Inflammatory response may influence how we see others

People who are sick typically feel fatigued, emotionally low and less inclined to socialise. This suite of "sickness behaviours" has been particularly linked to inflammation — the immune system's response to a trigger, such as a virus or an injury. 

However, recent work has found that higher levels of inflammation don't always lead to the same feelings and behaviours, and that other factors seem to play a role. Now, in a paper in Brain Behaviour and Immunity, Mallory J Feldman at the University of North Carolina and colleagues report hints that one such factor might be 'interoceptive ability', the ability to perceive sensory signals from inside the body. 

Interoception refers to the sensing of a wide range of bodily signals, such as signals from the heart each time it beats, bodily temperature, or various signals from the gut. Research shows that people vary in their interoceptive ability, and being good at sensing bodily signals has been linked to better emotional functioning. This led the team to wonder whether variations in interoceptive ability, in the presence of inflammation, may affect the impact of emotional cues on social processing, and so variations in some 'sickness behaviours'.

To explore this, they studied 30 healthy young adults. These participants completed an interoception questionnaire, which asked about their ability to tune into bodily states. Then they were given a version of an affect misattribution procedure (AMP). This involves viewing a series of emotionally positive, neutral or negative photos, with each one followed by an image of an emotionally neutral face. (The set of positive photos included a cute puppy, for example, while the negative set included a fire.) Each time, they had to rate the trustworthiness of the face. This task measured the extent to which emotional responses to the initial photo influenced their social judgements. 

The participants were then given the influenza vaccine, triggering an immune response and an increase in inflammation. Then, they completed a version of the AMP again. As expected, based on earlier research, the researchers found that neutral target faces were perceived as more trustworthy when they were paired with positive, rather than negative, images. 

However, their analysis also found that participants with higher levels of inflammation gave even lower trustworthiness ratings after viewing emotionally negative photos than in the initial AMP test. 

This represents some of the first evidence that inflammation may heighten the impact of emotional cues during social decision-making, the authors write. 

Their analysis also found a similar pattern for interoceptive ability: participants with greater interoceptive difficulty also gave lower trustworthiness ratings when neutral faces followed emotionally negative photos. This fits with the results of other work suggesting that people who are poorer at interoception may find it harder to identify exactly what emotion they are feeling, and what the specific source of that feeling is. As a result, these participants may have been more likely to misattribute negative feelings caused by seeing an unpleasant photo as relating to the paired face. 

The team had predicted that interoceptive difficulty would interact with levels of inflammation to influence the impact of emotional cues on trustworthiness judgements. They did not find this. 

But, their findings do suggest that increases in inflammation and also difficulties in perceiving bodily signals affect the extent to which emotion cues in the environment influence social perceptions. 

These findings may help to explain why inflammation seems to lead people to withdraw from some types of social interactions — with strangers, for example — but not from interactions with people they view positively.

The study does have a number of limitations. The sample size was small, and the team used a self-report survey of interoceptive ability, rather than an objective measure. These alone could limit the validity and generalisability of the results. Participants were all young and mostly female, so not exactly representative of the general population; older individuals, those with chronic health conditions involving inflammation, and other demographics may or may not experience the same changes. This research should be viewed as a preliminary study in a field of research that certainly warrants more investigation.

Read the paper in full: https://doi.org/10.1016/j.bbi.2023.05.011