's patients who were undergoing brain surgery while awake to take part in the ultimatum game during the operation.
The operation was carried out to implant electrodes for deep brain stimulation and thus alleviate the patient's symptoms. With the patients' consent, Batten and his team briefly inserted another electrode into the open brain and used it to measure the ratio of the brain messengers dopamine and serotonin while the test subjects made decisions in the ultimatum game. The team focused on the substantia nigra, which is involved in motor control and reward processing. The data was evaluated using machine learning.
Social context relevant
In the ultimatum game, the researchers either told the test subjects that they would play with a human or with a computer. This information played an important role in the decisions about the respective offers: "When people know that they are playing against a computer, they play perfectly, just like mathematical economists - they prefer small winnings to no winnings at all," explains Batten's colleague Read Montague . "But when they play against a human, they can't help it: they are often driven to punish the smaller offer by rejecting it." In fact, the test subjects also accepted small offers if they knew that their counterpart was a computer is. On the other hand, they rejected identical offers from humans. As expected, they took the social context into account in their decision.
The measurements of brain messengers reveal a biological mechanism for this phenomenon: dopamine levels vary depending on whether the current offer is better or worse than the previous one. Serotonin levels, on the other hand, respond to the value of the current offering, regardless of previous history. “Interestingly, overall dopamine levels are higher when people interact with another human than with a computer,” reports Bang. As soon as thoughts of fairness come into play, our brain evaluates the offers differently.
Research into Parkinson's disease
One of the fundamental features of Parkinson's disease is the loss of dopamine-producing neurons in the brainstem. While the current study did not focus on the brainstem, further experiments could also help to better understand the pathologically altered interactions between dopamine and serotonin in Parkinson's disease.
"There is already preclinical evidence that the progressive loss of the dopamine system tells the serotonin system, 'Hey, we have to do something.' But we have never been able to observe this dynamic before,” explains Montague. “Our current study is a first step, but there is hope that if we study enough people, we will be able to link the interplay of neurotransmitters to disease symptoms and make clinical statements about Parkinson's pathology make."
Source: Seth Batten (Virginia Tech) et al., Nature Human Behavior, doi: 10.1038/s41562-024-01831-w