Summary: A new study reveals that serotonin neurons in the brain may play a crucial role in helping us estimate the value of future rewards. Rather than simply responding to pleasure or pain, serotonin appears to encode a “prospective value” signal—telling the brain how good the near future is likely to be.
Using reinforcement learning models and neural recordings from serotonin-rich regions, researchers found these neurons respond most to unexpected rewards. This insight sheds new light on the complex role of serotonin in decision-making, learning, and emotion, potentially changing how we understand mood disorders and mental health treatments.
Key Facts:
- Prospective Value Code: Serotonin neurons encode expected future rewards, not just pleasure or pain.
- Surprise Matters: These neurons are especially activated by unexpected rewards.
- Broad Implications: Findings could reshape understanding of decision-making and mental health treatments.
Source: University of Ottawa
In our day-to-day lives, we’re constantly making a slew of decisions from immediate matters to prospects on the far horizon.
But the evolutionary nuts-and-bolts of how our brains weigh these numerous daily decisions and what role is played by the neurotransmitter serotonin has been shrouded in mystery.
Now, a new study led by an interdisciplinary uOttawa Faculty of Medicine team delivers fascinating findings on this big topic and potentially unravels a hidden aspect of what our nervous system’s extraordinarily complex serotonin system is really doing inside the enigmatic organ in our skulls.
Published in the journal Nature, this study from a highly impactful international collaboration was considered by one of the expert reviewers who evaluated the work to offer “broad implications across neuroscience, psychology, and psychiatry, enhancing our understanding of serotonin’s role in mood regulation, learning, and motivated behavior.”
The team’s innovative work merges ideas from reinforcement learning (RL) theory – used in neuroscience to better understand learning, behavior, and decision-making – with recent hard-won insights into the filtering properties of the brain’s dorsal raphe nucleus. That’s a region of the mammalian brainstem rich in serotonin-producing neurons.
Serotonin is often painted as the brain’s “pleasure chemical.” Antidepressant drugs such as selective serotonin reuptake inhibitors (SSRIs) famously target the serotonin system as part of a multi-billion-dollar industry.
However, serotonin’s precise role in the nervous system is ambiguous and perplexing: It’s implicated in everything from mood and movement regulation to appetite and sleep-wake cycles. The fact that it’s activated by pain, pleasure and surprise has long been a brain research puzzle.
With this study, the uOttawa-led researchers put forth a unifying perspective on serotonin they dub a “prospective code for value” – a biological code for how the brain places a value for future rewards.
This code essentially explains why serotonin neurons are activated in the brain in response to both rewards and punishments, with a preference for surprising rewards.
“Our work asks the question: What does serotonin tell the brain? In a nutshell, we find that its message closely matches the expectation of future rewards,” says senior author Dr. Richard Naud, associate professor at the Faculty of Medicine’s Department of Cellular and Molecular Medicineand the uOttawa Department of Physics.
Co-author Dr. Jean-Claude Béïque, professor in the Department of Cellular and Molecular Medicine, puts the main findings like this: “Your brain needs to compute the expected value of the actions you contemplate and undertake as you interact with a changing world, asking ‘What’s the value of this decision versus that decision in that particular environment?’ That’s a hard problem.
“So what we think serotonin actually does in the brain is encode the expected value of a particular environment or course of actions in order to ultimately guide everyday decisions.”
Drs. Béïque and Naud are both members of the uOttawa Brain and Mind Research Institute’s Centre for Neural Dynamics and Artificial Intelligence.
About this neuroscience research news
Author: Richard Naud
Source: University of Ottawa
Contact: Richard Naud – University of Ottawa
Image: The image is credited to Neuroscience News
Original Research: Closed access.
“A prospective code for value in the serotonin system” by Richard Naud et al. Nature
Abstract
A prospective code for value in the serotonin system
The in vivo responses of dorsal raphe nucleus serotonin neurons to emotionally salient stimuli are a puzzle. Existing theories centring on reward, surprise, salience and uncertainty individually account for some aspects of serotonergic activity but not others.
Merging ideas from reinforcement learning theory with recent insights into the filtering properties of the dorsal raphe nucleus, here we find a unifying perspective in a prospective code for value.
This biological code for near-future reward explains why serotonin neurons are activated by both rewards and punishments, and why these neurons are more strongly activated by surprising rewards but have no such surprise preference for punishments—observations that previous theories have failed to reconcile.
Finally, our model quantitatively predicts in vivo population activity better than previous theories.
By reconciling previous theories and establishing a precise connection with reinforcement learning, our work represents an important step towards understanding the role of serotonin in learning and behaviour.
