Scientists have made a significant leap in understanding how our brains process social interactions, specifically focusing on the roles of dopamine and serotonin, two critical chemical messengers in the brain. The study, led by Virginia Tech neuroscientists and published in Nature Human Behavior, involved Parkinson’s disease patients who were undergoing brain surgery while awake.
These patients participated in a game that tested their responses to financial offers from both humans and computers, revealing fascinating insights into the neurochemical underpinnings of social decision-making.
Background and Motivation
Dopamine and serotonin are two of the most well-known neurotransmitters in the human brain, playing crucial roles in regulating a wide array of physiological and psychological processes. Dopamine is often referred to as the “feel-good” neurotransmitter because of its association with pleasure, reward, and motivation.
Beyond this, dopamine is critical for motor control and is implicated in various neurological and psychiatric disorders, including Parkinson’s disease, where dopamine-producing neurons deteriorate, leading to motor symptoms.
Serotonin, on the other hand, is primarily involved in regulating mood, anxiety, and happiness. Often called the “stabilizing” neurotransmitter, it contributes to feelings of well-being and happiness and helps regulate the body’s sleep-wake cycles and internal clock. Serotonin also plays a role in appetite, digestion, and emotional states, and it has been linked to depression and anxiety.
The researchers embarked on this study to explore the relationship between dopamine and serotonin during social interactions, particularly how these chemicals influence our decision-making processes in different social contexts. They chose to study Parkinson’s disease patients undergoing deep brain stimulation surgery. This clinical scenario provided a rare opportunity to directly measure the brain’s neurochemical activity in real time during a controlled social interaction task.
Methodology: Deciphering the Dance of Dopamine and Serotonin
The core of the experimental design was the use of the “ultimatum game,” a well-established economic game in psychological research. In this game, participants were asked to respond to offers about splitting a sum of $20, made by either human or computer proposers. These offers varied, with some being fair (e.g., a $10-$10 split) and others being less so (e.g., a $16-$4 split in favor of the proposer).
The participants had the option to either accept the split, in which case both parties would receive the proposed amounts, or reject it, resulting in neither party receiving any money. This setup aimed to explore decision-making processes and the influence of social context on these decisions.
“You can teach people what they should do in these kinds of games — they should accept even small rewards as opposed to no reward at all,” said Virginia Tech computational neuroscientist Read Montague, the senior author of the study. “When people know they’re playing a computer, they play perfectly, just like mathematical economists – they do what they should do. But when they’re playing a human being, they cannot help themselves. They are often driven to punish the smaller bid by rejecting it.”
To measure the neurochemical dynamics during these tasks, the researchers employed an innovative approach using carbon-fiber electrodes that were implanted in the participants’ brains, specifically targeting the substantia nigra, a region known for its role in motor control and reward processing. These electrodes allowed for the detection of changes in dopamine and serotonin levels at a resolution of ten samples per second, offering a detailed view of the neurochemical fluctuations that occur during decision-making.
“The unique twist with our method is that it allows us to measure more than one neurotransmitter at a time — the impact of that should not be lost,” explained Seth Batten, a senior research associate in Montague’s lab and shared first author of the study. “We’ve seen these signaling molecules before, but this is the first time we’ve seen them dance. No one has ever seen this dance of dopamine and serotonin in a social context before.”
But deciphering the significance of the electrochemical signals obtained from patients during surgery presented a significant obstacle that required years to overcome.
“The raw data that we’re collecting from patients isn’t specific to dopamine, serotonin, or norepinephrine – it’s a mixture of those,” said Ken Kishida, a co-author of the study and an associate professor of translational neuroscience, and neurosurgery, at Wake Forest University School of Medicine. “We’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.”
Key Findings
The researchers found that dopamine appeared to function as a kind of continuous tracking system, closely monitoring and reacting to whether each new offer in the game was better or worse than the one before it. This suggests that dopamine may be integral to evaluating changes in our environment and adjusting our expectations and decisions accordingly.
In contrast, serotonin’s activity was more focused on the immediate value of the current offer, irrespective of past offers. This indicates that serotonin may be responsible for assessing the present situation on a case-by-case basis, without the influence of prior outcomes.
Moreover, the study revealed that the social context of these interactions—whether participants believed they were interacting with a human or a computer—had a significant impact on dopamine levels. Specifically, dopamine levels were consistently higher when participants were dealing with human proposers compared to when they were interacting with computer proposers.
This finding underscores the importance of social context in our neurochemical responses and suggests that our brains may be particularly attuned to fairness and the social implications of our decisions when we believe we are dealing with other humans.
Interestingly, while overall levels of dopamine varied with the social context, serotonin levels did not show a similar pattern. This specificity to dopamine suggests that it plays a pivotal role in our brain’s response to social interactions, potentially driving the emotional and cognitive processes that underlie our sense of social fairness and justice.
“We are shining a spotlight on various cognitive processes and finally receiving answers to questions in finer biological detail,” said study shared first author Dan Bang, associate professor of clinical medicine and Lundbeck Foundation Fellow at Aarhus University in Denmark, and an adjunct associate professor at the Fralin Biomedical Research Institute.
“Dopamine levels are higher when people interact with another human as opposed to a computer,” Bang said. “And here it was important that we also measured serotonin to give us confidence that the overall response to social context is specific to dopamine.”
Limitations and Future Directions
However, the study was not without its limitations. With only four participants, the sample size was quite small, raising questions about the generalizability of the findings. Additionally, the specific focus on Parkinson’s disease patients undergoing brain surgery means that the results may not directly apply to the broader population.
The researchers acknowledge these limitations and call for further studies to expand on their findings, particularly to explore how these neurochemical processes operate in everyday social interactions beyond the controlled environment of the surgery room.
Future research directions could include studying these neurochemical processes in a wider range of social contexts and with a larger and more diverse participant group. This could help to deepen our understanding of the neurobiological basis of social behavior, potentially leading to new insights into psychiatric disorders where social interaction is affected, such as schizophrenia or social anxiety disorder.
Implications for Parkinson’s Disease and Other Conditions
The insights gained from this study offer valuable implications for understanding and potentially addressing Parkinson’s disease. Parkinson’s disease is primarily characterized by its motor symptoms, such as tremors, stiffness, and bradykinesia, which are attributed to the degeneration of dopamine-producing neurons in the brain. This decline affects the striatum, a part of the brain significantly affected by dopamine levels. As dopamine decreases, serotonin terminals start to grow, indicating a sophisticated interplay.
“At some point, after we have evaluated enough people, we’re going to be able to address the Parkinson’s disease pathology that’s given us this window of opportunity,” Montague said. “Already there is pre-clinical evidence that the attrition of the dopamine system is telling the serotonin system, ‘Hey, we’ve got to do something.’ But we’ve never been able to watch the dynamics. What we’re doing now is the first step, but one would hope that once we get up to hundreds of patients, we’d be able to relate this to symptomatology and make some clinical statements about the Parkinson’s pathology.”
Michael Friedlander, executive director of the Fralin Biomedical Research Institute, who was not involved in the study, said, “This work is changing the entire field of neuroscience and our ability to query the human mind and brain — with a technology that was just not even imagined not many years ago.”
He added that psychiatry is an example of a medical specialty that could be significantly enhanced by this line of research.
“We have an enormous number of people in the world who suffer from a variety of psychiatric conditions, and, in many cases, the pharmacological solutions do not work very well,” said Friedlander, who is also Virginia Tech’s vice president of health sciences and technology. “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 I think we can be sure of is this work is going to be extremely important in the future for developing treatments.”
The study, “Dopamine and serotonin in human substantia nigra track social context and value signals during economic exchange,” was authored by Seth R. Batten, Dan Bang, Brian H. Kopell, Arianna N. Davis, Matthew Heflin, Qixiu Fu, Ofer Perl, Kimia Ziafat, Alice Hashemi, Ignacio Saez, Leonardo S. Barbosa, Thomas Twomey, Terry Lohrenz, Jason P. White, Peter Dayan, Alexander W. Charney, Martijn Figee, Helen S. Mayberg, Kenneth T. Kishida, Xiaosi Gu, and P. Read Montague.