How our immune system evolved to take control of our decision-making processes
Two new research papers are shedding light on the fascinating relationship between inflammation and behavior, suggesting our immune system can play a significant role in both our motivation and decision-making abilities.
An interesting body of evidence is growing around the idea that inflammatory activity in the brain can influence everything from behavior and mood, to self-regulation and decision-making. For example, several recent studies have contended a strong association between neuroinflammation and suicidal thoughts, implying major behavioral outcomes can be linked to physiological mechanisms.
One new study, led by scientists from the Texas Christian University, has found compelling evidence suggesting a link between proinflammatory cytokines and present-focused decision-making. The hypothesis presented is that higher levels of inflammation could be correlated with individuals preferring immediate gratification over delayed rewards. The idea is that we evolved this mechanism so that when the body is placed in a stressful situation it focuses on the immediate surrounding scenario as opposed to risking one's current well-being on the prospect of a future reward.
To experimentally test this hypothesis the researchers recruited 159 healthy subjects and induced increases of proinflammatory cytokines, measured in salivary levels, by exposing them to photographs of threats such as pathogens or images of physical harm. The cohort then completed a series of surveys designed to evaluate impulsivity, present focus, and their ability to delay gratification.
The results were intriguing, revealing inflammatory levels could be used to accurately predict how impulsive and present-focused a person's decisions would be. Essentially, the researchers saw higher levels of inflammatory markers correlating with subjects choosing immediate rewards over future, potentially larger, rewards.
Evolutionary scientists have long presented the idea that when organisms are placed in unpredictable or stressful environments they will exhibit a preference for immediate rewards over future potentials. The framework is often referred to as the risk-sensitive foraging theory (RSFT). What is exciting about this new study is that these decisions may be mechanistically guided by physiological processes in the body. Inflammation may directly exacerbate impulsive, present-focused decisions.
It is one thing to describe an association between inflammation and behavior, but another to figure out what is going on neurologically here.
A new paper from scientists at Emory University proposes a theoretical framework that hopes to explain how the immune system can directly disrupt the dopamine system in the brain to help regulate the body's energy resources. It is suggested this mechanism evolved to force a body into a rest state during times of acute stress.
"When your body is fighting an infection or healing a wound, your brain needs a mechanism to recalibrate your motivation to do other things so you don't use up too much of your energy," explains corresponding author Michael Treadway.
Treadway and his team are working towards developing a computational model designed to calculate the effects of chronic inflammation in relation to effort-based decision making. The implication is that this mechanism, which evolved to help our ancestors make better decisions in times of stress, is resulting in dysfunctional behaviors for us modern humans.
"While this mechanism may have been adaptive in ancestral times, the increased prevalence of chronic inflammation as a function of multiple factors including poor diet, limited exercise, and increased stress in Western societies may contribute to rising rates of depression and other disorders associated with motivational impairments," the researchers conclude in their recently published paper.
It's important to note that these studies are not trying to entirely explain away mental health conditions by suggesting inflammation is the sole cause of major depression or even schizophrenia. Instead, it is hoped these new understandings into how behaviors can be influenced by immune responses may lead to novel therapeutic approaches for a variety of mental health disorders.
"We're not proposing that inflammation causes these disorders," says Treadway. "The idea is that a subset of people with these disorders may have a particular sensitivity to the effects of the immune system and this sensitivity could contribute to the motivational impairments they are experiencing."
The inflammation/decision-making study was published in the journal Scientific Reports.
The dopamine behavior study was published in the journal Trends in Cognitive Sciences.