Deep inside the brain is a bundle of around 165,000 neurons, called the dorsal raphe nucleus. When prompted into action, the nucleus produces the neurotransmitter serotonin, which acts as a messenger to other areas of the brain. Two areas often in communication with the dorsal raphe nucleus sit just behind your eyes — the medial prefrontal cortex and the orbitofrontal cortex. Together, this communication channel appears responsible for the virtue of patience.
The Path of Serotonin
Past research has found a strong link between serotonin and patience. When the dorsal raphe nucleus is stimulated, serotonin floods the system and patience is displayed, while in those with low levels of serotonin impulsive behaviour is the norm. However, this effect appears to be maximised when two features are present — a high probability for a reward, and uncertainty about the timing of that reward.
In a 2018 study, Katsuhiko Miyazaki et al. hypothesised that serotonin plays a role in “resolving trade-offs, not only between immediate and delayed rewards, but also between sensory evidence and subjective confidence.” It could achieve this by influencing our judgements of the likelihood or probability of a reward.
Katsuhiko Miyazaki’s team set out to test the idea. They bred mice with light-sensitive serotonergic neurons, and implanted an optical fibre into four brain regions: the dorsal raphe nucleus, orbitofrontal cortex, medial prefrontal cortex, and nucleus accumbens. This allowed them to precisely control the release of serotonin.
Activity in the nucleus accumbens did not affect waiting, while both the dorsal raphe nucleus and the orbitofrontal cortex increased patience in similar ways. The medial prefrontal cortex, however, only promoted waiting when the timing of the reward was uncertain. This is not the first study to find a link between the medial prefrontal cortex and timing, but it does highlight the role this plays in reward-seeking.
In regards to the roles of the medial prefrontal cortex and the orbitofrontal cortex, Miyazaki told Medical News Today:
“This confirmed the idea that these two brain areas are calculating the probability of a reward independently from each other and that these independent calculations are then combined to ultimately determine how long the mice will wait.”
When’s the Reward?
Previous studies have highlighted that unexpected rewards are more effective at keeping us coming back. A classic example is social media, in which we can scroll through many unrewarding headlines and articles before anything of value pops up. Perhaps our willingness to continue for a reward whose both content and timing are uncertain is due in part to activity in our medial prefrontal cortex.
Meanwhile delayed gratification is most often highlighted by the marshmallow experiment in which children were tasked with resisting one sweet for the prospect of a second treat in the near future. The research spurred other psychologists to look into the mechanisms behind our ability to resist temptations, one of the bigger theories was ego-depletion.
Roy Baumeister ran several studies leading to his conclusion that willpower is reliant on a limited resource. The longer we have to resist something, the less likely we are to succeed; the more effort we invest in our decisions, the poorer our decisions will become. As our mental energy is drained, we need rest or glucose to replenish it.
However, the ego-depletion idea has suffered from the replication crisis. Attempts by other researchers to run the original studies came back with nothing, or such a small effect as to be largely negligible. And so the search for the source of self-control continues.
Perhaps Katsuhiko Miyazaki’s studies explain some of it — rather than a limited resource, willpower could be a product of serotonin, and the communication it allows between brain regions trying to estimate the timing and value of the expected rewards.
But that’s my own speculation. At the very least, the new studies by Katsuhiko Miyazaki give us a better understanding of how impulsive and patient behaviour forms in the brain, and may lead to better treatments for those who continually struggle with it.