Throughout human evolution, infectious diseases have become a primary cause of death.
The human immune system plays a major role to reduce the harm of infections. One part of the system is to avoid contagious individuals to improve biological fitness.
A recent study shows that human brain is very sensitive to detect disease in others, even before the disease breaks out.
The research shows that we can detect both facial and olfactory cues of sickness in others just hours after experimental activation of their immune system.
It also demonstrates that multisensory integration of these olfactory and visual sickness cues is a crucial mechanism for how we detect and socially evaluate sick individuals.
The findings are published in the Proceedings of the National Academy of Sciences.
Detecting cues about sickness and avoiding others who carry a disease is an adaptive way of coping with an environment fraught with pathogens.
In this process, the brain can perceive and integrate early cues of sickness just hours after the induction of immune system activation.
In the study, the team examined this detection, and the underlying neural mechanisms for this detection.
In the experiment, the immune system in 22 sample donors was transiently activated with an endotoxin injection [lipopolysaccharide (LPS)].
Facial photographs and body odor samples were taken from the same donors when “sick” (LPS-injected) and when “healthy” (saline-injected).
The facial photos and body odor samples subsequently were presented to a separate group of 30 participants who rated their liking of the presented person during fMRI scanning.
The results showed that faces were less socially desirable when sick, and sick body odors tended to lower liking of the faces.
In the brain, sickness status presented by odor and facial photograph resulted in increased neural activation of odor- and face-perception networks, respectively.
In addition, a superadditive effect of olfactory–visual integration of sickness cues was found in the intraparietal sulcus, which was functionally connected to core areas of multisensory integration in the superior temporal sulcus and orbitofrontal cortex.
The researchers suggest that the results outline a disease-avoidance model in which neural mechanisms involved in the detection of disease cues and multisensory integration are vital parts.
“Our study shows a significant difference in how people tend to prefer and be more willing to socialize with healthy people than those who are sick and whose immune system we artificially activated,” says one author.
“We can also see that the brain is good at adding weak signals from multiple senses relating to a person’s state of health.”